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Energy Converter

The calories on a food label are actually kilocalories — 1,000 times the scientist's calorie — while your electricity bill uses kilowatt-hours, a different unit again. This converter reconciles joules, kilojoules, calories, kilocalories, watt-hours, kWh, BTU, and electronvolts so the label, the bill, and the physics textbook finally agree.

In Kilojoules
0.001kJ
1 J = 0.001 kJ
Value
From
To
A food Calorie is a kilocalorie
The capital-C Calorie on a nutrition label is 1,000 small calories — a kcal. So "250 Calories" means 250 kcal, or 1,046 kJ; European labels lead with the kJ figure.
The unit on your power bill
Electricity is billed in kilowatt-hours: 1 kWh = 3.6 million joules. A BTU, common on heating and AC ratings, is about 1,055 joules.
All units
Joules1
Kilojoules0.001
Calories0.239
Kilocalories0.000239
Watt-hours0.0002778
Kilowatt-hours0.0000002778
BTU0.0009478
Electronvolts6.2422e18
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Uses your inputs above
0.001 in kilojoules. Want to try a variation?

The math

Reviewed 2026
Formula
Multiply or divide by the conversion factor to a base unit.

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Example: how energy is calculated

Step-by-step with default inputs

Suppose you put the default values into Energy Converter:

Value
1
From
Joules
To
Kilojoules

Plug those into the formula Multiply or divide by the conversion factor to a base unit. and the result is:

In Kilojoules
0.001kJ
Energy units and their conversion factors
UnitSymbolIn joules
JoulesJ1
KilojouleskJ1,000
Caloriescal4.184
Kilocalorieskcal4,184
Watt-hoursWh3,600
Kilowatt-hourskWh3,600,000
BTUBTU1,055.06
ElectronvoltseV1.602e-19

Source: NIST official conversions

How does the energy converter work?

Energy Converter uses the formula shown in the math card and cites NIST official conversions. Inputs are validated for sensible ranges; results are computed client-side for instant feedback and do not leave your browser.

References: NIST official conversions.

Last reviewed July 2, 2026 · Editorial policy

Frequently asked questions

Are food Calories the same as calories?

No — a food Calorie (capital C) is a kilocalorie, equal to 1,000 small calories or 4,184 joules. So a 2,000-Calorie diet is 2,000 kcal, not 2,000 cal.

How many joules are in a calorie?

4.184 joules, using the thermochemical calorie this converter is built on. A kilocalorie is therefore 4,184 joules.

How many kilocalories are in a kilowatt-hour?

About 860 kcal. One kWh is 3.6 million joules, and dividing by the 4,184 joules in a kilocalorie gives 860.4.

How accurate is this energy calculator?

The math is deterministic — the same inputs always produce the same output, and the formula is shown above. Accuracy of the answer for your situation depends on how well your inputs match reality and how well the formula models the question.

Why does Google show a slightly different conversion?

For most units the answer is identical to many decimal places. Differences usually come from currency rates (which fluctuate intraday — this tool shows the ECB's daily reference rates) or rounding. We display up to 4 decimals.

How do I share my result?

Hit Share at the top of the page. Every input you change is encoded in the URL, so a permalink reproduces exactly what you see. No account needed.