Kilocalorie to Kilojoule Converter

Convert kilocalories to kilojoules with our free online energy converter.

Quick Answer

1 Kilocalorie = 4.184 kilojoules

Formula: Kilocalorie × conversion factor = Kilojoule

Use the calculator below for instant, accurate conversions.

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All conversion formulas on UnitsConverter.io have been verified against NIST (National Institute of Standards and Technology) guidelines and international SI standards. Our calculations are accurate to 10 decimal places for standard conversions and use arbitrary precision arithmetic for astronomical units.

Last verified: February 2026|Reviewed by: Sam Mathew, Software Engineer

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How to Use the Kilocalorie to Kilojoule Calculator:

Enter the value you want to convert in the 'From' field (Kilocalorie).
The converted value in Kilojoule will appear automatically in the 'To' field.
Use the dropdown menus to select different units within the Energy category.
Click the swap button (⇌) to reverse the conversion direction.

How to Convert Kilocalorie to Kilojoule: Step-by-Step Guide

Converting Kilocalorie to Kilojoule involves multiplying the value by a specific conversion factor, as shown in the formula below.

Formula:

kJ = kcal × 4.184

Example Calculation:

1 kcal = 4.184 kJ

Disclaimer: For Reference Only

These conversion results are provided for informational purposes only. While we strive for accuracy, we make no guarantees regarding the precision of these results, especially for conversions involving extremely large or small numbers which may be subject to the inherent limitations of standard computer floating-point arithmetic.

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What is a Kilocalorie and a Kilojoule?

and Standards

The Kilocalorie (Large Calorie)

The kilocalorie (kcal), also called the large Calorie (Cal with capital C) or food Calorie, is defined as:

1 kcal = 1,000 cal = 4,184 joules (exactly) = 4.184 kilojoules

This represents the amount of energy required to raise the temperature of one kilogram of pure water by one degree Celsius (specifically from 14.5°C to 15.5°C) at standard atmospheric pressure (101.325 kPa).

Relationship to Small Calorie

The kilocalorie is exactly 1,000 times larger than the small calorie (gram calorie):

Small calorie (cal): Energy to raise 1 gram of water by 1°C = 4.184 J
Kilocalorie (kcal): Energy to raise 1 kilogram of water by 1°C = 4,184 J

This factor-of-1,000 difference is the source of widespread confusion when people use "calorie" (lowercase) to mean "Calorie" (uppercase, kcal) in everyday speech.

Food Calorie Equivalence

Food "Calories" = kilocalories (kcal)

Nutrition labels display energy in kilocalories but label them as:

"Calories" (capital C) in United States and Canada
"Cal" (abbreviated capital C) in some international contexts
"kcal" in scientific and some European contexts
"calories" (lowercase) in informal usage, despite technical incorrectness

All of these refer to the same unit: 1 kcal = 1,000 cal

Thermochemical vs. International Table kcal

Like the small calorie, multiple kilocalorie definitions existed historically:

Thermochemical kilocalorie (kcal_th): Exactly 4,184 J (most common today)
International Table kilocalorie (kcal_IT): Exactly 4,186.8 J
15°C kilocalorie (kcal₁₅): 4,185.5 J
Mean kilocalorie (kcal_mean): 4,190 J

The thermochemical kilocalorie (4,184 J) is now standard for nutrition labeling and most applications. Differences between definitions are less than 0.2%, negligible for practical nutrition purposes.

and Standards

The kilojoule is defined as exactly 1,000 joules:

1 kJ = 1,000 J = 1,000 kg·m²/s² = 1,000 N·m

Precise Equivalents

1 kJ = 1,000 J (exactly, by definition)
1 kJ = 0.239006 kilocalories (kcal, food Calories)
1 kJ = 0.947817 British thermal units (BTU)
1 kJ = 0.000277778 kilowatt-hours (kWh)
1 kJ = 6.242 × 10²¹ electronvolts (eV)
1 kcal = 4.184 kJ (exactly, thermochemical definition)

SI System Integration

The kilojoule is a standard SI unit formed by applying the metric prefix kilo- (1,000) to the base unit joule. Unlike the calorie, which has multiple definitions and creates confusion between small calories (cal) and large Calories (kcal), the kilojoule provides unambiguous measurement consistent across all scientific disciplines.

Nutrition Labeling Standards

International practice:

Australia, New Zealand: kJ only (mandatory since 1995/2003)
European Union: Both kJ and kcal required (kJ first, since 2016)
China, Japan, South Korea: Primarily kJ with optional kcal
Canada: Both kJ and kcal required
United States: kcal only (Calories)

Standard format: "Energy: 250 kJ (60 Cal)" or "Energy: 1,000 kJ / 239 kcal"

Note: The Kilocalorie is part of the imperial/US customary system, primarily used in the US, UK, and Canada for everyday measurements. The Kilojoule belongs to the imperial/US customary system.

History of the Kilocalorie and Kilojoule

and Evolution

Pre-Nutrition Era (1800s)

French chemist Nicolas Clément first defined the calorie in 1824 as the heat needed to raise one kilogram of water by one degree Celsius—equivalent to the modern kilocalorie. This original definition used the kilogram, not the gram.

