How this energy converter works
Energy comes up everywhere — the Calories on a food label, the kilowatt-hours on an electricity bill, the BTU rating on an air conditioner, even the electronvolts used in physics. They all measure the same quantity, just on wildly different scales. To convert accurately between them, this tool turns whatever you type into a single base unit — the joule — and then expresses that base value in every other unit. Converting through a base avoids the rounding errors that creep in when you chain conversions one after another.
The exact factors used (each expressed in joules) are: 1 kJ = 1,000 J, 1 cal = 4.184 J (thermochemical), 1 kcal = 4,184 J, 1 Wh = 3,600 J, 1 kWh = 3,600,000 J, 1 BTU = 1,055.05585 J, 1 ft·lb = 1.3558179483 J, and 1 eV = 1.602176634 × 10⁻¹⁹ J. The most-searched pairings — food Calories to kilojoules — are highlighted at the top so you can read them at a glance.
Common conversions
| From | Equals |
|---|---|
| 1 kilocalorie (kcal, food Calorie) | 4.184 kilojoules (kJ) |
| 1 kilowatt-hour (kWh) | 3,600,000 joules (3.6 MJ) |
| 1 BTU | 1,055.06 joules (≈ 0.000293 kWh) |
| 1 kilojoule (kJ) | 0.239 kilocalories (kcal) |
Reference note: the calorie used here is the thermochemical calorie (4.184 J exactly), the most common convention for nutrition. The IT calorie (4.1868 J) differs very slightly. Results are rounded for readability, and electronvolt-scale values are shown in scientific notation.
Frequently asked questions
- How do I convert calories to kilojoules?
- Multiply the number of thermochemical calories by 0.004184 to get kilojoules, or multiply food Calories (kcal) by 4.184. For example, 250 kcal × 4.184 ≈ 1,046 kJ. This converter does it instantly the moment you type a value.
- Is a food Calorie the same as a kcal?
- Yes. The Calorie shown on food labels (with a capital C) is a kilocalorie, equal to 1,000 small calories. So 1 food Calorie = 1 kcal = 4.184 kilojoules. Nutrition labels in many countries list both kcal and kJ.
- How many joules are in a kWh?
- One kilowatt-hour equals 3,600,000 joules, or 3.6 megajoules. That is because 1 kWh is 1,000 watts sustained for 3,600 seconds, and one watt is one joule per second, so 1,000 × 3,600 = 3,600,000 J.
- What is a BTU?
- A British thermal unit (BTU) is the energy needed to raise one pound of water by one degree Fahrenheit. One BTU equals about 1,055.06 joules, or roughly 0.000293 kWh. BTU is widely used to rate heating and air-conditioning equipment.
- How do I convert kJ to kcal?
- Divide the number of kilojoules by 4.184 to get kilocalories (food Calories). For example, 2,000 kJ ÷ 4.184 ≈ 478 kcal. To go the other way, multiply kcal by 4.184 to get kJ.
- How do I convert kWh to joules?
- Multiply the number of kilowatt-hours by 3,600,000 to get joules. For example, 2 kWh × 3,600,000 = 7,200,000 J, which is the same as 7.2 megajoules. This converter shows the result automatically as you type.