Energy shows up in more guises than almost any other quantity. Your electric bill counts kilowatt-hours, the gas bill counts therms, a furnace is rated in BTU, a nutrition label lists Calories or kilojoules, a physics problem wants joules, and a particle physicist talks in electronvolts. They all measure the same thing — the capacity to do work or produce heat — so any one can be converted to any other exactly. This converter handles 16 units and translates the result into something tangible: how long it would keep a 10-watt LED bulb lit.
How to use the energy converter
- Enter the amount of energy in Value.
- Choose its unit in From and the target unit in To.
- The tape shows the converted value, the factor in both directions and the LED-bulb runtime.
- The table lists the energy in every unit, from electronvolts to gigajoules.
Energy conversion formula
Every unit is expressed in joules, the SI unit (one newton acting over one meter):
The definitions behind the factors: 1 cal = 4.184 J (thermochemical), 1 Wh = 3,600 J because a watt is one joule per second, 1 BTU = 1,055.05585262 J (International Table), 1 therm = 100,000 BTU (US), 1 ft·lbf = 1.3558179483 J and 1 eV = 1.602176634 × 10⁻¹⁹ J, exact since the 2019 SI redefinition.
Worked example: a day’s food energy
A 2,000-Calorie daily diet, expressed in other units:
2,000 kcal × 4.184 kJ/kcal = 8,368 kJ
8,368,000 J ÷ 3,600,000 J/kWh = 2.32 kWh
8,368,000 J ÷ 1,055.056 J/BTU = 7,931 BTU
At 10 watts, 8,368,000 J would keep an LED bulb lit for 836,800 seconds, about 9 days and 16 hours. Put another way, a day’s worth of human food energy is worth roughly 30 to 40 cents of electricity at typical US residential rates.
Common energy conversions
| From | To | Multiply by |
|---|---|---|
| kilocalories | kilojoules | 4.184 (exact) |
| kilojoules | kilocalories | 0.2390057 |
| kilowatt-hours | megajoules | 3.6 (exact) |
| kilowatt-hours | BTU | 3,412.1416 |
| BTU | joules | 1,055.0559 |
| therms | kilowatt-hours | 29.300111 |
| kilocalories | BTU | 3.9656668 |
| foot-pounds | joules | 1.3558179 |
| electronvolts | joules | 1.602176634 × 10⁻¹⁹ (exact) |
Which unit for which job
- Joule (J), kJ, MJ, GJ: physics, engineering and nutrition outside the US.
- Calorie and kilocalorie: chemistry and US food labeling. The capital-C Calorie on a label is a kilocalorie.
- Watt-hour and kilowatt-hour: electricity and batteries. A phone battery stores roughly 15–20 Wh; an electric car battery 50–100 kWh.
- BTU and therm: US heating, cooling and natural gas. Furnace and water-heater ratings are BTU per hour (a power, not an energy).
- Foot-pound: mechanical work and torque-angle specifications in US engineering, and bullet muzzle energy.
- Electronvolt: atomic and particle physics, and photon energies.
Energy versus power
Energy is an amount; power is a rate. A 1,500 W space heater running for two hours uses 1,500 W × 2 h = 3 kWh of energy. Mixing the two is common when reading equipment labels: “BTU” on an air conditioner usually means BTU per hour, a power. Use the power converter for watts, horsepower and BTU/h.
Calorie and kilojoule figures on food labels are rounded estimates; actual energy absorbed varies by person and food. For daily needs, see the calorie calculator.
To compute the energy of a moving object, try the kinetic energy calculator, and for the energy equivalent of mass, the E = mc² calculator.
Frequently asked questions
How many joules are in a calorie?
The thermochemical calorie used here is exactly 4.184 J. A food Calorie (with a capital C) is a kilocalorie, 4,184 J. Some older tables use the International Table calorie of 4.1868 J, which differs by about 0.07%.
How many BTU are in a kilowatt-hour?
About 3,412.14 BTU. A kilowatt-hour is exactly 3.6 million joules, and an International Table BTU is 1,055.05585262 J.
Is a food Calorie the same as a kilocalorie?
Yes. The Calories on US nutrition labels are kilocalories, so a 2,000-Calorie diet supplies 2,000 kcal, or 8,368 kJ. Labels in Europe and Australia usually print kilojoules alongside or instead.
What is a therm?
A therm is 100,000 BTU, about 105.5 MJ or 29.3 kWh. US natural-gas utilities bill in therms or in hundreds of cubic feet of gas, which contain roughly one therm.