How this conversion works
The joule is the SI unit of energy. A watt-hour is one watt for one hour, or 3,600 J, so a kilowatt-hour is exactly 3.6 MJ.
Several units have more than one historical definition. This converter uses the thermochemical calorie (exactly 4.184 J), the International Table BTU (1,055.05585262 J) and the US therm (100,000 BTU as defined by US regulations, 105,480,400 J).
Formula: result = value × (base units per source unit) ÷ (base units per target unit)
Where this conversion is used
- Food energy on labels in kcal or kJ.
- Electricity bills in kWh compared with gas in therms.
- Heating and cooling capacity in BTU.
- Physics and chemistry problems using joules or electronvolts.
Worked example: A 2,000 kcal diet in kilojoules and kWh
- One kcal is 4.184 kJ.
- 2,000 × 4.184 = 8,368 kJ.
- Divide by 3,600 kJ per kWh: 8,368 ÷ 3,600 ≈ 2.324 kWh.
Unit definitions
| Unit | In joules | Note |
|---|---|---|
| Joule (J) | 1 | |
| Kilojoule (kJ) | 1,000 | |
| Megajoule (MJ) | 1,000,000 | |
| Calorie (thermochemical) (cal) | 4.184 | 4.184 J exactly. |
| Kilocalorie (food Calorie) (kcal) | 4,184 | |
| Watt-hour (Wh) | 3,600 | |
| Kilowatt-hour (kWh) | 3,600,000 | |
| British thermal unit (IT) (BTU) | 1,055.05585262 | |
| Therm (US) (thm) | 105,480,400 | 100,000 BTU (US definition, 105,480,400 J). |
| Foot-pound (ft·lbf) | 1.3558179483 | |
| Electronvolt (eV) | 1.60218e-19 |
Kilowatt-hours to BTU
| Kilowatt-hour (kWh) | British thermal unit (IT) (BTU) |
|---|---|
| 0.1 | 341.2142 |
| 0.5 | 1,706.0708 |
| 1 | 3,412.1416 |
| 2 | 6,824.2833 |
| 5 | 17,060.7082 |
| 10 | 34,121.4163 |
| 100 | 341,214.1633 |
| 1,000 | 3,412,141.6331 |
Kilocalories to kilojoules
| Kilocalorie (food Calorie) (kcal) | Kilojoule (kJ) |
|---|---|
| 50 | 209.2 |
| 100 | 418.4 |
| 200 | 836.8 |
| 250 | 1,046 |
| 500 | 2,092 |
| 1,000 | 4,184 |
| 2,000 | 8,368 |
| 2,500 | 10,460 |
Things to watch for
- A food “Calorie” with a capital C is a kilocalorie (1,000 calories).
Precision and sources
Results are calculated from the definitions above and rounded to 12 significant digits; the converter shows up to eight decimal places and the tables four. Reference: NIST Special Publication 811, Guide for the Use of the International System of Units, Appendix B.
Frequently asked questions
How many BTU are in a kilowatt-hour?
About 3,412.14 BTU (IT).
What is the difference between a calorie and a Calorie?
A small calorie (cal) is 4.184 J. The Calorie on food labels is a kilocalorie, 4,184 J.