Joule vs calorie — kitchen vs classroom
Why food labels use calories (or kilocalories) while physics uses joules.
1 J = 0.239 cal (SI, physics)
1 cal = 0.001 kcal = 0.001 "Calorie" (food label)
Three units do almost all the work in everyday energy measurements:
- Joule (J) — the SI unit. Used in physics and engineering. - Calorie (cal) — the energy to heat 1 g of water by 1 °C = 4.184 J. Used historically in nutrition and chemistry. - Kilocalorie (kcal) — 1,000 cal = 4,184 J. This is what food labels call a "Calorie" (with capital C). "One Calorie" on a US food label = 1 kcal = 4,184 J.
The trap: capitalisation matters.
- "calorie" (lowercase) = 4.184 J - "Calorie" (capital) = 1 kcal = 4,184 J
When a US label says "200 Calories" it means 200 kcal = 836,800 J. Most European labels now read "kJ" (kilojoule) for the same number, so 836.8 kJ ≈ 200 kcal.
Practical conversions: - 1 kcal = 4.184 kJ - 1 kJ = 0.239 kcal - 1 BTU (British thermal unit) ≈ 1,055 J ≈ 252 cal - 1 kWh = 3.6 MJ = 860 kcal - 1 kWh ≈ 3,412 BTU
In a sentence: a 100 W light bulb running 10 hours uses 1 kWh, which equals 860 kcal of energy — enough to heat a cup of water from 0 to 100 °C plus a bit extra.
The site converts across all five common units.
Frequently Asked Questions
Which should I use?
Use joules for physics, electrical engineering, mechanical engineering.
When is the alternative better?
Use (kilo)calories for nutrition, food labels, exercise science.
How is the conversion calculated?
Every conversion is a single factor (or an affine formula). The math is identical either direction; the difference is context.