Energy converter
Convert joules, kilojoules, calories, kcal, watt-hours, kWh, BTU, therms and electronvolts with exact definitions and a live formula.
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1 kcal = 4.184 kJ
Automatic formatting keeps up to 12 significant digits and switches to scientific notation only for numbers past 10¹⁸ or below 10⁻⁹. You can paste values with thousands separators, and fractions such as 3/4 or 1 1/2 are understood too.
kJ = kcal × 4.184
Your value: 1 × 4.184 = 4.184 kJ
Kilocalorie (food Calorie): The “Calorie” on a nutrition label is this one — 1,000 small calories.
Kilojoule: Food energy outside the US.
| kcal | kJ |
|---|---|
| 1 | 4.184 |
| 2 | 8.368 |
| 5 | 20.92 |
| 10 | 41.84 |
| 20 | 83.68 |
| 50 | 209.2 |
| 100 | 418.4 |
| 250 | 1,046 |
| 500 | 2,092 |
| 1,000 | 4,184 |
| 2,000 | 8,368 |
| 5,000 | 20,920 |
| kJ | kcal |
|---|---|
| 1 | 0.239005736138 |
| 2 | 0.478011472275 |
| 5 | 1.19502868069 |
| 10 | 2.39005736138 |
| 20 | 4.78011472275 |
| 50 | 11.9502868069 |
| 100 | 23.9005736138 |
| 250 | 59.7514340344 |
| 500 | 119.502868069 |
| 1,000 | 239.005736138 |
| 2,000 | 478.011472275 |
| 5,000 | 1,195.02868069 |
| Unit | Symbol | Value in J | Notes |
|---|---|---|---|
| JouleSI | J | 1 J = 1 J | One watt for one second. |
| KilojouleSI | kJ | 1 kJ = 1,000 J | Food energy outside the US. |
| MegajouleSI | MJ | 1 MJ = 1,000,000 J | — |
| Calorie (small)Nutrition & heat | cal | 1 cal = 4.184 J | Thermochemical calorie, exactly 4.184 J. |
| Kilocalorie (food Calorie)Nutrition & heat | kcal | 1 kcal = 4,184 J | The “Calorie” on a nutrition label is this one — 1,000 small calories. |
| Watt-hourElectrical | Wh | 1 Wh = 3,600 J | — |
| Kilowatt-hourElectrical | kWh | 1 kWh = 3,600,000 J | The unit on your electricity bill. |
| British thermal unitNutrition & heat | BTU | 1 BTU = 1,055.05585262 J | IT definition. Heating and air-conditioning ratings. |
| Therm (US)Nutrition & heat | thm | 1 thm = 105,480,400 J | The US gas-bill unit: 100,000 BTU measured at 59 °F, or 99,976 IT BTU. |
| Foot-poundMechanical | ft·lb | 1 ft·lb = 1.35581794833 J | Also the imperial unit of torque. |
| Newton-metreMechanical | N·m | 1 N·m = 1 J | Dimensionally identical to the joule. |
| ElectronvoltAtomic | eV | 1 eV = 1.602177 × 10⁻¹⁹ J | Exact since the 2019 SI redefinition. |
How to convert an energy value
- Type into either field — nutrition labels, electricity bills and physics homework all use the same control.
- Pick your units from the SI group, the nutrition and heat group, the electrical group, or the mechanical and atomic units.
- The formula panel shows the exact factor with your own number in it, so you can paste the working rather than just the answer.
Calories, kilocalories and the capital C
The word "calorie" means two different things and the difference is a factor of a thousand. The small calorie is the energy needed to warm one gram of water by one degree Celsius: exactly 4.184 joules. The kilocalorie is a thousand of those, and it is what nutrition labels mean when they print "Calories" with a capital C.
So a 500-Calorie meal contains 500 kcal, which is 2,092 kJ, which is 2,092,000 J. European labels usually print both kJ and kcal, and dividing the kJ figure by 4.184 gets you back to the number Americans quote. A rough mental conversion is to divide kJ by 4 and take off 5 %.
| Food | kcal | kJ |
|---|---|---|
| Apple | 95 | 397 |
| Slice of bread | 80 | 335 |
| Can of cola | 139 | 582 |
| Chocolate bar (50 g) | 260 | 1,088 |
| Adult daily intake guideline | 2,000–2,500 | 8,368–10,460 |
Kilowatt-hours and the electricity bill
A kilowatt-hour is one kilowatt drawn for one hour, so it is 1,000 W × 3,600 s = 3,600,000 joules. It is the unit your meter counts and the unit your tariff is priced in. Because power is a rate and energy is an amount, converting between them needs a time: a 2 kW heater run for three hours uses 6 kWh.
Useful anchors: 1 kWh is 3,412 BTU, 860 kcal, or 3.6 MJ. A typical UK home uses around 2,700 kWh of electricity a year; a US home uses closer to 10,500 kWh. An electric car takes roughly 15 to 20 kWh per 100 km.
BTU, therms and heating
Heating and air conditioning in the US are quoted in British thermal units — the energy to raise one pound of water by one degree Fahrenheit, which the international definition fixes at 1,055.05585262 J. Gas bills use therms, which are exactly 100,000 BTU, or about 29.3 kWh.
That makes therms the unit to convert when comparing gas and electricity prices. If gas costs 1.20 per therm, that is 4.1 cents per kWh of heat, against a typical electricity price several times higher — which is the whole economic argument for heat pumps, since they move three or four units of heat per unit of electricity consumed.
Reference: energy at different scales
| Quantity | In joules |
|---|---|
| One electronvolt | 1.602 × 10⁻¹⁹ J |
| Lifting an apple one metre | 1 J |
| One food Calorie (kcal) | 4,184 J |
| AA alkaline battery | about 10,000 J |
| One kilowatt-hour | 3,600,000 J |
| Litre of petrol burned | about 34,200,000 J |
| Electric car battery (60 kWh) | 216,000,000 J |
Because the range here spans nearly thirty orders of magnitude, the converter switches to scientific notation automatically at the extremes — or you can force plain decimal or scientific output from the number-format control.
Questions
How many kilojoules are in a kilocalorie?
4.184 kJ exactly. That is why a 500-Calorie meal is labelled 2,092 kJ in countries that use kilojoules on nutrition panels.
Is a food Calorie the same as a calorie?
No — the capital-C Calorie on a food label is a kilocalorie, 1,000 small calories. Both are in the list, so pick kcal for anything nutritional.
How much energy is a kilowatt-hour?
3.6 million joules: one kilowatt drawn for one hour. It is the unit your electricity meter counts, and equals 3,412 BTU.
What is a therm?
100,000 BTU (as measured at 59 °F; 99,976 in the IT BTU used here), or about 29.3 kWh. US gas bills are measured in therms, so this is the conversion to use when comparing gas and electricity prices.