What this calculator does
Energy units come from different fields that each invented their own before anyone realised they were measuring the same thing. The joule is the SI unit, the calorie came from heating water, the BTU from heating a pound of it, the kilowatt hour from selling electricity and the electronvolt from moving a charge. All of them convert exactly.
The one to be careful with is the calorie, because there are two. The calorie in physics heats one gram of water by one degree and is 4.184 joules. The Calorie on food labelling is a thousand of those, properly a kilocalorie, and is 4,184 joules. This converter lists both separately, and a factor of a thousand is not a mistake worth making on a nutrition figure.
The formula
Every unit has a fixed factor in joules, so a conversion multiplies to reach joules and divides to leave them. The factors that matter are exact by definition: a watt hour is 3,600 joules because a watt is a joule per second, and the thermochemical calorie is exactly 4.184 joules. The BTU has several competing definitions differing by a fraction of a percent, and the common one is used here.
| Term | Meaning |
|---|---|
| Joule (J) | The SI unit: one newton acting through one metre, or one watt for one second. |
| Calorie (cal) | The small calorie, 4.184 joules, enough to warm a gram of water by one degree. |
| Kilocalorie (kcal) | A thousand calories, and what food labels call a Calorie with a capital C. |
| Kilowatt hour (kWh) | A kilowatt sustained for an hour, 3.6 million joules, and the unit electricity is billed in. |
The inputs explained
| Field | What to enter |
|---|---|
| Value | The energy value to convert. |
| From | The unit it is currently in. Choose kcal for a food-label Calorie, not cal. |
| To | The unit you want the answer in. |
When to use it
Converting a food label between regions
Labels give kilojoules in Australia and much of Europe, and Calories in the United States and United Kingdom. Converting between them is a single factor of 4.184 and is otherwise easy to get badly wrong.
Comparing an appliance rating with a bill
Appliance energy figures appear in watt hours or joules and bills in kilowatt hours, so putting both into the same unit is the only way to see what something actually costs to run.
Working through a physics or chemistry problem
Thermochemistry mixes joules, calories and sometimes electronvolts in a single question, and converting first usually makes the arithmetic simpler than carrying the units through.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What is one of each energy unit in kilojoules?
A single unit of each kind, converted to kilojoules to compare their scale.
| From | In kilojoules |
|---|---|
| Joule | 0.001 kJ |
| Calorie (small) | 0.004184 kJ |
| Kilocalorie (food Calorie) | 4.184 kJ |
| Watt hour | 3.6 kJ |
| Kilowatt hour | 3,600 kJ |
| British thermal unit | 1.05506 kJ |
Converting 2,000 food Calories to other energy units
A day of food energy, roughly, expressed in each of the other units.
| To | Result |
|---|---|
| Joules | 8,368,000 J |
| Kilojoules | 8,368 kJ |
| Watt hours | 2,324.444444 Wh |
| Kilowatt hours | 2.32444444 kWh |
| BTU | 7,931.302485 BTU |
Questions
What is the difference between a calorie and a Calorie?
A factor of 1,000. The small calorie is 4.184 joules and is used in physics and chemistry. The Calorie with a capital C on food labelling is a kilocalorie, 4,184 joules. Because the capital is easily lost in ordinary writing, food energy is better handled in kilojoules where the ambiguity does not exist.
How many kilojoules in a Calorie?
4.184 kJ in one food Calorie. To convert a label from Calories to kilojoules multiply by 4.184, and to go the other way divide. A 250 Calorie item is 1,046 kJ.
Why is a kilowatt hour 3.6 million joules?
Because a watt is one joule per second and an hour is 3,600 seconds. A watt hour is therefore 3,600 joules, and a kilowatt hour is a thousand of those. The figure is exact, not a measured conversion.
Which BTU definition does this use?
The International Table BTU of about 1,055.06 joules, which is the most widely used. Several other definitions exist, differing by up to about a quarter of a percent depending on the water temperature assumed. The difference rarely matters, but it is there.
For power, which is energy per unit time, see the power converter. For temperature, which governs how much energy a heating job actually needs, see the temperature converter.