The joule is the base of everything here
A joule is the energy of a watt used for one second, or of a newton pushing something one meter. Every other unit on this page is a fixed multiple of it, so converting between any two is a single multiplication and nothing is rounded until the answer is shown.
- 1 kWh = 3,600,000 J = 3.6 MJ = 3,412.14 BTU = 860.42 kcal.
- 1 Wh = 3,600 J, because a watt for 3,600 seconds is 3,600 joules.
- 1 kcal = 4,184 J = 4.184 kJ = 1.1622 Wh.
- 1 BTU = 1,055.05585262 J = 1.05506 kJ = 0.29307 Wh.
- 1 ft⋅lbf = 1.35581795 J, worked out from the pound, the foot and standard gravity.
Energy is not power
A kilowatt hour measures energy, an amount. A kilowatt measures power, how fast energy is used. A 1.5 kW heater that runs for 2 hours uses 1.5 × 2 = 3 kWh, which is 10.8 MJ. Electricity bills are in kWh because that is the amount you used. The rating on an appliance is in watts or kilowatts, and the same goes for BTU per hour on an air conditioner, which is power too. This converter changes units of energy. To compare appliances, multiply their power by the hours of use first.
Calories, and which one is meant
The Calorie on a food label is a kilocalorie: 1 kcal = 4.184 kJ. The lowercase calorie of physics, which is exactly 4.184 J, is 1,000 times smaller. People write both as calories, so the units here are named kilocalorie (kcal) and calorie (cal) to keep them apart. There is also the International Table calorie, exactly 4.1868 J, which differs from the thermochemical one by about 0.07%. The calories to kilojoules page shows both kinds side by side.
Heat and fuel units
A BTU, the British thermal unit, is the heat that warms a pound of water by about one degree Fahrenheit. There are several slightly different BTU definitions, which differ by up to about half a percent. This converter uses the International Table BTU, 1,055.05585262 J. A US therm is 100,000 BTU of the 59 °F kind, 105,480,400 J or about 29.3 kWh, and it appears on natural gas bills. Because that BTU is a little different, a therm is 99,976 of the BTU used here, not exactly 100,000.
At the small end, the electronvolt (exactly 1.602176634 × 10⁻¹⁹ J) and the erg (10⁻⁷ J) are used in physics. At the large end, a ton of TNT is a convention of 4.184 gigajoules, used to describe explosions and impacts, not a measurement of any particular explosive.