Kilojoule (kJ) - Unit Information & Conversion

Quick Answer

What is a kilojoule? A kilojoule (kJ) is a metric unit of energy equal to 1,000 joules, used internationally for food energy labeling instead of Calories. One kilojoule equals approximately 0.239 kilocalories (food Calories), so 1 Calorie = 4.184 kJ. A medium apple contains about 250 kJ (60 Cal), a banana 400 kJ (95 Cal), and an average adult needs 8,400 kJ (2,000 Cal) daily. Kilojoules appear on nutrition labels in Australia, New Zealand, EU, and most countries worldwide. Use our energy converter for quick kJ conversions.

Quick Comparison Table

Energy Amount	Food Equivalent	Kilocalories (Calories)	Joules
100 kJ	Small apple slice	24 Cal	100,000 J
250 kJ	Medium apple	60 Cal	250,000 J
400 kJ	Banana	95 Cal	400,000 J
600 kJ	Can of soda (330ml)	143 Cal	600,000 J
1,000 kJ	Chocolate bar	239 Cal	1,000,000 J
2,000 kJ	Large meal	478 Cal	2,000,000 J
8,400 kJ	Daily intake (average adult)	2,000 Cal	8,400,000 J
10,500 kJ	Daily intake (active adult male)	2,500 Cal	10,500,000 J

Definition 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"

History 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.

Real-World Examples

Common Foods: Energy Content in kJ

Food Item	Serving Size	Energy (kJ)	Energy (kcal)
Apple	Medium (180g)	250	60
Banana	Medium (120g)	400	95
Orange	Medium (130g)	200	48
Slice of bread	1 slice (30g)	300	72
Egg	Large (50g)	300	72
Milk	1 cup (250ml)	650	155
Yogurt	1 cup (200g)	500	120
Chicken breast	100g cooked	700	167
Salmon	100g cooked	800	191
Rice	1 cup cooked (150g)	550	131
Pasta	1 cup cooked (140g)	650	155
Potato	Medium baked (170g)	650	155
Can of Coke	330ml	600	143
Beer	330ml	500	120
Chocolate bar	50g	1,100	263
Pizza slice	1/8 large pizza	1,200	287
Big Mac	1 burger	2,200	526
French fries	Medium serving	1,500	358
Ice cream	1 cup	1,000	239

Daily Energy Requirements

Demographic Group	Daily kJ	Daily kcal
Child (4-6 years)	5,000-6,000	1,200-1,400
Child (7-10 years)	6,500-8,000	1,550-1,900
Teen girl (14-18)	8,000-9,500	1,900-2,300
Teen boy (14-18)	10,000-12,500	2,400-3,000
Adult woman (sedentary)	7,500-8,000	1,800-1,900
Adult woman (moderate activity)	8,400-9,200	2,000-2,200
Adult woman (very active)	10,000-11,000	2,400-2,600
Adult man (sedentary)	8,800-9,600	2,100-2,300
Adult man (moderate activity)	10,500-11,300	2,500-2,700
Adult man (very active)	12,500-14,000	3,000-3,300
Pregnant woman	+1,400	+340 (2nd & 3rd trimester)
Breastfeeding woman	+2,100	+500
Athlete (training)	12,500-21,000	3,000-5,000

Note: Requirements vary by age, sex, body size, metabolism, and activity level. These are general estimates.

Exercise and Energy Expenditure

Activity	Intensity	Energy per hour (kJ)	Energy per hour (kcal)
Sleeping	-	250-350	60-85
Sitting, watching TV	-	350-450	85-110
Standing, light activity	-	500-650	120-155
Walking (slow, 3 km/h)	Light	800-1,000	190-240
Walking (brisk, 5 km/h)	Moderate	1,200-1,500	290-360
Jogging (8 km/h)	Moderate-vigorous	2,500-3,000	600-720
Running (12 km/h)	Vigorous	3,500-4,200	840-1,000
Cycling (leisurely)	Light-moderate	1,200-1,600	290-380
Cycling (vigorous)	Vigorous	2,500-3,300	600-800
Swimming (moderate)	Moderate	2,000-2,500	480-600
Swimming (vigorous)	Vigorous	2,900-3,800	700-900
Weight training	Moderate-vigorous	1,200-2,100	290-500
Yoga	Light-moderate	650-1,000	155-240
Dancing	Moderate	1,500-2,100	360-500
Basketball	Vigorous	2,500-3,300	600-800
Soccer	Vigorous	2,900-3,800	700-900

Per-kilometer estimates:

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

Macronutrient Energy Density

Macronutrient	Energy per gram (kJ)	Energy per gram (kcal)
Carbohydrate	17	4
Protein	17	4
Fat	38	9
Alcohol (ethanol)	29	7
Fiber	8	2 (partially digested)

Example: A food with 10g carbs, 5g protein, 8g fat contains:

• Energy = (10 × 17) + (5 × 17) + (8 × 38) = 170 + 85 + 304 = 559 kJ (134 kcal)

Chemistry: Heat of Combustion

Common substances and their combustion energies:

Substance	Energy per gram (kJ/g)	Energy per mole (kJ/mol)
Hydrogen (H₂)	142	286
Methane (CH₄)	55.5	890
Ethanol (C₂H₅OH)	29.7	1,367
Gasoline (avg)	47	-
Diesel	45	-
Coal (anthracite)	30-35	-
Wood (dry)	16-18	-
Glucose (C₆H₁₂O₆)	15.6	2,808
Sucrose (table sugar)	16.5	5,644
Palmitic acid (fat)	39.1	10,035

Human metabolism extracts similar energies from macronutrients via cellular respiration rather than combustion, but the principle is identical.

