Softonic

Energy Converter

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Food labels use kilocalories, heating systems use BTUs, electric bills use kilowatt-hours, physics textbooks use joules and particle physicists use electronvolts. Same quantity, different units for different industries. This converter goes both ways across every common energy unit with exact factors (no drift from rounding).

How to convert energy units

  1. 1

    Enter a value

    Type the energy in its current unit; decimals and scientific notation accepted.

  2. 2

    Pick source unit

    Joule, kilojoule, megajoule, calorie, kilocalorie (food calorie), watt-hour, kilowatt-hour, megawatt-hour, BTU, erg, foot-pound and electronvolt.

  3. 3

    Pick target unit

    The converted value updates instantly as you type.

  4. 4

    Swap or start over

    Swap the source and target units to convert the other way around, or start over with a new value.

Exact conversion factors to joule

The joule (J) is the SI unit of energy. Every other unit is defined relative to it.

Unit Joules
1 J 1
1 kJ 1,000
1 MJ 1,000,000
1 cal (thermochemical) 4.184
1 kcal (food calorie) 4,184
1 Wh 3,600
1 kWh 3,600,000
1 MWh 3,600,000,000
1 BTU (IT) 1,055.06
1 erg 10^-7
1 foot-pound 1.355818
1 eV 1.602176634 * 10^-19

Scale landmarks

  • 1 J: lifting an apple 1 m.
  • 1 kcal: the metabolic value of a peanut.
  • 1 kWh: running a 1 kW space heater for one hour.
  • 1 BTU: energy to raise 1 lb of water by 1 degree F (about 1 kJ).
  • 1 kiloton of TNT: 4.184 TJ; a small tactical nuclear yield.
  • 1 MeV: characteristic of nuclear processes.

Calorie confusion

  • calorie (small c, lowercase, “gram calorie”) = 4.184 J.
  • Calorie or kilocalorie (large C, food calorie) = 4,184 J = 1 kcal.

Food packaging outside the US often writes “kcal” explicitly to avoid confusion. In the US, “Calories” on a label means kilocalories. A “200 calorie” snack is actually 200 kilocalories, or 836,800 joules.

Engineering conversions

  • Heating: BTU/hour is power, not energy. A 10,000 BTU/h heater delivers 10,000 BTU per hour (~2.93 kWh/hour).
  • Food vs exercise: 100 kcal = 418 kJ. Running at moderate pace burns about 60 kcal per km for a 70 kg person, so a 7 km run is roughly 420 kcal.
  • Gasoline: 1 gallon = 33.7 kWh (the EPA equivalent for EV comparison).
  • Natural gas: 1 therm = 100,000 BTU = 29.3 kWh.

Why keep around so many units

Different industries fossilized their unit choices before SI unified things:

  • Electricity uses watt-hours because bills add up from meters reading instantaneous watts.
  • Heating and HVAC in the US use BTUs because the industry predates metric and retrofit costs prevent conversion.
  • Chemistry and nutrition use calories because 1 cal raises 1 g of water by 1 C; convenient for thermodynamic reasoning.
  • Particle physics uses electronvolts because joules are far too large for single-particle work.

Exact vs approximate

Some conversions are exact by definition (1 J = 10^7 erg, 1 kWh = 3.6 MJ). Others are defined values (1 cal = 4.184 J exactly by IUPAC thermochemical convention). A few are measured constants (1 eV tied to the elementary charge, exact since the 2019 SI redefinition). The converter uses defined values where available.

Frequently Asked Questions

There are several “BTU” definitions: BTU IT (1,055.06 J), BTU 59 F (1,054.80 J), BTU mean (1,055.87 J). The difference is small; the tool uses BTU IT, the most common international convention.

One food calorie (kcal) is the energy to raise 1 kg of water by 1 degree Celsius at standard pressure, equal to 4.184 kJ exactly. Food labels in the US use “calorie” to mean kilocalorie; the rest of the world uses “kcal” or “kJ”.

The tool uses defined values where they exist (joule, kilowatt-hour, thermochemical calorie) and high-precision measured values elsewhere. The result is computed with up to 12 decimal places, and trailing zeros are trimmed.

No. Your values are used only to perform the conversion and are not stored. In step-by-step mode the value and units travel in the page link so the next step can recalculate your result.

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