Energy Converter
Convert joules, calories, kilocalories, watt-hours and BTU.
Convert between units of energy used in nutrition, electricity and physics. Enter a value and choose the units — conversions run through the joule, the SI unit of energy.
Formula / method
Examples
4.184 kJ
3,600 kJ
Joules, calories and food Calories
The joule (J) is the SI unit of energy. The calorie is defined as the heat needed to warm one gram of water by one degree Celsius; the common value is 1 cal = 4.184 J. The big catch is nutrition labels: a food 'Calorie' with a capital C is really a kilocalorie, 1000 small calories, so 1 kcal = 4184 J. A 250 kcal snack therefore holds about 1,046,000 joules of chemical energy.
Watt-hours and BTU
A watt-hour is energy, not power: since one watt is one joule per second, running one watt for 3600 seconds gives 1 Wh = 3600 J. Your electricity bill uses the kilowatt-hour (1 kWh = 3,600,000 J), which pairs naturally with our Electricity Cost Calculator. The British thermal unit (BTU), common on heaters and air conditioners, is about 1055.06 J. Keeping these straight matters because a number that looks small in kWh can be enormous in joules.
Where it's used
- Converting food-label kilojoules to kilocalories, common outside the US
- Comparing an appliance's electricity use in kWh against joules or BTU
- Reading a heater or air-conditioner rating in BTU
- Physics and chemistry problems that need answers in joules
- Interpreting a battery capacity given in watt-hours
Real-world examples
- A 250 kcal snack holds about 1046 kJ of chemical energy
- 1 kWh, the unit on your electricity bill, equals 3.6 million joules or 3600 kJ
- 1000 BTU of heat is about 1055 kJ
A bit of history
The joule is named after James Prescott Joule, a 19th-century English physicist (and brewer) whose experiments established the mechanical equivalent of heat, a cornerstone of the first law of thermodynamics.
Did you know?
The 'Calorie' with a capital C on nutrition labels is really a kilocalorie, 1000 of the small calories used in physics, so a 100-Calorie snack actually contains 100,000 calories.
FAQ
Is a food Calorie the same as a kcal?
Yes — the "Calorie" on nutrition labels (capital C) is a kilocalorie (kcal), equal to 1,000 small calories.