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Energy Converter
Convert energy and work between joules, food Calories, electricity units and heating units. Enter a quantity and the two units, and read it in every other unit listed below the result.
The joule and the units built on it
The joule is the SI unit of energy: the work done by a force of one newton over one metre, or one watt of power for one second. Every other unit here is a fixed multiple of it.
- 1 watt-hour = 3,600 J, so 1 kWh = 3,600,000 J = 3.6 MJ
- 1 thermochemical calorie = 4.184 J exactly, so 1 kcal = 4,184 J
- 1 British thermal unit (International Table) = 1,055.056 J
- 1 foot-pound force = 1.3558179483 J
- 1 electronvolt = 1.602176634 × 10−19 J exactly
The kilowatt-hour, found on electricity bills, is therefore not a power unit: it is the energy used by a 1 kW load running for one hour.
Calories, kilocalories and Calories on food labels
The "Calorie" with a capital C on a food label means a kilocalorie (kcal). It is 1,000 small calories, and 1 kcal equals 4.184 kJ. A label showing 250 kcal therefore also carries about 1,046 kJ, and many countries print both. A daily intake of 2,000 kcal is 8,368 kJ.
Several different calories exist in older references. The International Table calorie is defined as 4.1868 J, whereas this converter uses the thermochemical calorie of 4.184 J, the value most often used for nutrition labelling. The difference is about 0.07%, which only matters in precise thermal work.
BTU, heating and appliance ratings
The BTU is used for gas appliances, boilers and air conditioners. This page uses the International Table value of 1,055.056 J; other BTU definitions differ by a fraction of a percent. One kilowatt-hour is about 3,412 BTU. A rating such as 12,000 BTU per hour describes a rate of energy transfer, which is power rather than energy; use the power converter for those figures.
| Energy | Equivalent |
|---|---|
| 100 W lamp for 10 hours | 1 kWh = 3.6 MJ |
| 1 kWh | about 860 kcal |
| 1 kcal | about 3.966 BTU |
Common mistakes
- Mixing energy and power. A kilowatt is a rate; a kilowatt-hour is a quantity. A 3 kW heater running for 2 hours uses 6 kWh.
- Dropping the factor of 1,000. A calorie and a kilocalorie differ by 1,000, and the capital-C Calorie is the large one.
- Treating energy units as interchangeable in cost comparisons. Fuel prices are usually per kWh, per therm or per litre; convert to a common unit before comparing.
Worked example
A daily food target is given as 2,000 kilocalories. How many kilojoules is that?
Inputs
| Value | 2000 |
|---|---|
| From | Kilocalories / food Calories (kcal) |
| To | Kilojoules (kJ) |
Result
2,000 kcal = 8,368 kJ
| Joules (J) | 8,368,000 |
|---|---|
| Megajoules (MJ) | 8.368 |
| Calories (thermochemical) (cal) | 2,000,000 |
| Watt-hours (Wh) | 2,324.4444 |
| Kilowatt-hours (kWh) | 2.324444 |
| British thermal units (IT) (BTU) | 7,931.3326 |
| Foot-pounds force (ft·lbf) | 6,171,920.0652 |
| Electronvolts (eV) | 5.222895e25 |
- Factors are exact definitions where one exists; results are shown to about six significant digits.
Accuracy note: Factors use the standard definitions stated on this page. Results are rounded for display; for engineering or legal work use the full-precision factors.
Frequently asked questions
How many joules are in a kilocalorie?
One kilocalorie is 4,184 joules, using the thermochemical calorie of 4.184 J.
How many kilojoules are in 2,000 kcal?
Multiply kilocalories by 4.184 to get kilojoules, so 2,000 kcal is 8,368 kJ. Divide by 4.184 to go the other way.
What is a kilowatt-hour in joules?
A kilowatt-hour is exactly 3,600,000 joules, or 3.6 megajoules, because one kilowatt is 1,000 joules per second for 3,600 seconds.
How many BTU are in a kilowatt-hour?
About 3,412.14 BTU (International Table), found by dividing 3,600,000 J by 1,055.056 J per BTU.
Why do calories differ between sources?
Several calorie definitions exist. The thermochemical calorie is 4.184 J, while the International Table calorie is 4.1868 J. The two differ by roughly 0.07%.