Calculator

Calories to kilojoules

The exact factor, both directions, and what the number on the packet actually means where you are — which is the part a plain unit converter leaves out.

Type in either box — the other follows.

If the figures above are per 100 g, this converts a whole serving.

500 kcal is

2,092kJ

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Of 8,700 kJ — Australia, NZ
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Of 8,400 kJ / 2,000 kcal — UK, EU
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Those two reference intakes are for comparing products on a shelf, not personal targets. For a figure that is actually yours, themaintenance calculator.

The exact conversion

One kilocalorie is 4.184 kilojoules, exactly — not approximately. The thermochemical calorie is defined as 4.184 joules, so the factor is a definition rather than a measurement. Going the other way, divide by 4.184, which is the same as multiplying by 0.239.

There is an older unit, the international steam-table calorie, worth 4.1868 J. Nutrition and thermochemistry use 4.184, so that is what this page uses and what food labelling is based on.

Is 4.2 close enough?

For mental arithmetic, yes. Dividing by 4.2 instead of 4.184 overestimates by 0.38% — about 33 kJ across a whole 8,700 kJ day, or 8 kcal in 2,079. You will see the error quoted as "under 0.07%" on some sites, which is wrong by roughly fivefold. It is fine in your head and not fine in a calculator, so this page uses the exact factor.

Why Australia and New Zealand use kilojoules

The Nutrition Information Panel in Australia and New Zealand gives energy in kilojoules, with kilocalories optional — so for a great many people the kJ figure is the only one on the packet. The percentage daily intake column, and the energy labelling on fast-food menu boards, both work from a reference of8,700 kJ for an average adult. That is 2,079 kcal.

8,700 and 8,400 are different references

In the UK and the EU, labels must declare energy in both kJ and kcal, in that order, per 100 g or 100 ml. The Reference Intake there is8,400 kJ / 2,000 kcal.

Which produces a small oddity worth knowing: 2,000 kcal × 4.184 is 8,368 kJ, not 8,400. The two printed figures are rounded separately and are not an exact conversion of each other. So a European pack and an Australian one are using different reference adults, and neither is yours. The US, Canada and India print kcal only.

Calories, Calories and kilocalories

A food "Calorie" with a capital C is a kilocalorie — a thousand small calories. The small calorie, the energy to warm a gram of water by one degree, is almost never what anyone means in a nutrition context. So kcal, Cal and "calories" on a menu are all the same unit, and the capital letter is the only thing distinguishing two units that differ by a factor of a thousand. It is a poor piece of notation and we are stuck with it.

Where the label number comes from

One last caveat, since precision is the point of a converter. The energy printed on a packet is not measured from that packet — it is calculated from the composition using standard factors, roughly 17 kJ per gram for protein and carbohydrate and 37 kJ for fat, and it carries legal rounding tolerances. Converting it to three decimal places is false precision applied to an estimate. Where a label prints both units, use the printed kcal rather than converting the kJ, since the two were rounded separately.