What the energy converter does
An energy converter turns an amount of energy in one unit into another: joules and kilojoules, calories and kilocalories, watt-hours and kilowatt-hours, or British thermal units. Type a value, choose the From and To units, then press Convert. Every unit is converted through the joule, the SI unit of energy, and shown to up to 8 decimal places.
Conversion factors
| Unit | Symbol | In joules |
|---|---|---|
| Joule | J | 1 |
| Kilojoule | kJ | 1,000 |
| Calorie (thermochemical) | cal | 4.184 |
| Kilocalorie | kcal | 4,184 |
| Watt-hour | Wh | 3,600 |
| Kilowatt-hour | kWh | 3,600,000 |
| British thermal unit (IT) | BTU | 1,055.05585262 |
The joule is one newton acting over one meter, defined in the BIPM SI Brochure. A watt-hour is one watt sustained for 3,600 seconds, so it's exactly 3,600 J, and the kWh is a thousand times that.
Which calorie, which BTU?
Both the calorie and the BTU started life as "the heat needed to warm a gram (or pound) of water by one degree", and because that amount changes slightly with the starting temperature, several versions exist. NIST Special Publication 811 lists them side by side.
- This converter's calorie is the thermochemical calorie, exactly 4.184 J. The International Table calorie (4.1868 J) is about 0.07% larger. For nutrition the difference is far below the uncertainty of any food label.
- The BTU here is the International Table BTU, 1,055.05585262 J, the one normally used for heating, cooling and fuel figures.
The kilocalorie on this page is the same as the food Calorie. If a recipe app shows 450 Calories, enter 450 kcal.
Quick reference
| From | To | Multiply by |
|---|---|---|
| kcal | kJ | 4.184 |
| kJ | kcal | 0.239006 |
| kWh | kJ | 3,600 |
| kWh | BTU | 3,412.14 |
| kWh | kcal | 860.421 |
| BTU | kJ | 1.05506 |
| kcal | BTU | 3.96567 |
Worked examples
- A 2,000 kcal daily diet in kilojoules: 2,000 × 4.184 = 8,368 kJ. Labels in Australia and New Zealand lead with kJ, so this is the conversion to use there.
- One kWh of electricity in food terms: 3,600,000 ÷ 4,184 = 860.42 kcal.
- A 15 kWh home battery in BTU: 15 × 3,412.14 = 51,182 BTU.
- A US gas bill of one therm (100,000 BTU) in kWh: 100,000 × 1,055.05585262 ÷ 3,600,000 = 29.31 kWh.
Energy vs power
Energy units on this page measure an amount. Power units (watts, horsepower, BTU per hour) measure how fast that energy is used. To get one from the other, multiply or divide by time: a 2 kW heater running for 3 hours uses 2 × 3 = 6 kWh. Air-conditioner ratings written as "12,000 BTU" almost always mean 12,000 BTU per hour, which belongs in the power converter.
Related converters and calculators
- Power converter: watts, kilowatts, horsepower and BTU/h.
- Calorie calculator: estimates daily calorie needs from age, size and activity.
- BMR calculator: the energy your body uses at rest.
- Temperature converter: Celsius, Fahrenheit and Kelvin.
Last updated