Toolcroft

Energy Converter

Unit Converters

Convert energy between Joules, kJ, MJ, calories, kcal, Wh, kWh, BTU, foot-pounds, and electronvolts in this free browser-based unit converter.

Equivalent values in all energy units

Based on the current From value in Kilocalorie (kcal).

Highlighted row is the current To unit.

SI

  • Joule (J)4184
  • Kilojoule (kJ)To4.184
  • Megajoule (MJ)0.004184

Thermal

  • Calorie (cal)1000
  • Kilocalorie (kcal)1
  • British Thermal Unit (BTU)3.96567

Electric

  • Watt-hour (Wh)1.16222
  • Kilowatt-hour (kWh)0.00116222

Other

  • Foot-pound (ft·lb)3085.96
  • Electronvolt (eV)2.61145e+22

How to use this tool

  1. Enter a value. Type the energy value you want to convert in the "From" field.
  2. Select units. Choose source and target energy units from the dropdown menus.
  3. Read the result. The converted energy appears instantly. Use the swap button to reverse the conversion.

Common energy conversions

ValueEquivalent
1 kcal4,184 J = 4.184 kJ
1 MJ1,000,000 J = 1,000 kJ = 0.278 kWh
1 kWh3,600,000 J = 3.6 MJ = 3,412.14 BTU
1 BTU1,055.06 J = 0.000293 kWh
1 kJ0.239 kcal = 0.278 Wh
1 Wh3,600 J = 3.412 BTU
1 ft·lb1.35582 J
1 eV1.602 × 10⁻¹⁹ J

How energy conversion works

All conversions pass through Joules (J) as the base SI unit. The input is multiplied by the source unit's Joule factor, then divided by the target unit's factor. Conversion factors use exact or NIST-defined values: 1 cal = 4.184 J (thermochemical calorie), 1 BTU = 1,055.05585 J (IT BTU), 1 eV = 1.602176634 × 10⁻¹⁹ J (exact by SI definition).

Food calories vs scientific calories

Nutritional labels use "Calories" (capital C), which are actually kilocalories (kcal): 1 food Calorie = 1,000 small calories = 4,184 J. Use the kcal option above for food energy conversion.

Human energy consumption

Everyday objects help make abstract energy units tangible. The human body at rest consumes roughly 80 W (watts) of power - about the same as an incandescent light bulb. Burning that bulb for one hour uses 1 Wh = 3,600 J = 860 cal = 0.86 kcal. A typical adult's daily energy budget of ~2,000 kcal equals roughly 8.4 MJ - enough to keep a 97 W appliance running continuously for 24 hours.

Power vs. energy

Energy is the total capacity to do work, measured in joules (J) or watt-hours (Wh). Power is the rate at which energy is used or produced, measured in watts (W). The relationship is: Energy = Power × Time. A constant 1,000 W load running for 2 minutes draws 1,000 W × 120 s = 120,000 J = 120 kJ ≈ 33.33 Wh ≈ 0.0333 kWh of energy. Understanding this distinction is essential when reading utility bills (kilowatt-hours) vs. appliance ratings (kilowatts).

Energy scale reference

EventApproximate energy
One photon of visible light~2–3 eV (≈ 3.2–4.8 × 10⁻¹⁹ J)
ATP hydrolysis (cellular energy unit)~30.5 kJ/mol
One large dietary Calorie (1 kcal)4,184 J
One Big Mac~2.3 MJ (~550 kcal)
Human daily energy metabolism~8–10 MJ (~2,000–2,400 kcal)
1 kWh of electricity3.6 MJ (~860 kcal)
Lightning bolt~1–5 GJ
Nuclear fission bomb (20 kt)~84 TJ

Frequently asked questions

How many joules are in a kilocalorie?

1 kilocalorie (kcal), also called a food calorie or "large calorie," equals 4,184 joules. The calorie you see on food labels is actually a kilocalorie. 1 small calorie (cal) = 4.184 joules.

How do I convert kWh to BTU?

1 kilowatt-hour (kWh) = 3,412.14 BTU. To convert kWh to BTU, multiply by 3,412.14. For example, 5 kWh = 17,060.7 BTU.

What is a joule?

A joule (J) is the SI unit of energy, equal to the work done by a force of one Newton over one meter. It equals one watt-second (W·s) or approximately 0.239 small calories.

What is the difference between a calorie and a Calorie?

A small calorie (cal) is the energy to raise 1 gram of water by 1 °C = 4.184 J. A food Calorie (Cal or kcal) is 1,000 small calories = 4,184 J. Nutrition labels always show kilocalories.

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