Energy converter — joules, calories, kWh and BTU

Food labels in kilocalories, electricity bills in kilowatt-hours, heating gear in BTU, physics in joules. Ten units that all measure the same quantity, with the exact factors printed below.

How to use it

1
Type into any row — say kJ or kcal. The results appear immediately, no button needed.
2
Need a combined figure? Fill in two rows, for example kJ and kWh, and they are added together before converting.
3
Set how many digits you want (4–12) and click a result to copy the plain number.

Exact conversion factors

These are the definitions the calculator uses. Nothing here is rounded for convenience — where a value is exact by definition, it says so.

1 kcal (food Calorie) = 4184 J = 4.184 kJ
1 kWh = 3 600 000 J = 3.6 MJ = 860.420650 kcal
1 Wh = 3600 J · 1 BTU = 1055.05585262 J
1 ft·lbf = 1.35581794833 J · 1 eV = 1.602176634 × 10⁻¹⁹ J

FormulaJ = kcal × 4184 · J = kWh × 3 600 000 · J = BTU × 1055.05585262 · kWh = MJ ÷ 3.6

Energy conversion table — every unit

One of each unit, converted into the three units people ask for most.

Unit	kJ	kcal	kWh
1 MJ	1 000 kJ	239.006 kcal	0.277778 kWh
1 kJ	1 kJ	0.239006 kcal	0.000277778 kWh
1 J	0.001 kJ	0.000239006 kcal	0.000000277778 kWh
1 kcal	4.184 kJ	1 kcal	0.00116222 kWh
1 cal	0.004184 kJ	0.001 kcal	0.00000116222 kWh
1 Wh	3.6 kJ	0.860421 kcal	0.001 kWh
1 kWh	3 600 kJ	860.421 kcal	1 kWh
1 BTU	1.05506 kJ	0.252164 kcal	0.000293071 kWh
1 ft·lbf	0.00135582 kJ	0.000324048 kcal	0.000000376616 kWh
1 eV	1.60218×10^-22 kJ	3.82929×10^-23 kcal	4.45049×10^-26 kWh

kJ → kcal, kWh — quick values

kJ	kcal	kWh
0.5 kJ	0.119503 kcal	0.000138889 kWh
1 kJ	0.239006 kcal	0.000277778 kWh
2 kJ	0.478011 kcal	0.000555556 kWh
3 kJ	0.717017 kcal	0.000833333 kWh
5 kJ	1.19503 kcal	0.00138889 kWh
10 kJ	2.39006 kcal	0.00277778 kWh
20 kJ	4.78011 kcal	0.00555556 kWh
25 kJ	5.97514 kcal	0.00694444 kWh
50 kJ	11.9503 kcal	0.0138889 kWh
100 kJ	23.9006 kcal	0.0277778 kWh
250 kJ	59.7514 kcal	0.0694444 kWh
1 000 kJ	239.006 kcal	0.277778 kWh

How this converter works

The food calorie is the classic source of confusion. What a label calls a Calorie is a kilocalorie: 4184 J. The small calorie used in chemistry is a thousand times smaller. Both rows are here, labelled plainly, so nothing is guessed.

On the electricity side, one kilowatt-hour is 3.6 MJ. That makes bill arithmetic easy: a 2 kW heater running three hours uses 6 kWh, or 21.6 MJ, and at typical tariffs you can price it straight from the kWh row.

Notes and abbreviations

1 kcal = 4184 J — the "Calorie" printed on food packaging
1 kWh = 3.6 MJ = 860.42 kcal
1 BTU = 1055.06 J — air conditioning and boiler ratings
eV — electronvolt, the unit of choice in particle and semiconductor physics

Questions

How many joules is one food calorie?

One food Calorie is one kilocalorie, or 4184 J. A 500 kcal meal is 2.092 MJ.

How much energy is a kWh?

3 600 000 J. Enough to run a 100 W bulb for ten hours.

Can I convert BTU to kW for an air conditioner?

BTU ratings on air conditioners are per hour, so use the power converter: 12 000 BTU/h is 3.517 kW.