maratoolHow to convert calories, joules, kWh, and BTU
Type an energy value in any unit — read joules, calories, kWh, BTU, or eV instantly.
Nutrition labels use kcal, utility bills use kWh, furnaces use BTU, and physics uses joules and electronvolts. The Energy Converter bridges all of them with exact NIST SP 811 factors.
How it works
- Pick units — "Kilocalorie / food Calorie (kcal)" to "Kilojoule (kJ)", or any other pair.
- Type a value — the result updates as you type, with swap and copy.
- Copy — one click, with "Copied!" feedback.
Energy conversion factors
Every unit converts through the joule (the base unit). To convert A → B, multiply by A's factor and divide by B's:
| Unit | = this many joules |
|---|---|
| Joule (J) | 1 |
| Kilojoule (kJ) | 1,000 |
| Megajoule (MJ) | 1,000,000 |
| Watt-hour (Wh) | 3,600 |
| Kilowatt-hour (kWh) | 3,600,000 |
| Calorie, thermochemical (cal) | 4.184 |
| Kilocalorie / food Calorie (kcal) | 4,184 |
| BTU, International Table (BTU) | 1,055.05585262 |
| Foot-pound (ft·lbf) | 1.355817948331 |
| Electronvolt (eV) | 1.6022 × 10^-19 |
When you need it
Useful anchors: a 2,000 kcal daily diet is 8.37 MJ or 2.32 kWh — about the energy in 7,900 BTU. The thermochemical calorie (4.184 J exactly) is used here; the slightly different International Table calorie (4.1868 J) appears in some older steam tables.
Sources & methodology
Conversion factors follow NIST Special Publication 811; exact-by-definition factors are marked in the tool itself. All math runs client-side in your browser — nothing is uploaded.
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