Later in the 19th century, chemists Pierre-Antoine Favre and Johann Silbermann redefined the unit for the gram (small calorie) to make laboratory measurements more practical. This created two units:

Small calorie (cal): 1 gram of water by 1°C
Large calorie or kilocalorie (kcal): 1 kilogram of water by 1°C

Nutrition Science Adoption (1890s-1900s)

American agricultural chemist Wilbur Olin Atwater (1844-1907) introduced the kilocalorie to nutrition science in the 1890s. Working at Wesleyan University and the USDA, Atwater:

Built the first human respiration calorimeter in the United States (1892), a room-sized chamber that measured the heat produced by human subjects to calculate their energy expenditure.

Measured caloric content of hundreds of foods using bomb calorimetry, burning food samples and measuring released energy.

Established the Atwater system (still used today):

Carbohydrates: 4 kcal/gram
Proteins: 4 kcal/gram
Fats: 9 kcal/gram
Alcohol: 7 kcal/gram

Published food composition tables (1896) listing kilocalorie values for common foods, enabling dietary planning.

Why Atwater chose kilocalories: Food portions typically contain hundreds to thousands of kilocalories, making kcal a more convenient unit than small calories (which would require values in the hundreds of thousands) or joules (which would require values in the millions).

The "Calorie" Naming Convention (1900s-1920s)

To distinguish the nutrition unit from the small calorie used in chemistry, nutrition scientists adopted the convention of capitalizing "Calorie" to mean kilocalorie:

calorie (lowercase c): Small calorie (cal), used in chemistry
Calorie (capital C): Kilocalorie (kcal), used in nutrition

This convention appeared in early nutrition textbooks and gradually became standard in American and Canadian usage. However, it created confusion because:

Many people don't notice the capitalization difference
Spoken language doesn't distinguish capital vs. lowercase
Modern usage often writes "calories" (lowercase) on labels despite technically meaning kcal

Global Food Labeling Standards (1960s-Present)

United States: The FDA mandated nutrition labeling in 1990 (Nutrition Labeling and Education Act), requiring "Calories" (kcal) on all packaged foods by 1994.

European Union: Required energy labeling in the 1990s, initially allowing kcal only, then requiring both kJ and kcal (Directive 90/496/EEC), with later regulations (EU 1169/2011) making kJ primary with kcal optional.

Australia/New Zealand: Required kilojoules (kJ) as primary unit with Calories (kcal) as secondary (Food Standards Code 2002).

Canada: Required both kcal and kJ on bilingual labels (English/French).

Most other nations: Adopted kcal-based labeling following US or EU models.

Resistance to SI Joule Adoption

Despite the joule becoming the official SI unit of energy in 1948, the kilocalorie persists in nutrition for several reasons:

Consumer familiarity: Generations learned to count "Calories" (kcal)
Established databases: Millions of recipes, diet plans, and food databases use kcal
Convenient numbers: Typical meals measure 300-800 kcal vs. 1.3-3.3 million joules
Global standardization: Most nations use kcal, creating network effects
Economic costs: Relabeling and re-education would cost billions

Only Australia and New Zealand successfully transitioned to kilojoules (kJ) as the primary unit, though even there, Calories (kcal) remain widely understood.

and Evolution

The Joule Foundation: Mechanical Equivalent of Heat (1840s-1850s)

The kilojoule's origins trace to James Prescott Joule's groundbreaking experiments establishing that mechanical work and heat are equivalent forms of energy. His famous paddle-wheel experiments (1843-1850) demonstrated that a specific amount of mechanical work always produces the same amount of heat, establishing the mechanical equivalent of heat: approximately 4.184 joules per calorie.

This work unified thermodynamics and mechanics, providing the foundation for energy measurement in consistent units rather than the historically separate "calories" for heat and "work" for mechanical energy.

SI Adoption and the Joule as Standard (1960)

The International System of Units (SI) adopted the joule as the standard unit of energy in 1960, derived from the base units kilogram, meter, and second. The kilojoule naturally followed as the convenient multiple for everyday energy measurements, particularly for food and human metabolism where joule values would be inconveniently large (a typical meal contains millions of joules but only thousands of kilojoules).

International Nutrition Standards: WHO and FAO (1970s-1980s)

The World Health Organization (WHO) and Food and Agriculture Organization (FAO) began recommending kilojoules for international nutrition standards in the 1970s-1980s. Reports on dietary energy requirements transitioned from kilocalories to kilojoules to align with scientific practice and eliminate confusion between calories and Calories.

Key milestone: The 1985 FAO/WHO/UNU Expert Consultation on Energy and Protein Requirements recommended expressing all energy values in both kilojoules and kilocalories, with kilojoules as the primary unit.

Australia: First Mandatory kJ Labeling (1995)

Australia became the first country to mandate exclusive kilojoule labeling on packaged foods in 1995, eliminating Calories entirely from nutrition panels. This bold move aimed to:

Align consumer nutrition information with scientific standards
Eliminate calorie/Calorie confusion
Educate the public in metric energy units
Standardize with international scientific practice

Reaction: Initial consumer confusion gave way to acceptance within 5-10 years. By 2005, surveys showed most Australians understood kilojoule values and could estimate daily requirements in kJ.

Menu labeling: In 2011, Australia extended kilojoule labeling to chain restaurant menus, requiring displayed kJ values for all items (but not Calories).

New Zealand and EU Adoption (2000s-2010s)

New Zealand followed Australia with mandatory kilojoule-only labeling in 2003.

