Energy Unit Converter
Convert energy between joules, kilojoules, calories, kilocalories (food Calories), watt-hours, kilowatt-hours, and BTU.
- Converted value
- 4.184
How it works
Every unit converts to joules first using its exact defined value, then from joules to the target unit — the same through-a-common-base-unit approach as this site's other unit-*.ts converters. A calorie is defined as exactly 4.184 J (so a kilocalorie, the 'food Calorie' printed on nutrition labels, is 4,184 J); a watt-hour is exactly 3,600 J, since a watt is one joule per second; and the International Table BTU is defined as exactly 1,055.05585262 J.
These are fixed definitions, not measurements that could go out of date — unlike a live exchange rate or a current price, a joule-to-calorie conversion has been the same fixed number since the units were formally defined, so it's safe to hardcode here the same way unit-angle.ts hardcodes π/180 for degrees-to-radians.
FAQ
Why does the food Calorie on a nutrition label use a capital C?
A nutrition label's 'Calorie' (capital C) is actually a kilocalorie (1,000 small calories) — a convention from early 20th-century nutrition science that stuck. This calculator's 'kcal' option is that same food Calorie, so converting 2,000 kcal to kilojoules gives the ~8,368 kJ figure sometimes printed alongside it on labels outside the US.
What's the difference between a watt-hour and a watt?
A watt measures power — the rate energy is used, right now. A watt-hour measures energy — power sustained for an hour. A 100-watt bulb left on for 1 hour uses 100 watt-hours of energy, the same as a 200-watt bulb left on for 30 minutes; this calculator converts the energy total, not the rate.