Kilocalories to kilojoules:
multiply by 4.184

How many kJ are in a kcal?

One kilocalorie is 4.184 kilojoules. The factor is exact, because the thermochemical calorie is defined as 4.184 joules, and a kilocalorie is a thousand of those. So 1 kcal equals 4,184 joules, or 4.184 kJ.

The kilocalorie is the unit most people mean when they say calories on food. A dietary Calorie with a capital C is the same thing as one kcal. Kilojoules are the metric (SI) unit of energy, which is why labels in many countries print both numbers side by side.

Kcal to kJ formula and worked example

For a mental estimate, multiply by 4 and add about 5 percent. A 350 kcal meal gives 1,400 plus 70, near 1,470 kJ, which is close to the exact 1,464.4.

1. 1Read the energy value in kcal from the label or plan.
2. 2Multiply it by 4.184.
3. 3Round to a whole number for food, since labels rarely show decimals.
4. 4Check that the kJ figure is a little over four times the kcal figure.

Chart: kcal to kJ for common amounts

These values come straight from the factor 4.184 and are rounded only where the digits run on.

A 2,000 kcal daily intake is 8,368 kJ, and a 500 kcal dinner is 2,092 kJ.

Where you need kcal to kJ in real life

• Nutrition panels that show only kJ while your diet app counts kcal, common on packaged food sold in Australia, New Zealand and parts of Europe.
• Comparing a recipe in a cookbook to the energy needs set by a dietitian who works in kilojoules.
• Sports science and lab reports, where energy is normally written in joules.

Going the other way matters just as much. A bar labelled 1,100 kJ is 1,100 ÷ 4.184 = 262.9 kcal. If you read that as 1,100 kcal you would overstate its energy more than fourfold.

Quick sanity checks for kilocalories to kilojoules

A kJ value should always be a bit more than four times the kcal value. If a label lists 140 kcal and 586 kJ, 140 × 4.184 = 585.76, so the two agree. If they did not, you would suspect a printing slip.

If you track energy in kJ but your app counts kcal, convert your target once instead of every food. A 9,000 kJ daily target becomes 9,000 ÷ 4.184 = 2,151 kcal.

Because the factor is exact, any rounding comes from the figures printed on the pack, which are usually whole numbers.

Common mistakes when converting kcal to kJ

• Mixing up small calories and kilocalories. A lower-case calorie (cal) is one thousandth of a kcal, so 1 cal is only 4.184 joules.
• Dividing instead of multiplying. kJ numbers are always larger than kcal numbers, so a smaller answer means you divided.
• Using 4.2 or 4 as the factor for exact work. These are fine for estimates, but the error reaches 4.5 percent with a factor of 4.
• Forgetting that macros are counted in kcal per gram. A snack with 120 kcal per serving and 4 servings has 480 kcal in total, which is 2,008.32 kJ, so convert the total rather than rounding each serving.

Results here are plain unit conversions and are not dietary advice. Ask a qualified professional about your own energy needs.