Many units for one quantity
Energy shows up in food, electricity, fuel and physics, and each field kept its own unit. Nutrition uses kilocalories, sold as Calories in the US, or kilojoules. Utilities bill electricity in kilowatt-hours and gas in therms or BTU. Science uses the joule, the SI unit, and atomic physics the tiny electronvolt. They all measure the same thing, so any of them converts exactly into the others.
How to use this converter
Type a value in any field and all the other units update as you type. Start from what you have: the energy on a food label, a furnace rating in BTU, a battery capacity in watt-hours or a utility bill in kWh. Each common pair below the converter also has its own page with the formula, an example and a chart.
Key equivalents
- 1 kcal = 4.184 kJ, and 1 kJ ≈ 0.239 kcal
- 1 Wh = 3,600 J, and 1 kWh = 3.6 MJ
- 1 kWh ≈ 3,412 BTU ≈ 860 kcal
- 1 therm = 100,000 BTU ≈ 29.3 kWh
- 1 BTU ≈ 1,055 J ≈ 0.252 kcal
Energy versus power
Power is the rate at which energy is used. Watts measure power; watt-hours measure energy. A 60 W bulb left on for 10 hours uses 600 Wh, or 0.6 kWh. To find the energy, multiply power by time; to compare appliances, look at their wattage and how long they run.
Comparing heating costs
To compare fuels, bring them to the same unit, usually kWh. Divide the price per therm by 29.3, or the price per million BTU by 293, to get a cost per kWh of heat. Then adjust for efficiency: a gas furnace may deliver 80 to 95% of the fuel energy as heat, while a heat pump delivers 2 to 4 kWh of heat per kWh of electricity.
Tips
For diets, stay in one unit to avoid mixing kJ and kcal, which differ by a factor of about 4.2. For physics problems, convert everything to joules first.
Energy units compared
| Unit | Symbol | In joules |
|---|---|---|
| Joule | J | 1 |
| Kilojoule | kJ | 1,000 |
| Megajoule | MJ | 1,000,000 |
| Calorie | cal | 4.184 |
| Kilocalorie | kcal | 4,184 |
| Watt-hour | Wh | 3,600 |
| Kilowatt-hour | kWh | 3,600,000 |
| British thermal unit | BTU | 1,055.06 |
| Therm | thm | 105,505,585 |
| Foot-pound | ft⋅lbf | 1.35582 |
| Electronvolt | eV | 1.60218 × 10⁻¹⁹ |
The units in detail
What is a joule?
The joule is the SI unit of energy: the work done by a force of one newton over one meter, or one watt for one second. It is small; lifting an apple about one meter takes roughly one joule.
What is a kilojoule?
The kilojoule equals 1,000 joules. Food labels in Europe, Australia and New Zealand list energy in kJ next to kcal; a typical adult reference intake is 8,400 kJ, or 2,000 kcal, per day.
What is a megajoule?
The megajoule equals one million joules, exactly the energy of 0.2778 kilowatt-hour. It is used for fuel energy content: a liter of gasoline holds about 32 to 34 MJ.
What is a calorie?
The small calorie, 4.184 joules, is the energy that warms one gram of water by one degree Celsius. In nutrition, the word Calorie with a capital C, as on US food labels, actually means a kilocalorie, 1,000 times more.
What is a kilocalorie?
The kilocalorie equals 1,000 small calories, or 4.184 kilojoules. It is the Calorie printed on US Nutrition Facts labels and used in diet and exercise tracking: an adult needs roughly 2,000 to 2,500 kcal a day.
What is a watt-hour?
The watt-hour is the energy used by a one-watt device in one hour, 3,600 joules. Battery capacities of phones, laptops and power banks are often given in Wh; airlines generally allow power banks up to 100 Wh in cabin baggage.
What is a kilowatt-hour?
The kilowatt-hour equals 1,000 watt-hours, or 3.6 megajoules. It is the unit on every electricity bill and for electric car batteries: a 2,000 W heater running for 3 hours uses 6 kWh.
What is a British thermal unit?
The British thermal unit, about 1,055 joules, is the heat needed to warm one pound of water by one degree Fahrenheit. US furnaces, water heaters, grills and natural gas are rated in BTU.
What is a therm?
The therm equals 100,000 BTU, about 29.3 kilowatt-hours. US and UK natural gas bills often charge per therm. The US therm used by some utilities differs from this value by only 0.02%.
What is a foot-pound?
The foot-pound is the work done by a one-pound force over one foot, about 1.356 joules. It measures the energy of bullets and air guns in the US; the same unit also expresses torque on wrenches and engines.
What is an electronvolt?
The electronvolt is the energy an electron gains across one volt, exactly 1.602176634 × 10⁻¹⁹ joule. Physicists and chemists use it for atoms, photons and particles; visible light photons carry about 2 to 3 eV.