The formula
kJ = kcal × 4.184
Where the factor comes from — each unit by its definition, then its size in SI:
1 kcal = 1000 × cal = 4,184 J1 kJ = 1000 × J = 1,000 J
Dividing the two SI sizes gives the factor: 4.184, exact — every step above is a definition, not a measurement. The reverse factor is 0.239005736138.
The SI unit of energy is the joule. Food labels use kilocalories, electricity bills kilowatt-hours, heating specs BTU and particle physics electronvolts — the same quantity in five costumes.
Worked examples
- A day's food: 2,000 kcal = 8,368 kJ — 2,000 × 4.184 = 8,368
- A 1 kW heater run for one hour (1 kWh): 860.421 kcal = 3,600 kJ — 860.421 × 4.184 = 3,600
- Lifting 1 kg through 1 m at standard gravity (9.80665 J): 0.00234385 kcal = 0.00980665 kJ — 0.00234385 × 4.184 = 0.00980665
- A visible-light photon (2 eV): 7.65859 × 10⁻²³ kcal = 3.20435 × 10⁻²² kJ — 7.65859 × 10⁻²³ × 4.184 = 3.20435 × 10⁻²²
Frequently asked questions
How many kilojoules is 1 kcal?
1 kcal = 4.184 kJ, exactly, from the unit definitions.
How do I convert kilojoules back to kilocalories (food Calories)?
Divide by 4.184 — or multiply by 0.239005736138: 1 kJ = 0.239005736138 kcal.