Energy Converter
Convert between joules, calories, kilowatt-hours, BTU, therms and electronvolts.
How to use this calculator
- 1Enter the value you want to convert.
- 2Pick the unit you are converting from and the one you want.
- 3The table below shows the same value in every other supported unit.
How the calculation works
result = (value × factor(from) ) / factor(to)- factor
- How many joules one unit represents
Every value is converted to joules first, then out to the target unit. This means n units need n factors rather than n² conversion pairs.
Energy, work and heat are the same physical quantity and share the same units — which is why a food Calorie, an electricity-bill kilowatt-hour and a mechanical foot-pound can all be converted into one another.
The calorie is genuinely ambiguous: the thermochemical calorie (exactly 4.184 J) is used here, while the International Table calorie is 4.1868 J. They differ by about 0.07%. The "Calorie" on food labelling is a kilocalorie — 1,000 of the small calories — which is why a 2,000 Calorie diet is 2,000 kcal, not 2,000 cal.
The therm is also ambiguous. This converter uses the US therm (105,480,400 J, based on the 59 °F BTU) used in American natural-gas billing; the EC therm is 105,506,000 J.
Worked example
1 kilowatt-hour to megajoule
- 1.One kilowatt-hour is 3,600,000 joules, so 1 kWh is 3,600,000 joules.
- 2.One megajoule is 1,000,000 joules, so divide: 3,600,000 ÷ 1,000,000 = 3.6.
- 3.Put another way, 1 kWh = 3.6 MJ.
Result: 3.6 MJ
One quantity, many units
Energy is the capacity to do work, and heat, work and energy are all the same physical quantity measured the same way. That single fact is what makes this converter possible at all: the chemical energy in food, the electrical energy on a utility bill, the heat output of a boiler and the kinetic energy of a moving object are not merely analogous, they are interchangeable quantities that happen to be measured in units inherited from different disciplines.
The joule is the SI unit, defined as the work done by one newton acting over one metre. Everything else in this converter is a rescaling of it — often one chosen because it produced convenient numbers for a particular industry before the SI system was universal.
Where each unit came from
Most energy units are historical artefacts of the field that coined them.
- Joule — the SI unit, named for James Prescott Joule, whose experiments established that mechanical work and heat are the same thing — the very equivalence this converter relies on.
- Calorie — originally defined as the heat needed to raise one gram of water by one degree Celsius. Retained in nutrition, largely abandoned in physics.
- Kilowatt-hour — an electrical-billing unit: the energy delivered by one kilowatt sustained for one hour. Convenient because household meters measure power draw over time directly.
- BTU and therm — the British Thermal Unit is the heat needed to raise one pound of water by one degree Fahrenheit; the therm is 100,000 of them. Both persist in heating, air-conditioning and natural-gas billing.
- Electronvolt — the energy an electron gains crossing a one-volt potential difference — vanishingly small in everyday terms, but the natural scale for atomic and particle physics.
The two ambiguous units to watch
Two units in this list do not have a single agreed value, and a converter that hides that is being misleading rather than helpful. The calorie exists in a thermochemical definition (exactly 4.184 J) and an International Table definition (4.1868 J), differing by roughly 0.07% — irrelevant for a nutrition label, but not for a precise calculation. The therm is worse: the US therm used for American gas billing is 105,480,400 J, while the EC therm is 105,506,000 J, a gap of about 0.02%.
This converter uses the thermochemical calorie and the US therm, and states both choices on the page. If your source specifies a different definition, the difference is small but real and worth applying deliberately rather than assuming.
What this assumes, and where it stops
Assumptions
- Conversion factors follow the internationally agreed definitions of each unit.
Limitations
- Converting between energy and other quantities that sound similar — power (energy per unit time) or temperature — is not possible here, because they are different physical quantities. A kilowatt is not a kilowatt-hour.
Common questions
Is a food Calorie the same as a calorie?
No — the "Calorie" on a nutrition label is a kilocalorie, meaning 1,000 of the small scientific calories. A 250 Calorie snack contains 250 kcal, which is 250,000 cal, or about 1.05 megajoules. Some countries label food in kilojoules instead, which avoids the ambiguity entirely.
What is the difference between a kilowatt and a kilowatt-hour?
A kilowatt is a rate — how fast energy is being used. A kilowatt-hour is an amount — the energy used by running one kilowatt for one hour. Your electricity bill charges for kilowatt-hours because it is billing the total energy consumed, not the rate at which you drew it.
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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