An energy converter that turns any amount between the nine units engineers, scientists and households actually use: joules (J), kilojoules (kJ), the calorie (cal) and kilocalorie (kcal), watt-hours (Wh) and kilowatt-hours (kWh), the British thermal unit (BTU), foot-pounds of force (ft·lb) and the physicist’s electronvolt (eV). Type a value once and read the equivalent in every unit at the same time — no two-field guessing, no rounding surprises.
It is built for real tasks: reading an electricity bill in kWh and wanting joules, checking how many kilocalories a kilojoule figure on a food label represents, sizing a heater quoted in BTU against a kettle quoted in kW, or sanity-checking a physics problem that mixes electronvolts with joules.
How it works
Energy is a single physical quantity, so every unit is just a different sized “ruler” measuring the same thing. The tool defines each unit by how many joules one of it equals, converts whatever you type into a joule baseline, then divides that baseline by the target unit’s factor. Because all nine units share one baseline, the reference table can show them all consistently with no accumulated rounding error.
The constants are exact or standard: 1 kWh = 3,600,000 J, 1 Wh = 3600 J, 1 kcal = 4184 J, 1 cal = 4.184 J, 1 BTU = 1055.05585262 J, 1 ft·lb = 1.3558179483314 J and 1 eV = 1.602176634 x 10⁻¹⁹ J. Values are shown to about ten significant figures, switching to scientific notation for very large or very small results so an electronvolt total stays readable.
Worked example
Take 1 kWh — the energy a 1000 W appliance uses in an hour. Converting:
- 3,600,000 J (3600 kJ)
- 860.42 kcal (so about 860 food Calories — roughly a large meal’s worth)
- 3412.14 BTU
- 2,655,224 ft·lb
- 2.247 x 10²⁵ eV
Flip the Swap button and the same machinery runs the other way: 3412 BTU comes straight back to ≈1 kWh.
Conversion reference table
| Unit | Symbol | Joules per unit | Typical use |
|---|---|---|---|
| Joule | J | 1 | SI base unit of energy |
| Kilojoule | kJ | 1,000 | Food labels (outside the US), heat loads |
| Calorie (thermochemical) | cal | 4.184 | Chemistry, small heat quantities |
| Kilocalorie / food Calorie | kcal | 4,184 | Nutrition, dieting |
| Watt-hour | Wh | 3,600 | Battery capacity |
| Kilowatt-hour | kWh | 3,600,000 | Electricity bills |
| British thermal unit | BTU | 1,055.05585262 | Heating and air-conditioning ratings |
| Foot-pound force | ft·lb | 1.3558179483 | Torque/work in imperial engineering |
| Electronvolt | eV | 1.602176634e-19 | Atomic and particle physics |
Every figure is calculated locally in your browser — no numbers are uploaded or stored.