The European Union required both kilojoules and kilocalories on nutrition labels from 2016 (under Regulation EU 1169/2011), with kilojoules listed first to emphasize the SI unit while maintaining familiarity for consumers accustomed to Calories.

United States: The Calorie Holdout

The United States remains the primary developed nation using kilocalories (Calories) exclusively on nutrition labels. The FDA updated nutrition fact panels in 2016 and 2020 without adopting kilojoules, citing consumer familiarity and the impracticality of changing established practice.

Scientific paradox: US nutrition scientists, exercise physiologists, and researchers routinely use kilojoules in academic publications while consumers encounter only Calories on products.

Global Status (2025)

Countries using kJ exclusively: Australia, New Zealand, Singapore (primary)

Countries using both kJ and kcal: European Union, Canada, China, Japan, South Korea, India, Brazil, Argentina, South Africa

Countries using kcal exclusively: United States, Myanmar

Population coverage: Over 85% of the world's population lives in countries where kilojoules appear on nutrition labels, either exclusively or alongside kilocalories.

Common Uses and Applications: kilocalories vs kilojoules

Explore the typical applications for both Kilocalorie (imperial/US) and Kilojoule (imperial/US) to understand their common contexts.

Common Uses for kilocalories

Nutrition Labeling and Diet Planning

The kilocalorie is the universal standard for quantifying food energy:

Nutrition Facts labels: Required in virtually all countries, listing energy per serving in kcal (or kcal + kJ in some regions)

Calorie counting apps: MyFitnessPal, Lose It!, Cronometer, Noom—all use kcal as the primary unit for tracking food intake

Diet programs: Weight Watchers (now WW), Jenny Craig, Nutrisystem—all based on kcal targets

Meal planning: Registered dietitians prescribe meal plans targeting specific kcal intakes based on individual needs, goals, and medical conditions

Restaurant menus: Chain restaurants in US, Canada, UK, Australia, and many other countries must display kcal values for menu items

Packaged foods: All packaged foods show kcal per serving and per 100g/100ml for comparison

Weight Management

Kilocalories are central to weight loss and gain strategies:

Creating deficits: To lose weight, consume fewer kcal than expended (TDEE - Total Daily Energy Expenditure)

Tracking intake: Apps and food diaries log kcal consumed throughout the day

Portion control: Understanding kcal density (kcal per 100g) helps identify high-calorie vs. low-calorie foods

Meal timing: Some approaches (intermittent fasting) manipulate meal timing while maintaining target kcal

Flexible dieting ("IIFYM"): "If It Fits Your Macros" approach allows any food that fits kcal and macronutrient targets

Bariatric surgery patients: Restricted kcal intakes (800-1,200 kcal/day) during weight loss phase

Exercise and Fitness Tracking

Kilocalories quantify energy expenditure during physical activity:

Fitness trackers: Apple Watch, Fitbit, Garmin, Polar devices estimate kcal burned using heart rate, movement, age, weight, and gender

Heart rate monitors: Convert heart rate to kcal/min based on validated equations (though accuracy varies ±20-40%)

Exercise equipment: Treadmills, ellipticals, stationary bikes, rowing machines display estimated kcal burned

Workout programs: CrossFit, Orangetheory, Peloton show kcal burned to motivate participants

Training logs: Athletes track both kcal consumed and kcal expended to maintain energy balance

Race fueling: Endurance athletes calculate kcal needed during marathons, triathlons, ultra-marathons (30-60g carbs/hour = 120-240 kcal/hour)

Clinical and Medical Nutrition

Healthcare providers use kilocalories for patient care:

Hospitalized patients: Calculated kcal needs (25-30 kcal/kg body weight/day for maintenance, more for hypermetabolic states)

Enteral nutrition (tube feeding): Formula prescribed in kcal/day and kcal/ml concentration

Parenteral nutrition (IV feeding): Total kcal delivered intravenously calculated for critically ill patients

Malnutrition treatment: Gradual kcal increases to restore weight (refeeding syndrome risk if too rapid)

Eating disorders: Anorexia nervosa treatment requires structured kcal increases (start 1,200-1,500, increase to 3,000-4,000 kcal/day for weight restoration)

Diabetes management: Consistent kcal intake helps stabilize blood glucose levels

Obesity treatment: Medical weight loss programs target 1,200-1,500 kcal/day for women, 1,500-1,800 kcal/day for men

Burn victims: Hypermetabolic state requires 4,000-6,000+ kcal/day for healing

Sports Nutrition

Athletes use kilocalories to optimize performance:

Energy availability: Female athletes need >45 kcal/kg lean body mass/day to avoid RED-S (Relative Energy Deficiency in Sport)

Weight class sports: Wrestlers, boxers, weightlifters manipulate kcal to make weight while preserving strength

Bodybuilding: Bulking phases (500+ kcal surplus) vs. cutting phases (500-1,000 kcal deficit)

Endurance training: Marathon/triathlon training increases kcal needs by 600-1,500+ kcal/day

Recovery nutrition: Post-workout meals timed and sized based on kcal expenditure during training

Food Science and Recipe Development

Kilocalories guide food formulation:

Product development: Companies create "light" or "reduced-calorie" versions targeting specific kcal values

Recipe analysis: Nutritionists calculate kcal per serving for cookbooks and cooking websites

Portion sizing: Restaurants design "500 kcal and under" menu sections

Reformulation: Reducing sugar/fat while maintaining taste and texture to lower kcal

Regulatory compliance: "Low calorie" claim requires ≤40 kcal per serving (FDA definition)

When to Use kilojoules

1. Nutrition Labeling and Dietary Planning

The kilojoule is the international standard for expressing food energy content on nutrition labels, enabling consumers to track energy intake and manage weight.

