⚡ Energy Converter
By ToolNimba Editorial Team · Updated 2026-06-19
Type a value in any box and every other unit updates instantly. All conversion runs through joules.
This energy converter switches a value between joules, kilojoules, calories, kilocalories, watt-hours, kilowatt-hours, BTU, electronvolts and foot-pounds. Type a number into any field and every other unit updates at once. Behind the scenes each entry is turned into joules (the SI base unit of energy) and then back out into every other unit, so the conversions stay consistent no matter which box you start from.
What is the Energy Converter?
Energy is the capacity to do work, and although it always measures the same physical quantity, different fields settled on different units. Physics and engineering use the joule (J), the SI unit, defined as one newton of force acting over one metre. Nutrition labels use the calorie or, more precisely, the kilocalorie (the food "Calorie" with a capital C is one kilocalorie). Electricity bills use the kilowatt-hour, heating and cooling systems use the British thermal unit, atomic physics uses the electronvolt, and older mechanical work is sometimes quoted in foot-pounds. They all describe energy, so any one can be converted into any other.
The converter works by anchoring everything to the joule. Each unit has a fixed factor: 1 calorie is 4.184 J (the thermochemical calorie), 1 kilocalorie is 4184 J, 1 watt-hour is 3600 J, 1 kilowatt-hour is 3,600,000 J, 1 BTU is 1055.06 J, 1 electronvolt is about 1.602 times ten to the minus nineteen joules, and 1 foot-pound is about 1.3558 J. To convert, the tool multiplies your input by its factor to get joules, then divides by the target factor. Converting calories to joules and joules to calories are simply the two directions of that single relationship.
A common point of confusion is the small calorie versus the large Calorie. The "calorie" in physics is the energy to raise one gram of water by one degree Celsius (4.184 J). The "Calorie" on food packaging is a kilocalorie, one thousand of those, which is why a 2000 Calorie daily diet is really 2000 kilocalories or about 8.4 million joules. This converter treats "calorie" as the small calorie and "kilocalorie" as the food Calorie, so pick the right row for nutrition figures.
When to use it
- Converting the kilojoules or kilocalories on a food label into the unit your country uses, since some labels show kJ and others show Calories.
- Turning a kilowatt-hour reading from your electricity meter into joules or BTU to compare appliances or heating sources.
- Checking physics and chemistry homework where energy is given in joules but the answer is expected in calories or electronvolts.
- Sizing heating and cooling equipment, where output is often quoted in BTU but you want the figure in kilowatts or kilojoules.
How to use the Energy Converter
- Find the unit you already have a value for among the nine input boxes.
- Type the number into that box.
- Read the converted value from every other box, all updated at the same time.
- Clear the box or type a new number to start a fresh conversion.
Formula & method
Worked examples
Convert 1 kilowatt-hour to joules.
- The factor for kilowatt-hour is 3,600,000 J per kWh.
- joules = value × factor = 1 × 3,600,000
- joules = 3,600,000 J (3.6 megajoules)
Result: 1 kWh = 3,600,000 J
Convert 500 kilocalories (a food Calorie figure) to kilojoules.
- First go to joules: 500 × 4184 = 2,092,000 J
- Now divide by the kilojoule factor (1000 J per kJ): 2,092,000 ÷ 1000
- result = 2092 kJ
Result: 500 kcal = 2092 kJ
Convert 2 kilowatt-hours to BTU.
