Energy Converter
Type an energy value once — see it instantly in joules, calories, kWh, BTU and more, kJ to calories included.
Enter a number to see it in every energy unit.
Nutrition labels list food energy as "Calories" (capital C) — that is actually kilocalories (kcal), 1,000 times larger than a small calorie (cal). This converter always uses the standard 1 kcal = 1000 cal ratio.
All units at once
| Joule (J) | — |
| Kilojoule (kJ) | — |
| Megajoule (MJ) | — |
| Calorie (cal) | — |
| Kilocalorie / food Calorie (kcal) | — |
| Watt-hour (Wh) | — |
| Kilowatt-hour (kWh) | — |
| British thermal unit (BTU) | — |
| Foot-pound (ft·lb) | — |
| Therm (US, 100,000 BTU) | — |
One converter for every everyday energy unit
Energy shows up in wildly different units depending on who is measuring it. Physicists and engineers use joules and kilojoules. Nutrition labels use calories and kilocalories. Electricity bills use kilowatt-hours. Air conditioners and furnaces in the US are rated in BTU. Mechanical work is sometimes given in foot-pounds. All of these measure the same physical quantity — energy — just scaled and named differently by the industry that adopted them. This converter keeps every unit on tap at once: type a number, pick where it starts and where it should end up, and the full table below fills in instantly so you can cross-check against any of the other seven units too.
How the conversion works
Every unit here converts through the joule (J), the SI base unit of energy, using a fixed multiplier:
| Unit | Symbol | Value in joules |
|---|---|---|
| Joule | J | 1 (base unit) |
| Kilojoule | kJ | 1,000 J |
| Calorie (small) | cal | 4.184 J |
| Kilocalorie (food Calorie) | kcal | 4,184 J |
| Watt-hour | Wh | 3,600 J |
| Kilowatt-hour | kWh | 3,600,000 J |
| British thermal unit | BTU | 1,055.06 J |
| Foot-pound | ft·lb | 1.356 J |
To convert any value, the tool multiplies by the From unit's joule value, then divides by the To unit's joule value. Under the hood that is exactly what you would do by hand — the converter just skips the two lookup steps and the arithmetic.
Worked examples
1. Electricity bill to food energy (1 kWh in kJ). 1 kWh = 3,600,000 J, and 1 kJ = 1,000 J, so 1 kWh = 3,600,000 ÷ 1,000 = 3,600 kJ. That is also 3.6 megajoules — useful when a utility bill (kWh) needs to be compared against a food or fuel figure normally printed in kJ.
2. Nutrition label kJ to kcal. A cereal bar lists 620 kJ per serving. Divide by 4.184 to get the more familiar Calorie count: 620 ÷ 4.184 ≈ 148.2 kcal. Many countries print both figures on packaging for exactly this reason.
3. A banana in kilojoules. A medium banana carries roughly 105 kcal of food energy. Multiply by 4.184: 105 × 4.184 ≈ 439.3 kJ, which is also about 122 Wh — enough to run a small 10 W LED bulb for around 12 hours.
4. Air conditioner sizing. A room air conditioner rated at 12,000 BTU/h is a common "1-ton" unit. In SI terms that is 12,000 × 1,055.06 ≈ 12.66 MJ per hour, or roughly 3.5 kW of cooling power — the BTU rating and the kW rating describe the same physical output, just in different unit systems.
The calorie vs. Calorie trap
This is the single most common mix-up in energy units. A calorie (cal, lowercase c) is a physics-textbook unit: the energy needed to raise 1 gram of water by 1°C. It is tiny — a single almond has roughly 7,000 of them. Nutrition science instead uses the kilocalorie (kcal), which food labels write as "Calorie" with a capital C, purely by convention. One kcal equals exactly 1,000 small calories. So when a label says "200 Calories," it really means 200 kcal, or 200,000 small calories. This converter always keeps cal and kcal as two separate, correctly-scaled units — pick the one you actually mean and the 1,000× difference is handled for you automatically.
Where each unit is normally used
- Joule / kilojoule (J, kJ): the SI standard, used in physics, engineering, and on nutrition labels outside the US.
- Calorie / kilocalorie (cal, kcal): food energy and some chemistry contexts (small calorie only).
- Watt-hour / kilowatt-hour (Wh, kWh): electricity consumption — what your utility meter and bill actually track.
- British thermal unit (BTU): heating and cooling equipment ratings, mostly in the US.
- Foot-pound (ft·lb): mechanical work and torque figures in US customary engineering.
Precision and limits
All conversions use standard reference values: the thermochemical calorie (4.184 J), the International Table BTU (1,055.05585262 J), and the foot-pound force (1.3558179483314004 J). Results are shown to up to 10 significant figures with trailing zeros trimmed, and switch to scientific notation for extremely large or small numbers so nothing gets silently rounded to zero.
Sources & further reading
Frequently asked questions
kJ to calories — how do I convert kilojoules to kilocalories?
Divide the kilojoule value by 4.184 to get kilocalories (kcal), the unit nutrition labels usually call "Calories". For example, 100 kJ ÷ 4.184 ≈ 23.9 kcal. The reverse works too: multiply kcal by 4.184 to get kJ. This converter does the division for you instantly — just pick kJ as the From unit and kcal as the To unit.
What is the difference between a calorie (cal) and a food Calorie (kcal)?
This is the classic "small calorie" trap. A calorie (cal, lowercase) is the energy needed to raise 1 gram of water by 1°C — a tiny amount. Nutrition labels use the kilocalorie (kcal), written as "Calorie" with a capital C, which equals 1,000 small calories. So a snack labeled "200 Calories" actually contains 200,000 small calories. This converter treats cal and kcal as separate units at their correct 1:1000 ratio so you never mix them up.
How many kJ are in 1 kWh?
1 kilowatt-hour (kWh) equals 3.6 megajoules, or exactly 3,600 kilojoules (kJ). This conversion matters when comparing an electricity bill (billed in kWh) with a food or fuel energy value (usually given in kJ or kcal) — for example, running a 1,500 W heater for one hour uses 1.5 kWh, the same energy as roughly 1,290 kcal of food.
How many kilojoules are in a banana?
A medium banana has about 105 kcal of food energy. Multiply by 4.184 to convert: 105 kcal × 4.184 ≈ 439.3 kJ. That is roughly the same energy as 122 Wh of electricity — enough to run a 10 W LED bulb for about 12 hours. Try entering 105 in this converter with kcal as From and kJ as To to see the exact figure.
Is my input sent to a server?
No. This energy converter runs entirely in your browser — the number you type is never uploaded anywhere. The only thing saved is your last chosen unit pair (for example kWh → kJ), kept in your browser's local storage so it is ready next time. The value itself is not stored.