Typical nutrition panel (Australia/NZ):

Nutrition Information
Servings per package: 5
Serving size: 30g

                Per Serving    Per 100g
Energy          600 kJ         2,000 kJ
Protein         3.0 g          10.0 g
Fat, total      5.0 g          16.7 g
- saturated     2.5 g          8.3 g
Carbohydrate    20.0 g         66.7 g
- sugars        5.0 g          16.7 g
Sodium          150 mg         500 mg

Weight management: To lose 0.5 kg per week, create a deficit of ~2,000 kJ (500 kcal) per day through reduced intake and increased activity.

2. Exercise Physiology and Sports Nutrition

Exercise scientists use kilojoules to quantify energy expenditure during physical activity and calculate fuel requirements for athletes.

Metabolic equivalent tasks (METs):

1 MET = resting metabolic rate ≈ 4.2 kJ per kg body weight per hour
Moderate exercise = 3-6 METs = 12.6-25.2 kJ/kg/hr
Vigorous exercise = 6+ METs = 25.2+ kJ/kg/hr

Example: A 70 kg person running at 8 METs for 1 hour:

Energy = 70 kg × 8 METs × 4.2 kJ/kg/hr = 2,352 kJ (562 kcal)

3. Chemistry: Reaction Enthalpies

Chemists use kilojoules to express heat of reaction (ΔH), combustion energies, and bond energies.

Examples:

Combustion of methane: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l), ΔH = -890 kJ/mol
Photosynthesis: 6CO₂ + 6H₂O + 2,870 kJ → C₆H₁₂O₆ + 6O₂
Cellular respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 2,870 kJ

Bond energies (typical values in kJ/mol):

C-H: 413 kJ/mol
C-C: 348 kJ/mol
C=C: 614 kJ/mol
C≡C: 839 kJ/mol
O-H: 463 kJ/mol
N-H: 391 kJ/mol

4. Thermodynamics and Heat Transfer

Engineers use kilojoules for heat transfer calculations, thermal energy storage, and efficiency analysis.

Specific heat capacity (energy to raise 1 kg by 1°C):

Water: 4.18 kJ/(kg·°C)
Air: 1.01 kJ/(kg·°C)
Aluminum: 0.90 kJ/(kg·°C)
Concrete: 0.88 kJ/(kg·°C)

Example: Heating 50 liters (50 kg) of water from 20°C to 60°C:

Energy = 50 kg × 4.18 kJ/(kg·°C) × (60-20)°C = 8,360 kJ (2.32 kWh)

5. Climate and Environmental Science

Climate scientists use kilojoules to quantify solar radiation, atmospheric energy budgets, and heat storage in oceans.

Solar irradiance: ~1,000 watts per square meter at Earth's surface = 3,600 kJ/m²/hr

Ocean heat content: Oceans store vast energy measured in zettajoules (10²¹ J), but marine biology and oceanography often express smaller values in kJ (e.g., energy available to marine organisms).

6. Consumer Energy Awareness

Kilojoule labeling on restaurant menus (mandatory in Australia, parts of US) helps consumers make informed choices.

Example chain restaurant menu (with kJ):

Grilled chicken salad: 1,500 kJ
Cheeseburger with fries: 4,200 kJ
Vegetarian wrap: 1,800 kJ
Large soft drink: 800 kJ

Daily context: If your target is 8,400 kJ/day, the cheeseburger meal consumes exactly half your daily budget.

Additional Unit Information

About Kilocalorie (kcal)

Is kcal the same as Calories on food labels?

Yes, exactly the same! Food "Calories" (capital C) are kilocalories (kcal).

When a label says:

"100 Calories" → means 100 kcal
"100 Cal" → means 100 kcal
"100 calories" (lowercase) → still means 100 kcal in nutrition context

This capitalization convention was intended to distinguish the food unit (kcal) from the small calorie (cal) used in chemistry, though modern usage often ignores the capitalization.

In other words: 1 Calorie = 1 kcal = 1,000 cal

How many kcal should I eat per day?

Estimated needs (sedentary to moderately active):

Adult women: 1,600-2,400 kcal/day
Adult men: 2,000-3,000 kcal/day

More precise calculation using Mifflin-St Jeor equation:

Men: BMR = (10 × weight in kg) + (6.25 × height in cm) - (5 × age) + 5 Women: BMR = (10 × weight in kg) + (6.25 × height in cm) - (5 × age) - 161

Then multiply by activity factor:

1.2 = sedentary (little/no exercise)
1.375 = lightly active (1-3 days/week)
1.55 = moderately active (3-5 days/week)
1.725 = very active (6-7 days/week)
1.9 = extremely active (athlete, physical job)

Example: 30-year-old woman, 65 kg, 165 cm, moderately active

BMR = (10 × 65) + (6.25 × 165) - (5 × 30) - 161 = 1,371 kcal
TDEE = 1,371 × 1.55 = 2,125 kcal/day

What's the difference between kcal and kJ?

