Popular energy conversions
Energy units
Energy is the capacity to do work. The SI unit is the joule. Food energy is labelled in kilocalories (Calories with a capital C) or kilojoules, electricity is billed in kilowatt-hours, and heating often uses BTU or therms.
| Unit | Symbol | In joules | Definition |
|---|---|---|---|
| Joule | J | 1 J | The SI unit of energy: one newton acting over one meter. |
| Kilojoule | kJ | 1,000 J | One thousand joules. Food labels in Australia and Europe show kJ alongside kcal. |
| Megajoule | MJ | 1,000,000 J | |
| Calorie | cal | 4.184 J | The small (thermochemical) calorie: exactly 4.184 joules. |
| Kilocalorie | kcal | 4,184 J | One thousand calories, the "Calorie" printed on food labels. |
| Watt-hour | Wh | 3,600 J | |
| Kilowatt-hour | kWh | 3,600,000 J | Exactly 3.6 megajoules: the "unit" on an electricity bill. |
| Megawatt-hour | MWh | 3,600,000,000 J | |
| Electronvolt | eV | 1.602176634 × 10⁻¹⁹ J | Exactly 1.602176634 × 10⁻¹⁹ joules, used in atomic and particle physics. |
| British thermal unit | BTU | 1,055.055853 J | About 1,055 joules (International Table value), used for heating and cooling. |
| Therm | thm | 105,480,400 J | The US therm: 100,000 BTU, used for natural gas billing. |
| Foot-pound | ft·lbf | 1.355817948 J | |
| Erg | erg | 0.0000001 J |
Calculate with energy
The smart calculator does arithmetic across energy units and picks a sensible unit for the answer. Try 2000 kcal to kJ.
Questions and answers
Which energy units can I convert?
All 13: joules, kilojoules, megajoules, calories, kilocalories, watt-hours, kilowatt-hours, megawatt-hours, electronvolts, British thermal units, therms, foot-pounds, ergs.
What is the SI unit of energy?
This converter uses the joule (J) as its base unit. Energy is the capacity to do work. The SI unit is the joule. Food energy is labelled in kilocalories (Calories with a capital C) or kilojoules, electricity is billed in kilowatt-hours, and heating often uses BTU or therms.
Are Calories on food labels the same as calories?
No. The food Calorie (capital C) is a kilocalorie: 1,000 small calories, or 4.184 kilojoules.
How precise are the results?
Factors are the exact legal definitions where one exists (an inch is exactly 2.54 cm, a pound exactly 0.45359237 kg). Results show 10 significant digits by default; the precision menu offers 4 to 15.
Does the energy converter work offline?
Yes. ConvertEasy is a progressive web app: after your first visit it keeps working with no connection, and you can install it to your home screen from the browser menu.