Common Uses

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.

Conversion Guide

Kilojoules to Kilocalories (Calories)

1 kJ = 0.239006 kcal (or approximately 0.24 kcal)

Formula: kcal = kJ × 0.239006 (or kJ ÷ 4.184)

Examples:

• 100 kJ = 23.9 kcal (≈ 24 Cal)
• 500 kJ = 119.5 kcal (≈ 120 Cal)
• 1,000 kJ = 239 kcal
• 2,000 kJ = 478 kcal
• 8,400 kJ = 2,007 kcal (≈ 2,000 Cal)

Quick mental math: Divide kJ by 4 to get a rough kcal estimate (slightly underestimates, but close).

kJ to Kilocalories converter →

Kilocalories to Kilojoules

1 kcal = 4.184 kJ (exactly, thermochemical definition)

Formula: kJ = kcal × 4.184

Examples:

• 100 kcal = 418.4 kJ (≈ 420 kJ)
• 250 kcal = 1,046 kJ (≈ 1,050 kJ)
• 500 kcal = 2,092 kJ (≈ 2,100 kJ)
• 2,000 kcal = 8,368 kJ (≈ 8,400 kJ)

Quick mental math: Multiply kcal by 4 to get approximate kJ (slightly underestimates).

Kilocalories to kJ converter →

Kilojoules to Joules

1 kJ = 1,000 J (exactly, by definition)

Formula: J = kJ × 1,000

Examples:

• 0.1 kJ = 100 J
• 1 kJ = 1,000 J
• 10 kJ = 10,000 J
• 100 kJ = 100,000 J
• 1,000 kJ = 1,000,000 J = 1 MJ

kJ to Joules converter →

Kilojoules to Kilowatt-hours

1 kWh = 3,600 kJ (exactly)

Formula: kWh = kJ ÷ 3,600

Examples:

• 3,600 kJ = 1 kWh
• 1,000 kJ = 0.278 kWh
• 10,000 kJ = 2.78 kWh

Context: Daily food intake (~8,400 kJ) = 2.33 kWh of chemical energy, though human metabolism operates at only ~25% efficiency in converting food to mechanical work.

kJ to kWh converter →

Kilojoules to BTU

1 kJ = 0.947817 BTU

Formula: BTU = kJ × 0.947817

Examples:

• 1 kJ = 0.948 BTU
• 10 kJ = 9.478 BTU
• 100 kJ = 94.78 BTU
• 1,000 kJ = 947.8 BTU

kJ to BTU converter →

Common Conversion Mistakes

❌ WRONG: Confusing kJ with kcal

"This snack has 500 kJ, which is 500 Calories."

✅ RIGHT: Using correct conversion factor

"This snack has 500 kJ, which is about 120 Calories (500 ÷ 4.184 = 119.5 kcal)."

Why it matters: Confusing kJ with kcal overstates energy by more than 4×. 500 kJ ≠ 500 Cal!

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❌ WRONG: Using incorrect conversion factor

"To convert kcal to kJ, multiply by 4." (Close, but imprecise)

✅ RIGHT: Using exact conversion factor

"To convert kcal to kJ, multiply by 4.184 exactly. 100 kcal = 418.4 kJ, not 400 kJ."

Why it matters: Using 4 instead of 4.184 introduces a 4.4% error, significant for dietary planning or scientific work.

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❌ WRONG: Assuming all nutrition labels use the same units

"This Australian product says 1,000 energy, so it must be 1,000 Calories."

✅ RIGHT: Checking the units on the label

"This Australian product says 1,000 kJ energy, which is about 240 Calories. Australian labels use kJ, not kcal."

Why it matters: Misreading kJ as Calories leads to massive overconsumption (4× more than intended).

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❌ WRONG: Forgetting that 1 kJ = 1,000 J

"A food with 250 kJ contains 250 joules."

✅ RIGHT: Applying the kilo- prefix correctly

"A food with 250 kJ contains 250,000 joules (250 × 1,000)."

Why it matters: The kilo- prefix means 1,000. Forgetting this understates energy by 1,000×.

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❌ WRONG: Mixing up small calories and Calories

"1 kJ = 239 calories, so this 1,000 kJ snack is only 239 calories—very low energy!"

✅ RIGHT: Distinguishing between cal and kcal (Calorie)

"1 kJ = 239 Calories (kcal), not 239 small calories. This 1,000 kJ snack is 239 Calories, which is moderate energy."

Why it matters: Small calories (cal) are 1/1,000 of a food Calorie (kcal). Using the wrong unit understates energy by 1,000×.

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❌ WRONG: Applying kcal→kJ conversion to kWh

"1 kWh is like 1 kcal, so 1 kWh = 4.184 kJ."

✅ RIGHT: Using correct kWh conversion

"1 kWh = 3,600 kJ (or 3.6 MJ). The kcal→kJ conversion (4.184) applies only to kilocalories, not kilowatt-hours."

Why it matters: kWh and kcal are different units with different conversion factors. Mixing them up creates 860× error!