Joule to Kilojoule Converter
Convert joules to kilojoules with our free online energy converter.
Quick Answer
1 Joule = 0.001 kilojoules
Formula: Joule × conversion factor = Kilojoule
Use the calculator below for instant, accurate conversions.
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Joule to Kilojoule Calculator
How to Use the Joule to Kilojoule Calculator:
- Enter the value you want to convert in the 'From' field (Joule).
- 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 Joule to Kilojoule: Step-by-Step Guide
Converting Joule to Kilojoule involves multiplying the value by a specific conversion factor, as shown in the formula below.
Formula:
kJ = J × 0.001Example Calculation:
1 J = 0.001 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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View all Energy conversions →What is a Joule and a Kilojoule?
and Standards
SI Definition
The joule is the SI derived unit of energy, work, and heat, defined as:
1 joule = the energy transferred when a force of 1 newton acts through a distance of 1 meter
Mathematically:
- 1 J = 1 N·m (newton-meter)
- 1 J = 1 kg·m²/s² (SI base units)
- 1 J = 1 W·s (watt-second)
- 1 J = 1 Pa·m³ (pascal-cubic meter)
Multiple Equivalent Definitions
Mechanical work: 1 J = work done moving an object 1 meter against 1 newton force
Electrical energy: 1 J = energy dissipated passing 1 ampere through 1 ohm resistance for 1 second (1 J = 1 A² · Ω · s = 1 V · A · s)
Kinetic energy: 1 J = kinetic energy of a 2 kg mass moving at 1 m/s (KE = ½mv² = ½ × 2 × 1² = 1 J)
Gravitational potential energy: 1 J = energy to lift ~102 grams by 1 meter on Earth (PE = mgh = 0.102 kg × 9.8 m/s² × 1 m ≈ 1 J)
Thermal energy: 1 J ≈ 0.239 calories (energy to raise 0.239 grams of water by 1°C)
2019 SI Redefinition
The 2019 SI revision redefined the joule based on exact fundamental constants:
- Planck constant (h): Exactly 6.62607015 × 10⁻³⁴ J·s
- Speed of light (c): Exactly 299,792,458 m/s
- Caesium hyperfine frequency: Exactly 9,192,631,770 Hz
Since the joule is defined in terms of kilogram, meter, and second, and these are now defined via fundamental constants, the joule has absolute stability independent of physical artifacts.
Standard Multiples
Kilojoule (kJ): 1 kJ = 1,000 J
- Typical for food energy (1 food Calorie = 4.184 kJ)
- Small appliance energy consumption
Megajoule (MJ): 1 MJ = 1,000,000 J = 1,000 kJ
- Vehicle fuel energy (1 liter gasoline ≈ 32 MJ)
- Industrial processes, home daily energy use
Gigajoule (GJ): 1 GJ = 1,000,000,000 J = 1,000 MJ
- Large-scale energy systems
- Natural gas (1 cubic meter ≈ 38 MJ, 1 therm = 105.5 MJ)
Terajoule (TJ): 1 TJ = 10¹² J
- Power plant output (per day/month)
- National energy statistics
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 Joule 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 Joule and Kilojoule
and Evolution
The Mechanical Equivalent of Heat (1840s-1850s)
James Prescott Joule (1818-1889), an English brewer and amateur scientist, conducted groundbreaking experiments demonstrating that mechanical work and heat are equivalent.
Famous paddle-wheel experiment (1843-1850): Joule built apparatus where falling weights drove paddles that stirred water in an insulated container. He precisely measured:
- Mechanical work input (weight × distance fallen)
- Temperature rise of water
- Established ratio: ~772 foot-pounds of work = 1 BTU of heat
- Modern equivalent: 4.184 joules = 1 calorie
Joule's work was initially met with skepticism but eventually revolutionized thermodynamics by unifying heat and mechanical energy under the first law of thermodynamics (conservation of energy).
Lord Kelvin (William Thomson) and Rudolf Clausius incorporated Joule's findings into the foundation of thermodynamics in the 1850s-1860s.
