How many joules in a kilocalorie?
1 kcal (or 1 food Calorie) equals exactly 4,184 joules (by definition for nutrition labeling, using the thermochemical calorie).
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Energy is the capacity to do work or produce heat. The SI unit is the joule (J), defined as 1 newton applied over 1 meter. Common energy units include the kilojoule (kJ), calorie (cal), kilocalorie or food Calorie (kcal = 4,184 J), kilowatt-hour (kWh = 3.6 MJ), BTU (1,055.06 J), and electron-volt (eV) for atomic-scale energies.
Example energy conversions
1 kJ1,000 J, 0.239 kcal, 0.948 BTU
1 kcal (1 food Calorie)4,184 J, 3.966 BTU
1 BTU (IT)1,055.06 J, 0.252 kcal
1 kWh3.6 MJ, 3,412.14 BTU, 859.85 kcal (exact)
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Joules and the SI energy hierarchy
The joule is the SI coherent unit for energy. 1 J equals 1 newton-meter (the work done lifting a 1-newton weight by 1 meter), 1 watt-second (the energy of 1 watt of power for 1 second), and 1 coulomb-volt (electrical work to move 1 coulomb across 1 volt). This compositional cleanness is why physics and engineering use the joule.
Decimal prefixes extend it across scales: 1 kJ (10³ J) is a small kitchen energy quantity, 1 MJ (10⁶ J) is the energy in a small high-calorie snack, 1 GJ (10⁹ J) is the energy in 28 liters of gasoline, 1 TJ (10¹²) is roughly a railcar's worth of coal.
Calories, kilocalories, and food Calories
The original 'calorie' was defined as the energy needed to raise 1 gram of water by 1°C. The kilocalorie (kcal, 1,000 calories) became the convenient unit for food because most foods deliver more energy than a small-calorie label could conveniently express. In nutrition labeling, the capital-C 'Calorie' equals 1 kcal or 4,184 joules.
This means a 200-Calorie snack delivers 200 kcal, or 200,000 small calories, or 836,800 joules, or about 793 BTU. The capitalization is the only thing distinguishing food Calories from small calories on labels, and the convention isn't universally followed, which is a frequent source of confusion in conversion writeups.
Practical energy contexts
Household electricity is billed in kilowatt-hours. 1 kWh equals 3.6 million joules or 3,412 BTU. A 1,500 W space heater running for an hour uses 1.5 kWh (5.4 MJ). Gas appliances are sometimes rated in BTU/hr; converting requires dividing by 3,412 to get kilowatts.
Atomic and molecular physics use electron-volts. 1 eV equals 1.602 × 10⁻¹⁹ J. Chemical bond energies are usually in the 1-10 eV range; gamma-ray photons span keV to MeV. Nuclear binding energies are in the MeV-per-nucleon range. The joule is the wrong unit at that scale because the numbers become inconveniently small.
Common energy conversions
| From | Equivalent |
|---|---|
| 1 kJ | 1,000 J, 0.239 kcal, 0.948 BTU |
| 1 kcal (1 food Calorie) | 4,184 J, 3.966 BTU |
| 1 BTU (IT) | 1,055.06 J, 0.252 kcal |
| 1 kWh | 3.6 MJ, 3,412.14 BTU, 859.85 kcal (exact) |
| 1 MJ | 10⁶ J, 239 kcal, 947.82 BTU |
| 1 eV | 1.602 × 10⁻¹⁹ J |
| 1 keV | 1.602 × 10⁻¹⁶ J |
| 1 ft-lb | 1.35582 J |
| 1 GJ | 10⁹ J, 947,817 BTU, 277.78 kWh |
Frequently asked questions
What is 1 kWh in joules?
1 kilowatt-hour equals 3,600,000 joules (3.6 MJ). That's 1,000 watts × 3,600 seconds.
Are food calories and physics calories different units?
Yes. A 'food Calorie' (capital C) is 1 kilocalorie (1,000 small calories). A 'calorie' (lowercase, as in the small unit) is 1/1,000 of that. Food labels nearly always use the kilocalorie scale even when written as 'Calories'.
How many BTU is 1 kWh?
1 kWh equals 3,412.14 BTU (using the IT BTU). HVAC and energy bills sometimes mix the two units, so the conversion comes up often.
When should I use electron-volts?
When working with atoms, molecules, or particles. 1 eV is the energy gained by an electron crossing 1 volt of potential. Bond energies are 1-10 eV; X-ray photons are keV; nuclear energies are MeV. The joule is too large at that scale.
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