Calories to Joules

The calorie was defined in the 19th century as the heat needed to raise one gram of water by one degree Celsius. Because that amount changes slightly with the water's starting temperature, several versions grew up: the 15 °C calorie of about 4.1855 J, the international table calorie of 4.1868 J used in steam engineering, and the thermochemical calorie of exactly 4.184 J. Chemistry and nutrition settled on the last one, and so does this converter.

The joule is the SI unit of energy: the work done by a force of one newton over one meter, or one watt for one second. Lifting a 102 g apple by one meter takes about 1 J, so a single calorie is roughly the energy to lift that apple four meters.

In the lab, heat is often calculated as q = m × c × ΔT. With water's specific heat of 1 cal/(g·°C), or 4.184 J/(g·°C), warming 250 g of water from 20 °C to 100 °C needs 250 × 80 = 20,000 cal, which is 83,680 J or 83.7 kJ. Doing the arithmetic in calories first and converting at the end avoids rounding on each step.

Older textbooks and some reference tables still give heats of reaction and combustion in kcal/mol. Multiply by 4.184 to get kJ/mol, the unit modern data uses: the 57.8 kcal/mol for forming water vapor from hydrogen and oxygen becomes 241.8 kJ/mol.

A food Calorie is 1,000 of these calories. A 100-Calorie snack therefore holds 418,400 J, not 418.4. If your result looks a thousand times too small for something edible, you started with kilocalories; use kcal to kJ instead. Electrical energy usually comes in kilowatt-hours, which kWh to joules converts.