Adoption as a Unit (1882)
The British Association for the Advancement of Science formally adopted "joule" as the unit of work at its 1882 meeting, honoring James Prescott Joule's contributions. The joule was defined as 10⁷ ergs (the CGS energy unit).
Early definition: 1 joule = 10⁷ erg = 10⁷ g·cm²/s²
This established the joule in the meter-kilogram-second (MKS) system, precursor to SI.
International System of Units (1960)
The 11th General Conference on Weights and Measures (CGPM) in 1960 established the International System of Units (SI), designating the joule as the official SI unit for energy, work, and heat.
Unified energy unit: The joule replaced multiple historical units:
- Calorie (thermochemistry, nutrition)
- Erg (CGS physics)
- Foot-pound (engineering)
- Watt-hour (electricity)
- BTU (heating/cooling)
Despite SI adoption, many of these units persist in specific contexts (food Calories, kWh for electricity billing, BTU for HVAC).
Modern Precision (1990s-2019)
1990 redefinition: Linked to the volt and ohm through quantum electrical standards (Josephson effect and quantum Hall effect), enabling unprecedented precision.
2019 SI revision: Redefined the joule via exact fundamental constants (Planck constant, speed of light), eliminating dependence on physical prototypes. This "New SI" provides ultimate stability—joule can be reproduced anywhere in the universe using only constants and measurement equipment.
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: joules vs kilojoules
Explore the typical applications for both Joule (imperial/US) and Kilojoule (imperial/US) to understand their common contexts.
Common Uses for joules
Physics and Science
The joule is the universal energy unit in scientific research:
Mechanics: Kinetic energy (KE = ½mv²), potential energy (PE = mgh), work (W = F·d)
Thermodynamics: Heat transfer, enthalpy, internal energy, entropy (S units: J/K)
Electromagnetism: Electrical energy (E = V·I·t), magnetic energy (E = ½LI²), capacitor energy (E = ½CV²)
Quantum mechanics: Energy levels in atoms/molecules (though electronvolts often preferred)
Relativity: Mass-energy equivalence E = mc² (in joules when using SI units)
Engineering Applications
Engineers use joules across all disciplines:
Mechanical engineering: Impact energy, spring energy (E = ½kx²), rotational energy (E = ½Iω²)
Civil engineering: Structural load energy, seismic energy absorption
Aerospace engineering: Rocket propulsion energy, orbital energy calculations
Chemical engineering: Heat of reaction, activation energy, process energy requirements
Materials science: Fracture energy, strain energy density
Electrical and Electronics
Joules quantify electrical energy consumption:
Power consumption: Energy (J) = Power (W) × Time (s)
- 100W light bulb for 1 hour = 100 × 3,600 = 360,000 J = 360 kJ
Battery capacity: Often listed in watt-hours (Wh), convert to joules by × 3,600
- 10 Wh battery = 10 × 3,600 = 36,000 J = 36 kJ
Capacitor energy: E = ½CV² (where C in farads, V in volts, E in joules)
Energy efficiency: Comparing joules input vs. joules output determines efficiency percentage
Nutrition and Food Science
Many countries mandate both kilojoules and Calories on nutrition labels:
Australia/New Zealand: Kilojoules (kJ) primary, Calories secondary
European Union: Often lists both kJ and kcal
Conversion: 1 food Calorie (kcal) = 4.184 kJ
Example: 500 kJ = 500 ÷ 4.184 ≈ 120 kcal (Calories)
Climate and Energy Policy
Joules (typically in TJ, PJ, EJ) measure large-scale energy:
National energy consumption: Reported in exajoules (EJ = 10¹⁸ J) annually
- World primary energy: ~600 EJ/year
- United States: ~100 EJ/year
Carbon footprint: kg CO₂ per GJ of energy source
Renewable energy: Solar irradiance (W/m² → J/m² over time)
Energy intensity: Economic output per joule consumed (GDP/GJ)
Sports and Exercise Science
Joules measure human energy expenditure:
Metabolic equivalent (MET): 1 MET = ~4.184 kJ/hour per kg body weight at rest
Cycling power meters: Measure work in kilojoules per ride
Weightlifting: Work = weight × height × 9.8 (in joules)
- Deadlift 100 kg to 1 meter height = 100 × 9.8 × 1 = 980 J
Running economy: Joules per meter traveled
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 Joule (J)
How do I convert joules to calories?
