Energy Converter
Convert joules, food calories, watt hours and BTU with exact definitions — the 4.184 J calorie is built right in.
| Reference | Exact value |
|---|---|
| 1 calorie (cal) | 4.184 J |
| 1 food calorie (kcal) | 4184 J |
| 1 kWh | 3,600,000 J |
| 1 BTU | 1055.05585262 J |
How to use
- Type an energy value and pick the from/to units.
- The result updates live as you type.
- Hit ⇅ to swap units and invert the conversion.
Frequently asked questions
Which energy units can I convert?
Eight: joule, kilojoule, calorie, kilocalorie, watt hour, kilowatt hour, British thermal unit (BTU) and foot-pound. Joule is the SI base for all conversions.
Why do food labels say calories but mean kilocalories?
The “calorie” of nutrition is historically the kilocalorie — 1000 small calories, exactly 4184 J. This tool uses the physics naming: 1 cal = 4.184 J, 1 kcal = 4184 J, so food “calories” appear under kcal.
How exact is the calorie definition?
The thermochemical calorie is defined as exactly 4.184 joules. Some fields use slightly different historical calories (the “large” 15°C calorie ≈ 4.1855 J), but 4.184 is the standard modern choice.
What is a BTU used for?
Heating and air-conditioning capacity, mostly in North America: 1 BTU ≈ 1055.06 J, originally the energy to raise one pound of water by one degree Fahrenheit.
How does a kWh compare to food energy?
One kilowatt hour is 3.6 million joules — about 860 food calories (kcal), or roughly a third of a person’s daily food intake. Human bodies are impressively energy-dense machines.