Energy Converter — joules, calories, kWh, BTU & more

How it works

Energy comes in many units, but each is a fixed multiple of the SI unit, the joule. Per NIST SP 811:

• 1 calorie = 4.184 J (the thermochemical calorie), so 1 kilocalorie = 4,184 J = 4.184 kJ. The Calorie on food labels (capital C) is this kilocalorie.
• 1 watt-hour = 3,600 J, so 1 kilowatt-hour (kWh) = 3,600,000 J = 3,600 kJ — the unit your electricity bill uses.
• 1 BTU ≈ 1,055.06 J ≈ 1.055 kJ, common for heating and air-conditioning.
• 1 electronvolt = 1.602×10⁻¹⁹ J, the tiny unit used in physics.

The calculator converts your value to joules and then to the unit you want, so any pairing works the same way.

Worked example

Converting 1 kilocalorie to kilojoules: 1 kcal = 4,184 J = 4.184 kJ. And 1 kilowatt-hour = 3,600,000 J = 3,600 kJ, while 1 BTU ≈ 1.0551 kJ.

Frequently asked questions

Is a food Calorie the same as a calorie?

A food **Calorie** (capital C) is a kilocalorie — 1,000 small calories, or 4,184 joules. So a 250-Calorie snack holds 250 kcal ≈ 1,046 kJ.

How many joules are in a kilowatt-hour?

Exactly 3,600,000. A kilowatt-hour is one kilowatt sustained for one hour, and 1,000 W × 3,600 s = 3,600,000 J = 3,600 kJ.

How do I convert calories to joules?

Multiply by 4.184: one thermochemical calorie is 4.184 joules. So 100 kcal is 100 × 4,184 = 418,400 J.

What is a BTU?

A British thermal unit is about 1,055 joules — historically the energy to warm one pound of water by one degree Fahrenheit. It is still used for heating and cooling capacity.

What is the SI unit of energy?

The joule (J), the energy of one newton acting over one metre. Every other energy unit converts to and from joules by a fixed factor.

Which calorie does this use?

The thermochemical calorie, 4.184 J exactly (NIST SP 811). The closely related international-table calorie (4.1868 J) differs only in the fourth decimal place.