Energy Converter (J, kWh, cal, BTU…)

Convert energy between joules, kJ, MJ, watt-hours, kWh, calories, kilocalories, BTU, and foot-pounds — one value into every unit, with a reference table.

Joule (J)–

Kilojoule (kJ)–

Megajoule (MJ)–

Watt-hour (Wh)–

Kilowatt-hour (kWh)–

Calorie (cal)–

Kilocalorie (food Calorie) (kcal)–

British thermal unit (BTU)–

Foot-pound (ft·lb)–

Energy conversion reference

1 unit	J	kJ	Wh	kcal
1 J	1	0.001	0.000277778	0.000239006
1 kJ	1 000	1	0.277778	0.239006
1 MJ	1 000 000	1 000	277.778	239.006
1 Wh	3 600	3.6	1	0.860421
1 kWh	3 600 000	3 600	1 000	860.421
1 cal	4.184	0.004184	0.00116222	0.001
1 kcal	4 184	4.184	1.16222	1
1 BTU	1 055.06	1.05506	0.293071	0.252164
1 ft·lb	1.35582	0.00135582	0.000376616	0.000324048

Converting energy units

Energy shows up in a lot of different units because different fields measure it their own way — the SI joule in physics, the kilowatt-hour on your electricity bill, the (kilo)calorie on food labels, and the BTU in heating and air-conditioning. They all measure the same thing, and this tool converts one value into every unit at once, working through the joule.

The relationships worth knowing

A watt-hour is one watt for one hour, which is 3600 joules; a kilowatt-hour is 3.6 million joules. The food "Calorie" (capital C) is actually a kilocalorie, 4184 joules, so a 100-Calorie snack holds about 418 kJ. A BTU is roughly 1055 joules. These fixed ratios make conversion exact.

Worked example

A 2000 W heater running for 3 hours uses 6 kWh — that's 6 × 3.6 = 21.6 MJ, or about 5162 kilocalories. On the food side, a 500-Calorie meal is 500 kcal = 2092 kJ, the energy a 60 W bulb would use in about 9.7 hours.

For the heat energy to warm a specific material, see theheat energy calculator; for RF signal levels rather than energy, thedBm to watts converter.

Which calorie, and which BTU?

Most of these conversions are exact, but a few units have competing definitions. The thermochemical calorie is exactly 4.184 J by convention while older sources use slightly different values, and the BTU exists in several variants that differ in the third significant figure. The food Calorie with a capital C is a kilocalorie — a factor of a thousand, which is the largest trap here. Where a figure matters, find out which definition your source used before chasing a discrepancy.