Enter an energy value, pick the unit you have and the unit you want, and read the answer instantly — plus the same value in every other unit at once. It covers the units on a food label (kcal, kJ), on an electricity bill (kWh) and in physics and heating (J, Wh, BTU). Everything runs in your browser.
In all units
- Joule (J)4,184 J
- Kilojoule (kJ)4.184 kJ
- Megajoule (MJ)0.004184 MJ
- Calorie (cal)1,000 cal
- Kilocalorie (kcal)1 kcal
- Watt-hour (Wh)1.162222 Wh
- Kilowatt-hour (kWh)0.001162 kWh
- British thermal unit (BTU)3.965667 BTU
How it works
Each unit is defined by how many joules it equals, and every conversion goes through the joule as a shared base. Your value becomes joules and then the target unit, so results stay consistent — converting kcal to kJ and back returns exactly what you started with.
The factors are exact by definition: 1 kJ = 1,000 J, 1 kcal = 4,184 J (the thermochemical calorie used on food labels), 1 Wh = 3,600 J, 1 kWh = 3,600,000 J, and 1 BTU ≈ 1,055.06 J. Note that the food “Calorie” with a capital C is actually a kilocalorie — this tool’s kcal is that unit.
Practical examples
Food label from kcal to kJ
A snack shows 250 kcal, which is 1,046 kJ — both appear on EU labels. Enter 250, from kcal, to kJ to check the two figures match.
Electricity use in kWh to kcal
Curious how much energy 1 kWh is? It’s about 860 kcal or 3,600 kJ. Convert kWh to kcal to compare an appliance’s energy with food energy.
Heating output in BTU to kW-hours
An air conditioner rated at 12,000 BTU (per hour) delivers about 3,517 watts. Convert BTU to joules or Wh to relate a US heating/cooling spec to metric figures.
Frequently asked questions
How many kilojoules are in a kilocalorie?
One kcal is 4.184 kJ. So a 500 kcal meal is 2,092 kJ. European food labels show both; multiply kcal by 4.184 to get kJ, which is exactly what the tool does.
Is a food Calorie the same as a calorie?
No — and it’s a common trap. The “Calorie” on food labels (capital C) is a kilocalorie: 1,000 small calories. This tool’s kcal is the food unit; its cal is the small calorie used in physics and chemistry.
How many joules are in a watt-hour?
Exactly 3,600, because a watt is one joule per second and an hour is 3,600 seconds. A kilowatt-hour is therefore 3,600,000 J (3.6 MJ) — the unit your electricity meter counts.
How much is 1 kWh in kcal?
About 860.4 kcal. It’s a handy sanity check: running a 1,000 W appliance for an hour uses roughly the food energy of a large meal.
What is a BTU?
The British thermal unit is the energy to heat one pound of water by one degree Fahrenheit — about 1,055 J. It’s common on US heating and air-conditioning equipment, which is why the tool includes it.
Which calorie definition does this use?
The thermochemical calorie, 1 cal = 4.184 J exactly, which is the one used for nutrition labels. Other historical calorie definitions differ by a fraction of a percent and are not used here.
Does it accept the decimal comma?
Yes. You can type 4,184 or 4.184 depending on your habit; both are read correctly, and grouping separators in pasted numbers are handled too.
How precise are the results?
Conversions run at full floating-point precision using the exact factors above and are shown to six decimal places with trailing zeros removed.
Are the values I enter sent anywhere?
No. The conversion runs entirely in your browser; nothing is uploaded and analytics never receives the numbers.
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