Energy Conversion Calculators

Enter an energy value and choose the units to convert between joules and its kilojoule, watt-hour, calorie, and BTU equivalents.

How to Use the Energy Conversion Calculators

Enter an energy value, choose its current unit, and choose the unit to convert to — joules, kilojoules, watt-hours, kilowatt-hours, calories, kilocalories, or BTU. The converted number appears as soon as you finish typing, and Swap Units flips the two selections.

Unlike the purely electrical converters elsewhere in this category, energy units here cross into everyday, non-electrical territory — food labels, utility bills, and HVAC ratings all quote energy in units with completely different origins, and converting between them isn't always the clean factor-of-ten arithmetic that SI prefixes give you.

base value (J) = entered value × from-unit factor
result = base value ÷ to-unit factor

1 Wh   = 3,600 J        (exact, by definition)
1 kWh  = 3,600,000 J    (exact, by definition)
1 cal  = 4.184 J        (thermochemical calorie)
1 kcal = 4,184 J
1 BTU  = 1,055.06 J     (International Table BTU)

The watt-hour conversions are exact — a watt is a joule per second, so an hour of it is defined as precisely 3,600 joules, no approximation involved. Calories and BTU are a different story: both have several competing historical definitions that differ by a fraction of a percent, and this calculator uses the thermochemical calorie (4.184 J) and the International Table BTU (1,055.06 J) specifically, since those are the versions most commonly cited today. A source using the 15°C calorie or the mean BTU instead would produce a very slightly different number.

One thing worth flagging since it trips people up constantly: the "Calories" printed on a nutrition label are actually kilocalories, not calories — food energy has used that shorthand so long that the capital-C convention is easy to miss, and it's a thousandfold difference if you get it wrong.