Joules to Calories Converter

How to convert joules to calories

The joule is the SI unit of energy: the work done when a force of one newton moves an object one meter. The calorie began as the heat needed to warm one gram of water by one degree Celsius, but because that amount varies slightly with the water's starting temperature, it was redefined in terms of the joule. The thermochemical calorie is exactly 4.184 J, so dividing by 4.184 is an exact conversion, not an approximation. The same joule-based rule links every unit on the energy converter, from kilojoules to kilowatt-hours and BTU.

The trap is the word itself. This page converts to small calories (cal). The Calorie on a food label is a kilocalorie, 1,000 times larger, so 1,000,000 J is 239,006 cal but only 239 food Calories. If you are working with nutrition figures, the kJ to kcal converter is the one you want. Steam tables and some engineering data use the International Table calorie of 4.1868 J instead; choose it with the Calorie switch above.

Step by step

1. Write down the energy in joules.
2. Divide it by 4.184 (or multiply by 0.239006).
3. Divide by a further 1,000 if you need kilocalories (food Calories).
4. Round to the precision of your measurement.

See it on a scale

How are joules and calories used in calorimetry?

Calorimetry finds heat from q = m × c × ΔT, and the calorie keeps that tidy because liquid water's specific heat is about 1 cal/g·°C, or 4.184 J/g·°C. That near-perfect 1 is why the unit survives in chemistry classrooms.

Example: warming 200 g of water by 15 °C takes 200 × 1 × 15 = 3,000 cal. In joules that is 3,000 × 4.184 = 12,552 J, or 12.552 kJ. Working in joules from the start with c = 4.184 J/g·°C gives the same result, which is a good way to check your answer.

Temperature changes are the same size in Celsius and kelvin, so ΔT of 15 °C equals ΔT of 15 K. If your data gives absolute temperatures, the Celsius to Kelvin converter helps, but for the difference you can skip it.

Bomb calorimeters and food

Food energy was originally measured by burning samples in a bomb calorimeter and reading the temperature rise of a water jacket. The results are reported in kilocalories or kilojoules, which is where the food Calorie comes from.

Common mistakes with joules and calories

• Treating food Calories as calories. A 250 Calorie snack holds 250,000 cal, or 1,046,000 J. Mixing the two is a 1,000-fold error.
• Using the wrong calorie. Steam tables often use the International Table calorie (4.1868 J). Converting 10,000 J with it gives 2,388.46 cal instead of 2,390.06 cal.
• Mixing energy and power. Joules are energy; watts are joules per second. A 60 W bulb does not contain 60 J, it uses 60 J every second.
• Multiplying instead of dividing. A calorie is bigger than a joule, so the number of calories is always smaller than the number of joules.

For large amounts of energy in billing units, the kWh to BTU converter covers heating and electricity. The full set of units lives on the energy converter.