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Calories to Joules Converter

Use this free calories to joules converter to change energy between calories, kilocalories, joules, kilojoules, BTU, kilowatt-hours, and foot-pounds. Enter an amount of energy, pick any two units, and the result appears instantly — along with the same energy in every other unit. It works both ways, so you can convert joules back to calories just as easily.

Energy units at a glance

How it works

Energy is measured in different units depending on where you meet it. Food labels use calories, science classes use joules, heating and cooling equipment uses BTU, electricity bills use kilowatt-hours, and mechanics use foot-pounds. They all measure the same thing — energy — so any one of them can be converted into any other with a fixed number.

The calorie is defined by heat: one calorie is the energy needed to raise one gram of water by one degree Celsius. The joule is the standard scientific unit of energy: one joule is the energy of pushing with a force of one newton over one meter. The link between them is fixed — one calorie equals 4.184 joules — and every other conversion on this page is built on that link. A kilocalorie is simply 1,000 calories, so one kilocalorie equals 4,184 joules.

The calorie-versus-kilocalorie confusion catches almost everyone. When a food label says a snack has 250 "calories", it really means 250 kilocalories — 250,000 small calories, or 1,046,000 joules. Scientists write the food unit with a capital C, "Calorie", to mean kilocalorie, but labels usually write it lowercase, so the safest rule is this: any energy number attached to food is in kilocalories. This converter keeps the two separate, so pick "kilocalories" for food and "calories" for the small scientific unit.

The other units each belong to a trade. A BTU, or British thermal unit, is the energy needed to raise one pound of water by one degree Fahrenheit, and it is the standard unit for furnaces, air conditioners, and water heaters — a typical home air conditioner is rated at 12,000 BTU per hour. A kilowatt-hour is the energy of running one kilowatt of power for one hour, and it is what the electricity meter counts: one kilowatt-hour equals 3,600,000 joules. A foot-pound is the energy of lifting one pound by one foot, and it shows up in torque and ballistics.

To convert any unit to any other, the converter first changes the value into joules, then divides by the size of the target unit in joules. For example, 250 kilocalories is 250 × 4,184 = 1,046,000 joules, and dividing by 1,055.06 gives about 991.41 BTU. Going to kilowatt-hours, divide 1,046,000 by 3,600,000 to get 0.2906 kWh. The same two steps work for every pair on this page, and the "All units" line underneath shows the result in all seven units at once.

These conversions turn up in everyday questions. A 30-minute run that burns 300 kilocalories uses about 1,255,200 joules of energy. A space heater rated at 1,500 watts running for one hour uses 1.5 kilowatt-hours, which is 5,400,000 joules or about 1,290.59 kilocalories. And a snack bar labelled 200 kilocalories holds the same energy as about 0.2326 kilowatt-hours — enough, in theory, to run a 100-watt bulb for a little over two hours. The numbers are exact conversions of energy; how much of that energy a body or a machine actually uses is a separate question.

Worked example

A recipe lists 500 calories of energy per serving. How many joules is that?

Step 1: Write down the conversion — 1 calorie = 4.184 joules.
Step 2: Multiply — 500 × 4.184 = 2,092 joules.
Step 3: Check the bigger unit — 2,092 ÷ 1,000 = 2.092 kilojoules, and 2,092 ÷ 1,055.06 = 1.9828 BTU.

So 500 calories equals 2,092 joules. Note that if the recipe meant 500 food calories — kilocalories — the answer would be a thousand times bigger, 2,092,000 joules, so always check whether a food number is in calories or kilocalories before converting.

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