Both measure energy but use different scales:

1 kcal = 4.184 kJ

Regional preferences:

United States, Canada: Exclusively use kcal (Calories)
Australia, New Zealand: Primarily use kJ, show kcal secondarily
European Union: Require both kJ and kcal on labels
Most other countries: Primarily use kcal

Conversion shortcuts:

kJ to kcal: divide by 4.2 (approximate) or 4.184 (exact)
kcal to kJ: multiply by 4.2 (approximate) or 4.184 (exact)

Example: 500 kJ = 500 ÷ 4.184 = 119.5 kcal

How accurate are kcal counts on food labels?

FDA regulations allow ±20% margin of error in the United States. Similar tolerances exist globally.

Typical accuracy:

Packaged foods: Usually within ±5-10% of stated value
Restaurant foods: Often 10-20% higher than stated (studies show average 18% underestimation)
Chain restaurants: Generally more accurate due to standardized portions
Independent restaurants: Can vary significantly (±30-50%)

Sources of inaccuracy:

Natural variation in ingredients
Portion size variation
Measurement method limitations (Atwater system uses averages)
Rounding rules on labels
Fiber calculation differences

Advice: Use label values as reasonable estimates, but expect some variation. Over time, tracking averages out errors.

How many kcal to lose weight?

Safe weight loss rate: 0.5-2 pounds (0.25-1 kg) per week

Calorie deficit needed:

500 kcal/day below TDEE → ~1 pound lost per week
1,000 kcal/day below TDEE → ~2 pounds lost per week (maximum recommended)

Minimum intake guidelines:

Women: 1,200 kcal/day minimum (1,000 kcal with medical supervision)
Men: 1,500 kcal/day minimum (1,200 kcal with medical supervision)

Why minimums matter: Very low calorie diets (<1,000-1,200 kcal) risk:

Nutrient deficiencies
Muscle loss
Metabolic adaptation (slowed metabolism)
Difficulty sustaining long-term
Potential health complications

Better approach: 10-25% deficit from TDEE, combined with resistance training and adequate protein (0.7-1g per pound body weight) to preserve muscle mass.

Do athletes need more kcal?

Yes, significantly more. Athletic training increases energy expenditure substantially:

Recreational athletes (3-5 hours/week):

Add 200-500 kcal/day above sedentary baseline

Serious athletes (10-15 hours/week):

Add 500-1,500 kcal/day above baseline
Total: 2,500-4,000 kcal/day typical

Elite endurance athletes (20+ hours/week):

Total needs: 3,500-6,000+ kcal/day
Tour de France cyclists: 5,000-8,000 kcal/day during race stages
Ultra-marathon runners: 6,000-10,000 kcal/day during multi-day events

Female athletes special consideration: Must maintain >45 kcal/kg lean body mass/day to avoid Relative Energy Deficiency in Sport (RED-S), which causes hormonal disruption, bone loss, and impaired performance.

Are all kcal the same for weight loss?

For weight loss, yes (calorie balance matters most). For health, no (nutrient quality matters).

Weight loss perspective (energy balance):

100 kcal from candy = 100 kcal from broccoli in terms of energy
"A calorie is a calorie" for weight change (supported by metabolic ward studies)

Health and satiety perspective (food quality):

Protein: Higher thermic effect (20-30%), increases satiety, preserves muscle
Fiber: Promotes fullness, slows digestion, feeds beneficial gut bacteria
Processed foods: Less satiating, easier to overeat, may affect hunger hormones
Whole foods: More filling per kcal, provide micronutrients and phytonutrients

Practical reality: While kcal determine weight change, food choices affect:

Hunger and adherence to diet
Muscle retention during weight loss
Overall health markers (cholesterol, blood pressure, inflammation)
Energy levels and workout performance

Best approach: Hit kcal targets using mostly nutrient-dense whole foods, with flexibility for treats within your budget.

What is "empty calories"?

"Empty calories" refers to foods providing kcal but minimal essential nutrients (vitamins, minerals, protein, fiber).

Common empty calorie sources:

Sugary drinks: Soda, sports drinks, sweet tea (only carbohydrates, no micronutrients)
Alcohol: 7 kcal/g with almost no nutritional value
Added sugars: Candy, pastries, desserts (primarily carbs, minimal nutrients)
Refined grains: White bread, white rice (stripped of fiber, vitamins, minerals during processing)

Why it matters: Empty calorie foods contribute to total energy intake without providing nutrients needed for health, making it easier to overconsume kcal while undernourishing the body.

Contrast with "nutrient-dense" foods: Foods providing high amounts of vitamins, minerals, protein, and/or fiber relative to their kcal content:

Vegetables (especially dark leafy greens)
Fruits (especially berries)
Lean proteins (chicken, fish, legumes)
Whole grains (oats, quinoa, brown rice)
Nuts and seeds

Dietary guidelines recommend: Limit empty calories to <10-15% of total intake.

Can you eat too few kcal?

