Energy Converter (J, kJ, kWh, calories, BTU, ...)
Convert energy between joules, calories, watt-hours, kilowatt-hours, and BTU. Useful for reading nutrition labels, understanding electricity bills, and comparing heating and cooling specifications across regions. Everything runs in your browser.
How the conversion works
Every unit on this page is defined by how many joules it represents. The joule is the SI unit of energy, defined as the work done by one newton of force acting over one meter. The input is converted to joules, then from joules into the target unit:
result = value × (joules per source unit) ÷ (joules per target unit)
Energy is not power
These two are constantly confused, and the difference matters for understanding electricity bills.
Power is a rate: how fast energy is used. It is measured in watts. Energy is the total amount consumed over time, measured in joules or kilowatt-hours.
A 2000 W heater running for 3 hours uses 2000 × 3 = 6000 Wh = 6 kWh of energy. The wattage tells you how hard it draws at any moment, the kilowatt-hours tell you what appears on your bill. This is also why "kWh" is a unit of energy despite containing "watt": it is power multiplied by time.
For converting the rate itself, see the power converter.
The calorie problem
There are two units called "calorie" that differ by a factor of 1000.
The small calorie (cal) is the energy needed to raise one gram of water by one degree Celsius, and equals 4.184 J.
The large Calorie (kcal, sometimes written with a capital C) is 1000 small calories, or 4184 J. This is what food labels mean. A 250-Calorie snack contains 250 kcal, which is 250,000 small calories or 1046 kJ.
European nutrition labels sidestep the ambiguity by printing kilojoules alongside kilocalories. When converting a food figure, assume kcal unless the source is explicitly a physics or chemistry context.
Quick reference table
| From | Joules | Kilojoules | kcal | kWh |
|---|---|---|---|---|
| 1 J | 1 | 0.001 | 0.000239 | 0.00000028 |
| 1 kJ | 1000 | 1 | 0.239006 | 0.000278 |
| 1 MJ | 1,000,000 | 1000 | 239.006 | 0.277778 |
| 1 cal | 4.184 | 0.004184 | 0.001 | 0.00000116 |
| 1 kcal | 4184 | 4.184 | 1 | 0.001162 |
| 1 Wh | 3600 | 3.6 | 0.860421 | 0.001 |
| 1 kWh | 3,600,000 | 3600 | 860.421 | 1 |
| 1 BTU | 1055.06 | 1.05506 | 0.252164 | 0.000293 |
Practical examples
Reading nutrition labels
European labels give energy per 100 g in both kJ and kcal. To convert kcal to kJ, multiply by 4.184. A 2000 kcal daily reference is 8368 kJ. If a label shows only kilojoules, divide by 4.184 to get the more familiar kilocalorie figure.
Electricity bills
Household electricity is billed in kilowatt-hours. To estimate an appliance's consumption, multiply its power rating in kilowatts by the hours of use. A 150 W refrigerator running continuously uses 0.15 × 24 = 3.6 kWh per day, roughly 1314 kWh per year. Multiply by your rate per kWh for the annual cost.
Heating and air conditioning
Heating and cooling capacity is quoted in BTU (or BTU per hour) in the US and in kilowatts elsewhere. A 12,000 BTU/h air conditioner is a 3.5 kW unit. Since 1 kWh = 3412 BTU, dividing a BTU/h rating by 3412 converts it to kilowatts.
Comparing energy sources
Different fuels are sold in different units, which makes cost comparison awkward. Converting everything to kWh gives a common basis: natural gas is often billed per cubic meter (roughly 10 kWh per m³ depending on composition), heating oil per liter (roughly 10 kWh per liter), and electricity directly in kWh. Only after converting to a shared unit can prices per unit of usable energy be compared meaningfully.
Frequently asked questions
Is a food Calorie the same as a physics calorie?
No. A food-label Calorie is a kilocalorie, meaning 1000 small calories. A 500-Calorie meal contains 500 kcal or 2092 kJ.
How many joules is 1 kWh?
Exactly 3,600,000 J (3.6 MJ). This follows from 1 kW = 1000 W and one hour being 3600 seconds.
How many BTU is a kilowatt-hour?
3412.14 BTU. Conversely, 1 BTU is about 0.000293 kWh.
What is the difference between kW and kWh?
kW is power, the rate of energy use at a given moment. kWh is energy, the total consumed over time. A 2 kW appliance running for 30 minutes uses 1 kWh.
How do I convert kilojoules to kilocalories?
Divide by 4.184. For 1000 kJ: 1000 ÷ 4.184 = 239 kcal.
Why do European food labels show both kJ and kcal?
EU regulations require energy to be declared in kilojoules, the SI unit, while kilocalories remain more familiar to consumers. Showing both satisfies the regulation and stays readable.
Related converters
- Power converter, for the rate at which energy is used
- Temperature converter, for the thermal side of energy calculations
- All unit converters