Energy Converter
Convert energy between joules, kilojoules, calories, kilocalories, watt-hours, kilowatt-hours and BTU — for food, electricity and physics.
How energy conversion works
All energy units measure the same physical quantity, so each has an exact definition in joules — the SI unit
(one newton-metre, or one watt-second):
- 1 calorie (thermochemical) = 4.184 J exactly · 1 kcal (food Calorie) = 4,184 J
- 1 Wh = 3,600 J · 1 kWh = 3.6 MJ (the electricity-bill unit)
- 1 BTU = 1,055.06 J (heating and air-conditioning)
The units cluster by field: nutrition labels use kcal (or kJ in Europe and Australia), utilities bill in kWh,
HVAC equipment is rated in BTU, and physics works in joules. Converting between them connects everyday
quantities — a 500 kcal meal is 0.58 kWh, roughly what a laptop uses in a working day.
Handy approximations
- kcal → kJ: multiply by 4.2 (100 kcal ≈ 420 kJ; exact 418.4)
- kJ → kcal: divide by 4.2 (2,000 kJ ≈ 478 kcal)
- kWh → kcal: multiply by 860
- BTU → kJ: multiply by 1.055 (10,000 BTU ≈ 10,551 kJ)
Worked example
A snack of 100 kcal: 100 × 4,184 ÷ 1,000 = 418.4 kJ. The same energy is
116.2 Wh (0.116 kWh) or 396.6 BTU — enough to run a 60 W bulb for just under two hours.
Frequently asked questions
How many kJ is 100 kcal?
100 kcal = 418.4 kJ, since 1 kcal = 4.184 kJ exactly. To go from kJ on a food label back to calories, divide by 4.184 (or by 4.2 for a quick estimate).
Are food "calories" the same as calories?
A food Calorie (capital C) is actually a kilocalorie — 1,000 small calories. When a label says 250 calories it means 250 kcal = 1,046 kJ. This converter’s kcal unit is the food calorie.
How many joules are in a kWh?
1 kWh = 3,600,000 J (3.6 MJ) exactly — one kilowatt sustained for 3,600 seconds. It is the unit on your electricity bill: a 2 kW heater running for 30 minutes uses exactly 1 kWh.
How many calories is 1 kWh?
1 kWh = 860.4 kcal (3,600,000 ÷ 4,184). So one unit of electricity carries roughly the energy of a third of an average adult’s daily food intake.
What is a BTU?
The British Thermal Unit is the energy to heat one pound of water by 1 °F: 1 BTU = 1,055.06 J = 0.2931 Wh. Air conditioners and boilers are rated in BTU/h — a 12,000 BTU/h AC unit moves about 3.5 kW of heat.