Yes. Chronic undereating has serious health consequences:

Short-term effects (days to weeks):

Fatigue, weakness, difficulty concentrating
Irritability, mood changes
Reduced exercise performance
Increased hunger and food preoccupation

Medium-term effects (weeks to months):

Hair loss, brittle nails, dry skin
Feeling cold constantly (reduced metabolic rate)
Menstrual irregularities or loss of period (women)
Reduced testosterone (men)
Sleep disturbances

Long-term effects (months to years):

Nutrient deficiencies (iron, B vitamins, calcium, etc.)
Bone loss (osteopenia/osteoporosis)
Muscle wasting
Metabolic adaptation (slowed metabolism)
Increased risk of eating disorders
Cardiovascular complications (severe cases)
Immune system suppression

Who's at risk:

Chronic dieters repeatedly eating <1,000-1,200 kcal
Athletes with Relative Energy Deficiency in Sport (RED-S)
Individuals with eating disorders (anorexia nervosa, atypical anorexia)

Minimum intakes: Generally 1,200 kcal/day for women and 1,500 kcal/day for men, though individual needs vary. Consult healthcare providers before going below these thresholds.

How were food kcal values originally determined?

Bomb calorimetry was the original method and remains the gold standard for measuring food energy:

Process:

Food sample dried and weighed
Sample placed in sealed steel "bomb" chamber filled with pure oxygen (30 atmospheres pressure)
Bomb submerged in precisely measured water bath
Electrical ignition burns sample completely
Temperature rise in water measured with precision thermometer
kcal calculated: Energy (kcal) = water mass (kg) × temperature rise (°C)

Atwater system refinement (1890s): Wilbur Atwater recognized human digestion doesn't extract 100% of energy from food, so he adjusted bomb calorimetry values:

Carbohydrates: 4.1 kcal/g in bomb → 4.0 kcal/g physiologically available
Protein: 5.6 kcal/g in bomb → 4.0 kcal/g physiologically available (nitrogen not fully oxidized)
Fat: 9.4 kcal/g in bomb → 9.0 kcal/g physiologically available
Fiber: Partially indigestible, yields ~2 kcal/g vs. 4 kcal/g for other carbs

Modern food databases (USDA FoodData Central, etc.) compile bomb calorimetry measurements and Atwater calculations for tens of thousands of foods, enabling nutrition label accuracy.

About Kilojoule (kJ)

How do I convert kilojoules to Calories?

Formula: Calories (kcal) = kJ ÷ 4.184

Quick approximation: Divide kJ by 4 (slightly underestimates)

Examples:

100 kJ ≈ 24 Cal
500 kJ ≈ 120 Cal
1,000 kJ ≈ 239 Cal
2,000 kJ ≈ 478 Cal
8,400 kJ ≈ 2,007 Cal (daily intake)

Exact vs. approximate:

1,000 kJ ÷ 4 = 250 Cal (approximate, 4.6% high)
1,000 kJ ÷ 4.184 = 239 Cal (exact)

For dietary purposes, the approximate conversion (÷4) is usually sufficient.

How many kilojoules should I eat per day?

Daily kilojoule requirements depend on age, sex, body size, and activity level:

Adult women:

Sedentary: 7,500-8,400 kJ/day (1,800-2,000 Cal)
Moderately active: 8,400-9,600 kJ/day (2,000-2,300 Cal)
Very active: 10,000-12,000 kJ/day (2,400-2,900 Cal)

Adult men:

Sedentary: 8,800-10,000 kJ/day (2,100-2,400 Cal)
Moderately active: 10,500-12,000 kJ/day (2,500-2,900 Cal)
Very active: 12,500-14,500 kJ/day (3,000-3,500 Cal)

Children and teens: 5,000-12,500 kJ/day depending on age and activity

Athletes: 12,500-21,000 kJ/day (3,000-5,000 Cal) or more during intense training

General guideline: Most adults need 8,000-10,000 kJ daily (~2,000-2,400 Cal).

Why do some countries use kJ instead of Calories?

Scientific consistency: Kilojoules are SI units, consistent with all other scientific measurements (meters, kilograms, seconds). Calories are non-SI legacy units.

Eliminates confusion: The calorie system is inherently confusing:

Small calorie (cal) = 4.184 J
Large Calorie (kcal, "food Calorie") = 1,000 cal = 4,184 J
Many people don't realize "Calories" on food labels are actually kilocalories

Kilojoules eliminate this ambiguity—1 kJ always equals 1,000 J.

International standardization: The WHO, FAO, and international scientific community use kilojoules as the standard for nutrition research and policy.

Countries using kJ:

Exclusive kJ: Australia, New Zealand, Singapore
Both kJ and kcal: EU (27 countries), Canada, China, Japan, India, Brazil, Argentina, South Africa
Only kcal: United States (and a few small nations)

Result: Over 85% of the world's population sees kilojoules on food labels.

How many kilojoules do I burn exercising?

Energy expenditure varies by activity intensity, body weight, fitness level, and efficiency:

General estimates (per hour for 70 kg person):

Walking (5 km/h): 1,200-1,500 kJ (290-360 Cal)
Jogging (8 km/h): 2,500-3,000 kJ (600-720 Cal)
Running (12 km/h): 3,500-4,200 kJ (840-1,000 Cal)
Cycling (leisure): 1,200-1,600 kJ (290-380 Cal)
Cycling (vigorous): 2,500-3,300 kJ (600-800 Cal)
Swimming (moderate): 2,000-2,500 kJ (480-600 Cal)
Weight training: 1,200-2,100 kJ (290-500 Cal)
Yoga: 650-1,000 kJ (155-240 Cal)

Per-distance estimates:

Walking: 200-250 kJ/km (50-60 Cal/km)
Running: 300-350 kJ/km (70-85 Cal/km)
Cycling: 100-150 kJ/km (25-35 Cal/km)

Formula (approximate): kJ/hr ≈ Body weight (kg) × MET value × 4.2

Where MET = metabolic equivalent (1 MET = resting, 3-6 = moderate, 6+ = vigorous)

Are kilojoules more accurate than Calories for nutrition?

