About the Energy Converter
This energy converter changes a value between twelve units: joule, kilojoule, megajoule, calorie, kilocalorie, watt-hour, kilowatt-hour, BTU, US therm, foot-pound force, electronvolt and erg. Type a number, choose the unit you have and the unit you want, and see the answer along with the same amount in every other unit. Calorie means the thermochemical calorie of 4.184 joules, and the BTU is the International Table BTU. Older tables use other calories and several BTU and therm definitions exist, so figures from other sources can differ slightly. Results are floating-point approximations, and very small or very large answers switch to scientific notation.
Updated September 2026
How to use the Energy Converter
- 1Type the amount of energy in the Value box.
- 2Choose the unit you have in From and the unit you want in To.
- 3Use the swap button to reverse the direction.
- 4Read the result, or scroll to the table for every unit at once.
- 5Press Copy on any row to copy that number.
Energy units in plain words
Energy is the capacity to do work or produce heat. The joule is the standard unit: roughly the energy needed to lift a small apple one metre. Because a joule is small, larger amounts are written in kilojoules and megajoules. Other units grew up in particular fields, such as calories for food and heat, kilowatt-hours for electricity, BTU for heating and cooling, and electronvolts for atoms and particles.
This converter passes every value through joules, so any unit can reach any other in one step.
Food calories vs physics calories
The word calorie is used for two sizes. The small calorie is 4.184 joules. The nutrition Calorie, often written kcal or Cal, is 1,000 small calories, or 4.184 kilojoules. A snack of 250 kcal therefore holds about 1,046 kJ. Countries that label food in kilojoules use the same energy, only in different units. If you are planning meals, the Calorie Calculator estimates daily needs.
Electricity: Wh, kWh and joules
Electricity bills count kilowatt-hours, and battery sizes are often quoted in watt-hours.
- 1 Wh = 3,600 J
- 1 kWh = 1,000 Wh = 3.6 MJ
- A 100 W bulb left on for 10 hours uses 1 kWh
- A phone battery of 15 Wh holds 54,000 J
Heating: BTU and therms
In heating and air conditioning, energy is often measured in British thermal units. One BTU is about 1,055 joules, and a US therm is 100,000 BTU of the 59 °F kind, defined here as 105,480,400 joules. Gas bills in some countries are in therms. For general conversions of lengths, weights and volumes, use the Unit Converter, for percentage changes the Percentage Calculator, and for any sum along the way the Online Calculator.
Features
- Twelve energy units, from erg to therm
- Every unit shown in one table with Copy buttons
- Thermochemical calorie and food Calorie handled clearly
- Swap button
- Scientific notation for tiny and huge results
- Never shows NaN or negative zero
- Runs in your browser
Frequently asked questions
What is the difference between a calorie and a Calorie?
A calorie (small c) is the heat needed to warm one gram of water by one degree, defined here as 4.184 joules. A food Calorie (capital C) is a kilocalorie, which is 1,000 of those, or 4,184 joules. Nutrition labels that say Calories or kcal mean the larger one.
How many joules are in a kWh?
One kilowatt-hour is 3,600,000 joules, or 3.6 megajoules. It is 1,000 watts used for 3,600 seconds.
How do I convert kcal to kJ?
Multiply by 4.184. So 100 kcal is 418.4 kJ. Choose kcal in From and kJ in To, or type the number and read the kJ row of the table.
How many BTU are in a kWh?
One kilowatt-hour is about 3,412.14 BTU using the International Table BTU of 1,055.056 joules. Other BTU definitions give slightly different totals.
What is an electronvolt?
It is the energy an electron gains crossing one volt, about 1.602 × 10⁻¹⁹ joules. It is common in physics, and this page shows tiny values like this one in scientific notation.
Why do different sites give slightly different BTU values?
There is more than one BTU. The International Table BTU is 1,055.056 J, while thermochemical and 59 °F versions differ in the fourth digit. This page states the definition it uses, and the same applies to the therm.
Is a watt-hour the same as a joule?
No. A watt-hour is 3,600 joules, because a watt is one joule per second and an hour has 3,600 seconds.