⚡ Energy Converter

Convert an energy value between joules, kilojoules, calories, kilocalories, kilowatt-hours and BTU.

Value

From Unit

The factors, all relative to the joule

Your value is converted to joules first, then out to every other unit:

1 cal  = 4.184 J           (thermochemical calorie)
1 kcal = 4184 J           = 1 food "Calorie"
1 kJ   = 1000 J
1 kWh  = 3,600,000 J      = 1 kW held for one hour
1 BTU  = 1055.05585262 J  (the IT BTU)

A 2,000 kcal day is 8,368 kJ, or 2.324 kWh - about what a fan oven draws in an hour. Energy and power are different things: a watt is a joule per second, and a kilowatt-hour is what a kilowatt delivers over 3,600 seconds.

One of each, side by side

1 unit of	Joules	kcal	kWh	BTU
Kilojoule	1,000	0.239	0.000278	0.9478
Kilocalorie	4,184	1	0.001162	3.9657
Kilowatt-hour	3,600,000	860.42	1	3,412.14
BTU	1,055.06	0.2520	0.000293	1

Air conditioners and boilers are rated in BTU per hour, so a 12,000 BTU/h unit is 3.517 kW of cooling - divide BTU/h by 3,412 for kW.

There are several slightly different calories and BTUs. This tool uses the thermochemical calorie of 4.184 J and the International Table BTU of 1055.056 J. The rival definitions differ by under 0.1%, which never matters on a nutrition label and can matter in a gas supply contract.

Frequently asked questions

How many calories are in a kilojoule?

239 small calories, or 0.239 kilocalories. Australian and European labels print kJ, so divide by 4.184 to reach the kcal figure used on US and UK packaging.

Is a food Calorie the same as a calorie?

No. The Calorie on a nutrition label is a kilocalorie, 1,000 of the calories used in physics. A 250 Calorie chocolate bar is 250 kcal, which is 1,046 kJ.

How many BTU are in a kWh?

3,412.14 BTU. Heating and cooling equipment is usually rated in BTU per hour, so dividing that rating by 3,412 gives the kilowatts it draws or moves.