Energy Converter
Convert energy between joules, kilojoules, calories, kilocalories, watt-hours, kilowatt-hours and BTU. One tool for food labels, electricity bills and physics.
How to use this calculator
- Enter the energy value.
- Choose the from and to units.
- Press Convert to see all equivalents at once.
Formula used
Food "Calories" (capital C) are actually kilocalories — 1 Cal = 1 kcal = 4,184 J. The kilowatt-hour on your electricity bill is 3.6 million joules; the BTU heats air conditioners and furnaces in the US.
Example calculation
A 500 kcal meal: 500 × 4.184 = 2,092 kJ — the value printed on labels in metric countries. It also equals 0.58 kWh: enough electricity to run a laptop for ~10 hours.
One energy, many units
Energy is energy — food, electricity, heat and motion all measure in the same physical quantity, just with different traditional units. Food labels use kcal or kJ, utility bills use kWh, US heating and cooling use BTU, and science uses joules.
Perspective anchors: a 2,000 kcal daily diet is 8.4 MJ or 2.3 kWh — remarkably, a human runs on about the energy of one cheap kWh of electricity per day.
Why use this calculator?
- Convert between kcal and kJ on food labels from different countries.
- Relate appliance kWh usage to real energy amounts.
- Translate BTU ratings of ACs and heaters into watts and kilojoules.
Frequently asked questions
How many kJ is a calorie?
1 kilocalorie (food Calorie) = 4.184 kJ. Food labels showing both use this factor — 250 kcal ≈ 1,046 kJ.
What exactly is a kWh?
The energy of using 1,000 watts for one hour = 3.6 million joules. A 100 W device running 10 hours uses exactly 1 kWh.
What is a BTU?
British Thermal Unit — the heat to warm one pound of water by 1 °F, about 1,055 J. Air conditioners are rated in BTU/hour: 12,000 BTU/h ≈ 3.5 kW of cooling.