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Energy Converter

Convert energy between joules, calories, kcal, kWh, BTU, foot-pounds and electronvolts with exact definitions and the formula for each pair.

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Formula:

≈ means the result was rounded to the chosen number of significant figures.

cal and kcal are thermochemical calories (1 cal = 4.184 J), the ones used on food labels; the International Table calorie (4.1868 J) is listed separately. BTU is the International Table BTU (1,055.05585262 J).

{value} in every unit

UnitResult

Quick conversion table: {from} to {to}

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Common values

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Unit definitions

Unit

How to use the energy converter

  1. Type an amount of energy in Value. A decimal point or a decimal comma both work.
  2. Pick the starting unit in From and the target unit in To, for example kilocalorie (food Calorie) and kilojoule.
  3. The result updates as you type, with the complete line below it (“1 kcal = 4.184 kJ”). Copy copies that line.
  4. Swap units (⇄) reverses the direction.
  5. Significant figures controls the rounding; “≈” means the value shown is rounded. Very large or very small results, such as joules in electronvolts, are written in scientific notation.

The page also shows your value in every unit, a quick table for the selected pair, everyday references (a food Calorie, a smartphone battery, a litre of petrol) and each unit expressed in joules.

Definitions and sources

  • Joule (J): the SI unit of energy, 1 newton over 1 metre, or 1 watt for 1 second.
  • Calorie (cal): this tool uses the thermochemical calorie, exactly 4.184 J. It is the value used in nutrition and chemistry. The older International Table calorie (cal_IT), fixed at the 1956 International Conference on the Properties of Steam, is exactly 4.1868 J; it is listed separately because the 0.07 % difference matters in engineering tables.
  • Kilocalorie (kcal): 1,000 cal = 4,184 J. A dietary “Calorie” with a capital C is one kilocalorie.
  • Watt-hour and kilowatt-hour: 1 Wh = 3,600 J and 1 kWh = 3.6 MJ exactly, the unit on your electricity bill.
  • British thermal unit (BTU): the International Table BTU, exactly 1,055.05585262 J. It derives from the IT calorie, the pound and the Fahrenheit degree.
  • Foot-pound (ft·lbf): the work of one pound-force over one foot, 1.3558179483 J.
  • Electronvolt (eV): the energy an electron gains across 1 volt. Since the 2019 redefinition of the SI the elementary charge is fixed, so 1 eV = 1.602176634 × 10⁻¹⁹ J exactly.

Handy equivalents

  • 1 kWh ≈ 860.42 kcal ≈ 3,412.14 BTU
  • 1 kcal = 4.184 kJ ≈ 3.966 BTU
  • 1 BTU ≈ 1.055 kJ ≈ 0.252 kcal
  • 1 MJ ≈ 0.27778 kWh

Energy or power?

Energy is an amount; power is a rate (energy per unit of time). A 2 kW kettle running for 3 minutes uses 0.1 kWh, which is 360 kJ or about 86 kcal. To convert watts, kilowatts or horsepower, use a power converter instead: a kilowatt and a kilowatt-hour are different quantities.

Frequently asked questions

How many kilojoules are in a Calorie?

A food Calorie is one kilocalorie, so 4.184 kJ. A 2,000 kcal daily intake is 8,368 kJ, which is why labels in Europe and Australia show both figures.

Why are there two different calories?

Historically, the calorie was the heat needed to warm 1 g of water by 1 °C, which depends on the starting temperature. Chemists fixed it at 4.184 J (thermochemical), steam engineers at 4.1868 J (International Table). This tool uses the thermochemical calorie for cal and kcal and offers the IT calorie as its own unit.

How do I convert kWh to BTU?

Multiply by about 3,412.14. Heating and air-conditioning equipment is often rated in BTU, or in BTU per hour for power.

What is an electronvolt used for?

For energies at the scale of atoms and particles. Ionising a hydrogen atom takes 13.6 eV, a visible photon carries about 2 to 3 eV, and particle accelerators work in MeV, GeV and TeV.

Can I convert a negative energy?

Yes. The factors are linear, so a negative value, such as an energy released or lost, converts like any other number.

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