Formula: calories = joules ÷ 4.184
Examples:
- 4,184 J = 1,000 small calories = 1 kcal (food Calorie)
- 100 J = 100 ÷ 4.184 ≈ 23.9 calories
- 1,000 J = 1,000 ÷ 4.184 ≈ 239 calories
For food Calories (kcal): joules ÷ 4,184 = kcal
- 418,400 J ÷ 4,184 = 100 kcal (food Calories)
How do I convert joules to kWh?
Formula: kWh = joules ÷ 3,600,000
Why 3,600,000?: 1 kWh = 1,000 watts × 3,600 seconds = 3,600,000 joules
Examples:
- 3,600,000 J = 1 kWh
- 1,800,000 J = 0.5 kWh
- 108,000,000 J = 30 kWh (typical home daily use)
How much energy is 1 joule?
Physical examples:
- Lifting 100 grams (small apple) 1 meter against Earth's gravity
- 1 watt of power for 1 second
- Kinetic energy of 2 kg mass moving at 1 m/s
- Energy to heat 0.24 grams of water by 1°C
Perspective: 1 joule is small—about the energy in a single heartbeat. Practical applications typically use kilojoules (kJ), megajoules (MJ), or kilowatt-hours (kWh).
What is the relationship between watts and joules?
Power (watts) = Energy (joules) ÷ Time (seconds)
Or equivalently: Energy (joules) = Power (watts) × Time (seconds)
Definition: 1 watt = 1 joule per second (W = J/s)
Examples:
- 100W light bulb for 10 seconds = 100 × 10 = 1,000 J = 1 kJ
- 1,000W microwave for 2 minutes = 1,000 × 120 = 120,000 J = 120 kJ
- 1W for 1 hour = 1 × 3,600 = 3,600 J = 3.6 kJ
Takeaway: Watts measure the rate of energy use; joules measure the total amount of energy.
Why do energy bills use kWh instead of joules?
Historical reasons: Kilowatt-hours (kWh) became standard for electricity billing before SI joule adoption (1960).
Convenient numbers: Home electricity typically measures 10-50 kWh/day, equivalent to 36-180 MJ/day. Using joules would require large numbers (millions).
Industry inertia: Billions of meters and billing systems use kWh; switching would require enormous infrastructure investment.
Conversion: 1 kWh = 3,600,000 J = 3.6 MJ
Both valid: kWh and MJ are both acceptable SI-compatible units for electrical energy.
How many joules are in a food Calorie?
1 food Calorie (capital C, kilocalorie) = 4,184 joules = 4.184 kilojoules
Important distinction:
- Small calorie (cal): 4.184 J
- Food Calorie (Cal, kcal): 4,184 J = 4.184 kJ
Examples:
- 100 Calorie snack = 418,400 J = 418.4 kJ
- 2,000 Calorie daily intake = 8,368,000 J = 8.368 MJ
- Banana (105 Calories) = 439,320 J ≈ 439 kJ
Labeling variation: Some countries (Australia, EU) show energy in both kJ and kcal.
What's the difference between joules and electronvolts?
Both measure energy but use vastly different scales:
Joule (J): SI unit for macroscopic energy (mechanical, thermal, electrical)
Electronvolt (eV): Atomic-scale energy unit (energy gained by electron crossing 1 volt)
Conversion: 1 eV = 1.602 × 10⁻¹⁹ J (extremely tiny in macroscopic terms)
Usage:
- Joules: Everyday physics, engineering, household energy
- Electronvolts: Atomic physics, chemistry, particle physics, semiconductors
Example: Visible light photon ≈ 2 eV ≈ 3.2 × 10⁻¹⁹ J
How do joules relate to kinetic energy?
