What this converter does
This converter turns energy between joules, calories, kilocalories, kilojoules, BTU, kilowatt-hours, megajoules and foot-pounds — instantly and with exact factors. Type a value, pick the units, and the answer updates as you type.
One calorie is defined as exactly 4.184 joules (the thermochemical calorie). Food “Calories” are actually kilocalories. For electrical energy over time, see the kWh to kW converter.
The units it covers
Every unit measures energy, so each converts through the joule with a fixed ratio.
View all units & their values
| Unit | Symbol | Value | Mainly used |
|---|---|---|---|
| Joule | J | 1 | SI unit of energy |
| Calorie | cal | 4.184 | Chemistry, heat (thermochemical) |
| Kilocalorie | kcal | 4184 | Food energy (“Calories”) |
| Kilojoule | kJ | 1000 | Food labels, metric energy |
| British thermal unit | BTU | 1055.06 | HVAC, US heating energy |
| Kilowatt-hour | kWh | 3600000 | Electricity billing |
| Megajoule | MJ | 1000000 | Fuels, large energy |
| Foot-pound | ft·lb | 1.35582 | Mechanical work, US units |
The formula
Each unit has a fixed value in joules, so any conversion goes through the joule once:
result = value × factor_from ÷ factor_toWhere:
- value = the number you typed, in the “from” unit
- factor_from = the “from” unit’s value in joules
- factor_to = the “to” unit’s value in joules
Worked example
Convert 1000 joules to calories.
1 cal = 4.184 J1000 ÷ 4.184 = 239.01 calSo 1000 J is about 239 calories, or 0.239 kilocalories.
The units in this example
The SI unit of energy — one newton-metre of work, or one watt for one second. The base for all other energy units here.
- 1 J ≈ 0.239 cal
- 1 J = 0.001 kJ
- 1 J ≈ 0.000948 BTU
- 1 J ≈ 0.7376 ft·lb
The thermochemical calorie, exactly 4.184 joules — the heat to warm one gram of water by about 1 °C. Food “Calories” are kilocalories.
- 1 cal = 4.184 J
- 1 kcal = 1,000 cal
- 1 kcal = 4,184 J
- 239 cal ≈ 1,000 J