Joules, calories and watt-hours explained
Energy is the capacity to do work, and the SI unit is the joule (J). One kilojoule (kJ) is 1,000 joules. Other units come from specific fields. A calorie is a unit of heat, and a kilocalorie (kcal) is 1,000 calories. A watt-hour (Wh) is the energy used by a one watt device running for an hour, which is 3,600 joules.
The word "calorie" causes confusion. Food labels that say "Calories" with a capital C normally mean kilocalories. In the strict physics sense, a calorie is the small unit, and a kilocalorie is a thousand of them.
This converter uses these values in joules: 1 calorie is 4.184 J, 1 kilocalorie is 4184 J, 1 kilojoule is 1000 J and 1 watt-hour is 3600 J. It supports five units: Joules, Kilojoules, Calories, Kilocalories and Watt-hours.
Why you might need an energy conversion
Nutrition is the most common reason. Some countries print energy on food packaging in kilojoules, others in kilocalories, and many show both. Converting lets you compare products from different regions or track intake against a target set in another unit.
Electricity is another area. Small batteries and devices are often rated in watt-hours, while heat and physics problems use joules. If you want to compare a battery's capacity to the energy in a snack, you need a shared unit.
Students also use it for physics and chemistry homework, where results are given in joules or kilojoules but reference data lists calories.
How to convert energy with this tool
- Type a value into the number box under From. It starts at 1.
- Choose your starting unit from the From drop-down: Joules, Kilojoules, Calories, Kilocalories or Watt-hours.
- Choose the unit you want from the To drop-down.
- Read the result in the To box. It updates as you type or switch units.
It opens on Joules to Kilojoules, showing 0.001. Results are rounded to six decimal places, and your numbers are processed in your browser only.
Worked examples and things to watch
Some checks: 1 kilocalorie equals 4.184 kilojoules, 250 kilocalories equals 1046 kilojoules, and 100 watt-hours equals 360 kilojoules. Going the other way, 1 watt-hour equals about 0.860421 kilocalories.
The most common mistake is mixing up Calories and Kilocalories in this tool. If you are converting a food label figure, choose Kilocalories, not Calories, otherwise the answer will be 1,000 times too small. The Calories option here is the small calorie.
Because the calorie value used is the 4.184 joule definition, results will match standard nutrition conversions. Very small results can round to 0 at six decimals, so convert to a smaller unit if you need a visible number. For body measurements that often accompany nutrition tracking, the BMI Calculator may be useful as well.