The formula
cal = J × 0.2390057361
Where the factor comes from — each unit by its definition, then its size in SI:
1 J is the SI unit of energy.1 cal = 4.184 × J = 4.184 J
Dividing the two SI sizes gives the factor: 0.239005736138 (rounded for display; the underlying ratio is fixed by the definitions). The reverse factor is 4.184.
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 (2000 kcal): 8,368,000 J = 2,000,000 cal — 8,368,000 × 0.2390057361 = 2,000,000
- A 1 kW heater run for one hour (1 kWh): 3,600,000 J = 860,421 cal — 3,600,000 × 0.2390057361 = 860,421
- Lifting 1 kg through 1 m at standard gravity: 9.80665 J = 2.34385 cal — 9.80665 × 0.2390057361 = 2.34385
- A visible-light photon (2 eV): 3.20435 × 10⁻¹⁹ J = 7.65859 × 10⁻²⁰ cal — 3.20435 × 10⁻¹⁹ × 0.2390057361 = 7.65859 × 10⁻²⁰
Frequently asked questions
How many calories (thermochemical) is 1 J?
1 J = 0.239005736138 cal.
How do I convert calories (thermochemical) back to joules?
Divide by 0.239005736138 — or multiply by 4.184: 1 cal = 4.184 J.