Energy Converter
Convert energy units fast, between joules, kilojoules, calories, kilocalories, watt hours, and electronvolts. Great for science and nutrition.
Enter the Details
Result will appear here...
What this converter does
Energy gets counted three different ways depending on who is asking: calories on a food label, kilowatt-hours on the electricity bill, joules in a physics class. This converter moves between all of them, plus watt-hours, electronvolts, and the therm.
How to use it
- Type your energy value into the value box.
- Set From to the unit you have, and To to the unit you want.
- Press Calculate.
The result reads back in plain words, like "1 kcal is equal to 4184 J." Use Swap to flip the two units, and Reset to clear.
How it works
Every unit is tied to the joule, the SI unit of energy. Your value is converted to joules and then to the target unit, using standard factors: a calorie is 4.184 joules, a kilowatt-hour is 3.6 million. Displayed results are rounded to nine significant figures.
Common energy conversions
The ones that come up most:
| Conversion | Value |
|---|---|
| 1 kilocalorie (kcal) | 4,184 J (4.184 kJ) |
| 1 food Calorie | 1 kcal (4,184 J) |
| 1 kilowatt-hour (kWh) | 3,600,000 J (3.6 MJ) |
| 1 watt-hour (Wh) | 3,600 J |
| 1 calorie (cal) | 4.184 J |
| 1 kilojoule (kJ) | 1,000 J |
The calorie confusion: why a food Calorie is really a kilocalorie
Here is the one that catches almost everyone out. The "Calories" on a nutrition label are not the same as the calories in a physics textbook. A food Calorie, written with a capital C, is actually a kilocalorie: one thousand of the small calories scientists use. So a snack listed as 200 Calories really holds 200 kilocalories, which is about 837,000 joules.
This converter offers both the small calorie and the kilocalorie, so if you are working from a food label, the kilocalorie (kcal) is the one you want. For a sense of scale, a medium banana carries around 105 kilocalories, and a typical day's eating of 2,000 Calories is 2,000 kilocalories, or roughly 2.3 kilowatt-hours of energy.
Energy or power: which one you need
Energy and power get mixed up constantly, and the difference is worth pinning down. Energy is a total amount, the whole tank: how much was used or stored. Power is a rate: how fast that energy flows, measured in watts.
Your electricity bill shows both. A kilowatt (kW) is power, the rate an appliance draws. A kilowatt-hour (kWh) is energy, what you actually pay for, and a 1,000-watt heater running for one hour uses exactly one kilowatt-hour. So if you are converting watts, kilowatts, or horsepower, you want the Power Converter. If you are converting joules, calories, or kilowatt-hours, you are in the right place.
Questions people ask
How many joules are in a calorie?
One small calorie is 4.184 joules. One kilocalorie, the food Calorie, is 4,184 joules.
Is a food Calorie the same as a calorie?
No. A food Calorie, with a capital C, is a kilocalorie, which is 1,000 of the small calories used in science. Nutrition labels mean kilocalories even when they just write "Calories."
How many joules are in a kilowatt-hour?
3,600,000 joules, or 3.6 megajoules. A kilowatt-hour is the energy a 1,000-watt appliance uses in one hour.
What is an electronvolt?
A very small unit of energy used in atomic and particle physics, equal to about 1.602 × 10⁻¹⁹ joules. It is handy for measuring energies at the scale of single particles.
How do I convert kilocalories to kilojoules?
Multiply by 4.184. One kilocalorie is 4.184 kilojoules, so a 250 kcal snack is about 1,046 kilojoules.
References
- National Institute of Standards and Technology (NIST), Office of Weights and Measures. The International System of Units (SI) and the joule. https://www.nist.gov/pml/owm/metric-si/si-units
Okan Atalay is a results driven senior operations manager and a graduate of Industrial Engineering from Bilkent University. With over 22 years of experience in textile manufacturing and integrated operations, he has led large scale business process improvements and strategic planning initiatives. Currently, he serves as a top mathematics expert for a global ed tech platform, where he applies his analytical expertise to solve complex mathematical problems. At Eon Tools, he reviews converter and maths tools.