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Energy Converter

Convert between joules, kilojoules, calories, kilocalories, watt-hours, kilowatt-hours and BTUs.

Energy Converter

Professional grade conversion for energy units.

Joule
Select Unit
0.001
Kilojoule
Select Unit

Conversion Formula

1 Joule ≈ 0.001 Kilojoule

What Energy Converter does

Energy is measured in different units by different industries: joules in science, kilowatt-hours on electricity bills, calories on food labels and BTUs in heating and air conditioning.

The most common source of confusion is the calorie. The energy figure on a food label, written as a Calorie in the United States, is actually a kilocalorie: 1000 small calories.

  • Joule and kilojoule
  • Calorie and kilocalorie
  • Watt-hour and kilowatt-hour
  • BTU for heating and cooling equipment

How to use Energy Converter

  1. 1

    Enter the value

    Type the amount you want to convert. The result updates as you type.

  2. 2

    Select the units

    Choose what your value is in and what you want it in, for example kilocalories to kilojoules.

  3. 3

    Copy the result

    The copy button gives you the unformatted number, without thousands separators, ready to paste into a spreadsheet or code.

Example

A food label in two units

Input

250 kcal (as written on a label)

Output

= 1046 kJ

European labels usually print both. The kJ figure is always the larger number, by a factor of about 4.184.

Limits and known behaviour

  • A food Calorie with a capital C is a kilocalorie. Mixing the two produces an error of a thousand times.
  • The BTU has several slightly different definitions; the value used here is the common thermochemical one.
  • Air-conditioning equipment is often rated in BTU per hour, which is a power rating rather than an energy amount.
  • Arithmetic uses double-precision floating point, so a converted value can end in a small representation residue. That is a property of binary arithmetic, not of the conversion factor.

Privacy and data handling

Runs entirely in your browser

  • The conversion is arithmetic performed in this page. Nothing you enter is transmitted or stored.
  • The tool works offline once the page has loaded.

Site-wide data handling, including analytics and advertising, is described in the privacy policy.

Frequently asked questions

Are the calories on a food label the same as calories in physics?

No. A food label Calorie is a kilocalorie, or 1000 physics calories. A 250 Calorie snack contains 250,000 physics calories.

How much energy is a kilowatt-hour?

3,600,000 joules — one kilowatt sustained for one hour. It is the unit electricity is billed in.