Kinetic energy formula: KE = ½mv²
Where:
- m = mass (kg)
- v = velocity (m/s)
- KE = kinetic energy (joules)
Examples:
- 1 kg at 1 m/s: KE = ½ × 1 × 1² = 0.5 J
- 10 kg at 10 m/s: KE = ½ × 10 × 10² = 500 J
- 1,000 kg (small car) at 20 m/s (72 km/h): KE = ½ × 1,000 × 20² = 200,000 J = 200 kJ
- 70 kg person at 3 m/s (jogging): KE = ½ × 70 × 3² = 315 J
Note: Kinetic energy grows with velocity squared, so doubling speed quadruples energy.
Can you create or destroy joules?
No, energy (in joules) is conserved according to the first law of thermodynamics (conservation of energy).
Energy can only transform:
- Mechanical → thermal (friction converts motion to heat)
- Chemical → thermal (combustion releases heat)
- Electrical → mechanical (motors convert electricity to motion)
- Nuclear → thermal → mechanical → electrical (nuclear power plants)
Total joules remain constant in isolated systems, though energy may become less useful (second law: entropy increases).
Mass-energy equivalence: Einstein's E = mc² shows mass can convert to energy and vice versa, but total mass-energy (in joules) is conserved.
How many joules does a human use per day?
Basal metabolism (resting): ~7,000,000-9,000,000 J (7-9 MJ = 1,700-2,200 kcal)
Total daily energy expenditure (including activity):
- Sedentary: ~8,000,000-10,000,000 J (8-10 MJ = 2,000-2,400 kcal)
- Moderate activity: ~10,000,000-12,000,000 J (10-12 MJ = 2,400-3,000 kcal)
- Very active: ~12,000,000-16,000,000 J (12-16 MJ = 3,000-4,000 kcal)
- Elite athlete training: ~16,000,000-33,000,000 J (16-33 MJ = 4,000-8,000 kcal)
Energy sources: Carbohydrates (17 kJ/g), proteins (17 kJ/g), fats (37 kJ/g)
Efficiency: Human body ~20-25% efficient at converting food energy to mechanical work; rest becomes heat.
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: Joule to Kilojoule
| Joule (J) | Kilojoule (kJ) |
|---|---|
| 0.5 | 0.001 |
| 1 | 0.001 |
| 1.5 | 0.002 |
| 2 | 0.002 |
| 5 | 0.005 |
| 10 | 0.01 |
| 25 | 0.025 |
| 50 | 0.05 |
| 100 | 0.1 |
| 250 | 0.25 |
| 500 | 0.5 |
| 1,000 | 1 |
People Also Ask
How do I convert Joule to Kilojoule?
To convert Joule to Kilojoule, enter the value in Joule 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.
Learn more →What is the conversion factor from Joule to Kilojoule?
The conversion factor depends on the specific relationship between Joule 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 Joule?
Yes! You can easily convert Kilojoule back to Joule by using the swap button (⇌) in the calculator above, or by visiting our Kilojoule to Joule converter page. You can also explore other energy conversions on our category page.
Learn more →What are common uses for Joule and Kilojoule?
Joule 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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All Energy Conversions
Other Energy Units and Conversions
Explore other energy units and their conversion options:
- Megajoule (MJ) • Joule to Megajoule
- Gigajoule (GJ) • Joule to Gigajoule
- Watt-hour (Wh) • Joule to Watt-hour
- Kilowatt-hour (kWh) • Joule to Kilowatt-hour
- Megawatt-hour (MWh) • Joule to Megawatt-hour
- Calorie (cal) • Joule to Calorie
- Kilocalorie (kcal) • Joule to Kilocalorie
- British Thermal Unit (BTU) • Joule to British Thermal Unit
- Therm (therm) • Joule to Therm
- Foot-pound (ft⋅lb) • Joule to Foot-pound
Verified Against Authority Standards
All conversion formulas have been verified against international standards and authoritative sources to ensure maximum accuracy and reliability.
National Institute of Standards and Technology — Fundamental physical constants and unit conversions
Bureau International des Poids et Mesures — International System of Units (SI) definitions and standards
International Organization for Standardization — Quantities and units international standards series
Last verified: February 19, 2026