Same accuracy, different units: Both kJ and kcal measure the same physical quantity (energy) with equal precision. The accuracy of nutrition labeling depends on measurement methods (bomb calorimetry, Atwater system), not the choice of units.

Advantages of kilojoules:

SI consistency: Compatible with all other scientific units
No ambiguity: 1 kJ always means 1,000 J (unlike calorie/Calorie confusion)
International standard: Used in scientific literature worldwide
Educational: Connects food energy to physics and chemistry

Advantages of Calories (kcal):

Familiarity: Deeply established in US consumer culture
Smaller numbers: 2,000 Cal/day sounds less intimidating than 8,400 kJ/day
Historical data: Decades of dietary research in kcal

Scientific community consensus: Kilojoules are preferred for publications, while kcal persists in US consumer contexts due to inertia.

Can I lose weight by tracking kilojoules?

Yes—identical to tracking Calories: Weight loss requires a sustained energy deficit (consuming fewer kJ than you expend). Whether you count in kJ or kcal makes no difference physiologically.

Creating a deficit:

0.5 kg/week loss: Deficit of ~2,000 kJ/day (500 kcal/day)
1 kg/week loss: Deficit of ~4,000 kJ/day (1,000 kcal/day)

Practical approach:

Calculate maintenance: Estimate daily kJ needs (8,000-12,000 kJ for most adults)
Set deficit: Reduce intake by 1,500-2,500 kJ/day (moderate, sustainable)
Track intake: Log food kJ using labels or apps
Monitor progress: Adjust based on weekly weigh-ins

Example: If you need 10,000 kJ/day to maintain weight, eating 8,000 kJ/day creates a 2,000 kJ deficit = ~0.5 kg loss per week.

How accurate are kilojoule values on food labels?

Regulatory tolerances allow some variation:

Australia/NZ: ±20% tolerance for energy values (same as nutrients)

EU: Energy values must be calculated using standard conversion factors (Atwater system) or measured via bomb calorimetry. Tolerances apply.

Sources of inaccuracy:

Natural variation: Agricultural products vary in composition
Rounding: Labels round to nearest 10-100 kJ
Calculation methods: Atwater factors (17 kJ/g carb, 38 kJ/g fat) are averages
Serving size: Manufacturers sometimes understate serving sizes, making total kJ appear lower

Reality: Most packaged food kJ values are reasonably accurate (within ±10-15%), but restaurant/takeaway items may vary more significantly. Track trends over weeks rather than obsessing over individual items.

What is the relationship between kJ and metabolism?

Basal metabolic rate (BMR): The energy your body needs at rest to maintain vital functions (breathing, circulation, cell production) is typically:

Women: 5,000-6,500 kJ/day (1,200-1,550 kcal/day)
Men: 6,500-8,000 kJ/day (1,550-1,900 kcal/day)

Total daily energy expenditure (TDEE): BMR × activity factor

Sedentary (little exercise): BMR × 1.2
Lightly active (1-3 days/week): BMR × 1.375
Moderately active (3-5 days/week): BMR × 1.55
Very active (6-7 days/week): BMR × 1.725
Extra active (twice daily, heavy): BMR × 1.9

Metabolic efficiency: Humans convert food energy to mechanical work at only ~25% efficiency. The other ~75% becomes heat. This is why intense exercise makes you hot!

Example: Running requires ~300 kJ/km of mechanical work, but your body actually burns ~1,200 kJ/km of food energy to produce that work (300 kJ useful, 900 kJ heat).

How do kilojoules relate to weight gain or loss?

Energy balance equation:

Weight maintenance: Energy in (kJ consumed) = Energy out (kJ expended)
Weight gain: Energy in > Energy out → Surplus stored as fat/muscle
Weight loss: Energy in < Energy out → Deficit drawn from body stores

Fat energy density: 1 kg body fat ≈ 0.87 kg pure fat ≈ 33,000 kJ (7,700 kcal)

Weight loss calculation:

To lose 0.5 kg/week: Create 4,700 kJ weekly deficit = 670 kJ/day deficit
To lose 1 kg/week: Create 9,400 kJ weekly deficit = 1,340 kJ/day deficit

Practical example:

Maintenance: 10,000 kJ/day
Target: Lose 0.5 kg/week
Daily intake: 10,000 - 670 = 9,330 kJ/day (2,230 kcal/day)
Or: Eat 10,000 kJ but burn extra 670 kJ through exercise

Sustainable approach: Combine moderate deficit (1,500-2,000 kJ/day) with regular activity for gradual, maintainable weight loss.

Is 8,400 kJ per day a standard recommendation?

8,400 kJ (~2,000 kcal) is a common reference value for nutrition labeling (e.g., "% daily value" on labels), representing an average adult's needs. However:

Not universal: Individual needs vary by:

Age: Older adults typically need less
Sex: Men generally need more than women
Body size: Larger bodies need more energy
Activity level: Active people need significantly more
Metabolism: Varies 10-20% between individuals

Who needs ~8,400 kJ/day?

