Energy Calculator
Convert energy between joules, calories, kilocalories, watt-hours, kWh, BTU, foot-pounds and electronvolts, from nutrition labels to electricity bills. Uses the thermochemical calorie (4.184 J) and the International Table BTU, and handles values as small as 1 eV (1.602×10⁻¹⁹ J).
What does the Energy Calculator do?
It converts an amount of energy into 15 units grouped by where they are used: SI joules (J, kJ, MJ, GJ), calories for food and heat (cal, kcal), watt-hours for electricity (Wh, kWh, MWh, GWh), BTU and foot-pounds from the imperial system, and electronvolts for atomic and particle physics (eV, keV, MeV). Enter a value anywhere and the other rows are recalculated in your browser.
How to use
- Type a value in the row of its unit, for example 2000 in Kilocalorie (a typical daily food intake). The other rows show 8.368 MJ and ≈ 2.32444 kWh.
- Set rounding digits to choose the number of decimals. Very small values such as eV in joules are shown with significant digits instead of rounding to 0.
- Turn on Description to see definitions, Ratio to compare each unit with the top row, and Separators for thousands separators.
- Drag rows by the handle to reorder them. The top row is the (base unit).
- Press Up / Down in a row to step the value by 1 of that unit.
Units and definitions
| Unit | Symbol | Definition |
|---|---|---|
| Joule | J | SI unit; 1 N·m = 1 W·s |
| Calorie | cal | Thermochemical calorie, 4.184 J |
| Kilocalorie | kcal | 1,000 cal = 4,184 J (the food "Calorie") |
| Watt-hour | Wh | 3,600 J |
| Kilowatt-hour | kWh | 3.6 MJ |
| British thermal unit | BTU | International Table BTU, 1,055.05585262 J |
| Foot-pound | ft-lb | ≈ 1.35582 J |
| Electronvolt | eV | 1.602176634×10⁻¹⁹ J (exact) |
Multiples kJ, MJ, GJ, MWh, GWh, keV and MeV are also included.
Conversion examples
- 1 kcal = 4.184 kJ
- 1 kWh = 3.6 MJ ≈ 860.421 kcal ≈ 3,412.14 BTU
- 1 BTU ≈ 1.05506 kJ ≈ 0.293071 Wh
Features
- 15 energy units across nutrition, electricity, heating and physics
- Convert from any row in either direction
- 40-digit decimal precision for both very large (GWh) and very small (eV) values
- Optional ratios, thousands separators and unit descriptions
- Unit order, value and options are saved in this browser
FAQ
Which calorie does this use?
The thermochemical calorie, 4.184 J, which is also the basis of food labeling. The International Table calorie (4.1868 J) and the 15 °C calorie are slightly different and are not listed separately.
Can I convert watts to kWh?
Watts measure power, not energy. Multiply the power by the time first (for example 1,500 W × 2 h = 3,000 Wh), then enter 3000 in Watt-hour.
Is my data sent anywhere?
No. All calculations run locally in the browser.