Type a value into any field and instantly see it converted across joules, kilojoules, calories, kilocalories, watt-hours, kilowatt-hours and BTU.
Runs entirely in your browser — nothing you enter is uploaded or stored.
Joules, kJ, calories, kcal, Wh, kWh and BTU all update together.
Uses the defined thermochemical calorie (4.184 J) and International Table BTU.
No accounts, no uploads — every calculation stays on your device.
Enter an energy amount in whichever unit you already have, and the rest fill in automatically.
Energy shows up in very different-looking units depending on the field: physics and engineering use joules and kilojoules, nutrition uses calories (really kilocalories), household electricity uses watt-hours and kilowatt-hours, and heating and cooling equipment in the US is rated in BTU. They all measure the same physical quantity, just at different scales and historical conventions.
This converter routes every value through joules as the SI base unit, using exact definitions: 1 calorie (thermochemical) = 4.184 J, 1 kilocalorie = 4,184 J, 1 watt-hour = 3,600 J, 1 kilowatt-hour = 3,600,000 J, and 1 BTU (International Table) = 1,055.05585262 J.
A nutrition label's "Calories" (capital C) are actually kilocalories — 1,000 times a small calorie.
A kilowatt-hour (3.6 million joules) is a convenient everyday scale for household energy bills.
Air conditioners and furnaces in the US are rated in BTU or BTU per hour; 1 BTU equals about 1,055 joules.
Every other unit here is defined as an exact multiple of the joule, so conversions carry no compounding rounding error.
Common questions about converting between joules, calories, kWh and BTU.
One (small, thermochemical) calorie equals exactly 4.184 joules. A food Calorie, written with a capital C, is actually one kilocalorie, or 4,184 joules.
A kilocalorie (kcal) is 1,000 small calories, or 4,184 joules. Nutrition labels list Calories that are really kilocalories, which is a common source of confusion.
One kilowatt-hour equals exactly 3,600,000 joules (3.6 megajoules), since a watt-hour is a watt sustained for one hour and 1 W × 3600 s = 3600 J.
One British Thermal Unit (International Table definition) equals exactly 1,055.05585262 joules.
A joule is a very small amount of energy, so household electricity use in joules would be an unwieldy number. A kilowatt-hour (3.6 million joules) matches the scale of typical daily electricity consumption.