Sedentary to moderately active women
Sedentary men
Older adults

Who needs more?

Active/very active men (10,000-14,000 kJ/day)
Athletes (12,500-21,000+ kJ/day)
Teenagers (especially boys)

Who needs less?

Children (5,000-7,500 kJ/day depending on age)
Very sedentary older adults (7,000-7,500 kJ/day)

Recommendation: Use 8,400 kJ as a starting point, then adjust based on whether you're maintaining, losing, or gaining weight.

Conversion Table: Kilocalorie to Kilojoule

Kilocalorie (kcal)	Kilojoule (kJ)
0.5	2.092
1	4.184
1.5	6.276
2	8.368
5	20.92
10	41.84
25	104.6
50	209.2
100	418.4
250	1,046
500	2,092
1,000	4,184

How do I convert Kilocalorie to Kilojoule?

To convert Kilocalorie to Kilojoule, enter the value in Kilocalorie in the calculator above. The conversion will happen automatically. Use our free online converter for instant and accurate results. You can also visit our energy converter page to convert between other units in this category.

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What is the conversion factor from Kilocalorie to Kilojoule?

The conversion factor depends on the specific relationship between Kilocalorie and Kilojoule. You can find the exact conversion formula and factor on this page. Our calculator handles all calculations automatically. See the conversion table above for common values.

Can I convert Kilojoule back to Kilocalorie?

Yes! You can easily convert Kilojoule back to Kilocalorie by using the swap button (⇌) in the calculator above, or by visiting our Kilojoule to Kilocalorie converter page. You can also explore other energy conversions on our category page.

Learn more →

What are common uses for Kilocalorie and Kilojoule?

Kilocalorie and Kilojoule are both standard units used in energy measurements. They are commonly used in various applications including engineering, construction, cooking, and scientific research. Browse our energy converter for more conversion options.

For more energy conversion questions, visit our FAQ page or explore our conversion guides.

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Verified Against Authority Standards

All conversion formulas have been verified against international standards and authoritative sources to ensure maximum accuracy and reliability.

NIST Reference on Constants, Units and Uncertainty

National Institute of Standards and Technology — Fundamental physical constants and unit conversions

BIPM SI Units

Bureau International des Poids et Mesures — International System of Units (SI) definitions and standards

ISO 80000

International Organization for Standardization — Quantities and units international standards series

Last verified: February 19, 2026

Kilocalorie to Kilojoule Converter

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Kilocalorie to Kilojoule Calculator

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How to Convert Kilocalorie to Kilojoule: Step-by-Step Guide

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What is a Kilocalorie and a Kilojoule?

The Kilocalorie (Large Calorie)

Relationship to Small Calorie

Food Calorie Equivalence

Thermochemical vs. International Table kcal

Precise Equivalents

SI System Integration

Nutrition Labeling Standards

History of the Kilocalorie and Kilojoule

Pre-Nutrition Era (1800s)

Nutrition Science Adoption (1890s-1900s)

The "Calorie" Naming Convention (1900s-1920s)

Global Food Labeling Standards (1960s-Present)

Resistance to SI Joule Adoption

The Joule Foundation: Mechanical Equivalent of Heat (1840s-1850s)

SI Adoption and the Joule as Standard (1960)

International Nutrition Standards: WHO and FAO (1970s-1980s)

Australia: First Mandatory kJ Labeling (1995)

New Zealand and EU Adoption (2000s-2010s)

United States: The Calorie Holdout

Global Status (2025)

Common Uses and Applications: kilocalories vs kilojoules

Common Uses for kilocalories

Nutrition Labeling and Diet Planning

Weight Management

Exercise and Fitness Tracking

Clinical and Medical Nutrition

Sports Nutrition

Food Science and Recipe Development

When to Use kilojoules

1. Nutrition Labeling and Dietary Planning

2. Exercise Physiology and Sports Nutrition

3. Chemistry: Reaction Enthalpies

4. Thermodynamics and Heat Transfer

5. Climate and Environmental Science

6. Consumer Energy Awareness

Additional Unit Information

About Kilocalorie (kcal)

Is kcal the same as Calories on food labels?

How many kcal should I eat per day?

What's the difference between kcal and kJ?

How accurate are kcal counts on food labels?

How many kcal to lose weight?

Do athletes need more kcal?

Are all kcal the same for weight loss?

What is "empty calories"?

Can you eat too few kcal?

How were food kcal values originally determined?

About Kilojoule (kJ)

How do I convert kilojoules to Calories?

How many kilojoules should I eat per day?

Why do some countries use kJ instead of Calories?

How many kilojoules do I burn exercising?

Are kilojoules more accurate than Calories for nutrition?

Can I lose weight by tracking kilojoules?

How accurate are kilojoule values on food labels?

What is the relationship between kJ and metabolism?

How do kilojoules relate to weight gain or loss?

Is 8,400 kJ per day a standard recommendation?

Conversion Table: Kilocalorie to Kilojoule

People Also Ask

How do I convert Kilocalorie to Kilojoule?

What is the conversion factor from Kilocalorie to Kilojoule?

Can I convert Kilojoule back to Kilocalorie?

What are common uses for Kilocalorie and Kilojoule?

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