- To joules: 2 × 3,600,000 = 7,200,000 J
- Divide by the BTU factor (1055.06 J per BTU): 7,200,000 ÷ 1055.06
- result ≈ 6824.73 BTU
Result: 2 kWh ≈ 6824.73 BTU
Energy units expressed in joules (the conversion factors used by this tool)
| Unit | Symbol | Value in joules |
|---|---|---|
| Joule | J | 1 |
| Kilojoule | kJ | 1000 |
| Calorie (thermochemical) | cal | 4.184 |
| Kilocalorie / food Calorie | kcal | 4184 |
| Watt-hour | Wh | 3600 |
| Kilowatt-hour | kWh | 3,600,000 |
| British thermal unit | BTU | 1055.06 |
| Electronvolt | eV | 1.602176634 × 10⁻¹⁹ |
| Foot-pound | ft·lb | 1.3558179 |
Handy everyday energy conversions
| From | To | Result |
|---|---|---|
| 1 kcal | kilojoules | 4.184 kJ |
| 1 kJ | calories | 239.01 cal |
| 1 kWh | kilocalories | 860.42 kcal |
| 1 BTU | joules | 1055.06 J |
| 1 BTU | calories | 252.16 cal |
| 1 kWh | BTU | 3412.14 BTU |
Common mistakes to avoid
- Mixing up the calorie and the food Calorie. The "calorie" in science is the small calorie (4.184 J), while the "Calorie" on nutrition labels is a kilocalorie, one thousand times larger. Use the kilocalorie row for diet and food figures, not the calorie row, or your numbers will be off by a factor of 1000.
- Confusing watts with watt-hours. A watt is power (a rate), while a watt-hour is energy (power multiplied by time). A 100 watt bulb running for one hour uses 100 watt-hours. This tool converts energy units, so convert power to energy by multiplying by the run time first.
- Assuming one fixed value for the calorie or BTU. There are slightly different definitions (thermochemical, international table, mean). This tool uses the thermochemical calorie of 4.184 J and the BTU of 1055.06 J, which are the common choices. Other references may differ in the third or fourth decimal place.
- Reading huge electronvolt results as an error. Because one electronvolt is a tiny fraction of a joule, even a small joule value becomes an enormous number of electronvolts. That is expected, not a bug, the electronvolt is sized for single atoms and particles.
Glossary
- Joule (J)
- The SI unit of energy, equal to one newton of force acting over one metre. Every other unit here is defined relative to it.
- Calorie (cal)
- The small calorie, roughly the energy to warm one gram of water by one degree Celsius. This tool uses 4.184 J per calorie.
- Kilocalorie (kcal)
- One thousand calories, equal to the food "Calorie" with a capital C used on nutrition labels. 1 kcal = 4184 J.
- Kilowatt-hour (kWh)
- The energy of one kilowatt of power sustained for one hour, the unit electricity is billed in. 1 kWh = 3,600,000 J.
- BTU
- British thermal unit, the energy to raise one pound of water by one degree Fahrenheit. Common in heating and cooling. 1 BTU = 1055.06 J.
- Electronvolt (eV)
- The energy gained by an electron moving through a one volt potential difference, used in atomic and particle physics.
Frequently asked questions
How do I convert joules to calories?
Divide the number of joules by 4.184, because one calorie equals 4.184 joules. For example 1000 J ÷ 4.184 = 239.01 cal. For food Calories (kilocalories) divide by 4184 instead. Type your joule value into the converter and the calorie and kilocalorie boxes fill in automatically.
How many joules are in a kilowatt-hour?
One kilowatt-hour is 3,600,000 joules (3.6 megajoules). It comes from 1000 watts times 3600 seconds in an hour. So a 2 kWh reading is 7,200,000 J. The tool does this multiplication for you when you type into the kWh box.
Is a calorie the same as a Calorie on food labels?
No. The "calorie" in physics is the small calorie (4.184 J). The "Calorie" with a capital C on food packaging is a kilocalorie, one thousand small calories. So a 250 Calorie snack contains 250 kilocalories, or about 1,046,000 joules. Use the kilocalorie row for nutrition figures.
How do I convert kJ to kcal on a nutrition label?
Divide kilojoules by 4.184 to get kilocalories, since 1 kcal = 4.184 kJ. For example 2092 kJ ÷ 4.184 = 500 kcal. Many labels print both, so you can use this to check that a label is consistent.
What is a BTU and how does it relate to kilowatts?
A BTU (British thermal unit) is 1055.06 joules, common in heating and cooling. Because power equals energy over time, 1 kWh equals about 3412 BTU. To compare a BTU-per-hour rating with kilowatts, divide the BTU per hour by roughly 3412.
Why is the electronvolt value so large?
An electronvolt is an extremely small amount of energy, about 1.602 times ten to the minus nineteen joules, so it takes a huge number of them to equal even one joule. When you convert any everyday value into electronvolts the result is naturally enormous, which is correct.