kJ to Calories Converter

A kilojoule (kJ) is 1,000 joules, the metric unit of energy. A food Calorie, written with a capital C or as kcal, is 1,000 small calories. Labels in Australia, New Zealand, the UK and the EU show energy in kJ, while many diet apps count Calories, so you often need to switch between the two.

What This Conversion Tells You

The tool changes the unit, not the energy. 1,000 kJ and 239 Calories are the same amount. The "Calorie" on food is a kilocalorie, and in everyday use the "kilo" is usually dropped¹. This tool does not tell you how many calories you burn.

The Formula

Calories (kcal) = kJ / 4.184

Calories (kcal) = kJ x 0.239006 (rounded)

Small calories (cal) = kJ x 239.006 (rounded)

The 4.184 comes from the thermochemical calorie, which NIST lists as exactly 4.184 joules², and it is the factor the FAO uses for food energy³.

How This Calculator Computes Your Result

Type an amount in kJ, or paste a label figure such as "8 700 kJ". The tool divides by 4.184 and rounds only at the end. Calories show to 1 decimal place, or 2 decimals below 10 kcal and 3 below 1 kcal. A second line shows small calories (cal), the unit used in chemistry, as a whole number. Amounts from 0.1 to 1,000,000 kJ are accepted. The swap button opens our Calories to kJ Converter.

What Your Result Means

The result is the same energy written in food Calories. Here are common label amounts. The last column shows each as a share of 8,700 kJ, the figure Australian and New Zealand labels use for percentage daily intake⁴.

For a quick check in your head, divide the kJ by 4 and take off about 4.5 percent (1,000 kJ gives 250 minus 11, or 239). If your Calorie number is bigger than the kJ number, the units are the wrong way round.

Reading Labels: 8,700 kJ, 8,400 kJ and 2,000 kcal

In Australia and New Zealand, labels must show energy in kJ and may add kcal⁴. Labels that show a percentage daily intake base it on "an average adult diet of 8 700 kJ"⁴, which is about 2,079 Calories. UK and EU labels that show reference intakes use 8 400 kJ / 2 000 kcal for an average adult⁵. Those two numbers are each rounded, since 8,400 kJ is really 2,007.6 kcal.

Why Your Answer May Not Match the Label's Calories

Labels do not convert kJ into kcal with 4.184. In the UK and EU, each figure is calculated from its own per-gram factors: carbohydrate and protein 17 kJ or 4 kcal, fat 37 kJ or 9 kcal, alcohol 29 kJ or 7 kcal, and fiber 8 kJ or 2 kcal⁵. These pairs are not exactly 4.184 apart. So 100 g of sugar comes out as 1,700 kJ and 400 kcal on a label, but 1,700 kJ converts to 406.3 kcal. 100 g of oil comes out as 3,700 kJ and 900 kcal, but converts to 884.3 kcal. For most foods the gap is within about 2 percent.

Australia and New Zealand use the same kJ factors⁶. When a label there also shows kcal, the rules work it out by dividing kJ by 4.18⁶. That is about 0.1 percent off 4.184, so 8,700 kJ shows as 2,081 rather than 2,079. Neither number is wrong. If the label prints Calories, use that number.

Two Kinds of Calorie

A small calorie is roughly the energy needed to warm 1 gram of water by 1 °C¹. There are two exact modern versions. The thermochemical calorie is 4.184 joules, and the International Table calorie is 4.1868 joules². The International Table calorie was set at a 1956 conference on the properties of steam¹. The two differ by about 0.07 percent, so 1,000 kJ is 239.0 kcal or 238.8 kcal. Nutrition uses 4.184: the FAO gives 1 kJ = 0.239 kcal and 1 kcal = 4.184 kJ³.

kJ in Cycling Apps: Why 1 kJ of Work Is About 1 Calorie Burned

A power meter measures the work you put into the pedals. Average watts times seconds, divided by 1,000, gives kJ. One hour at 200 watts is 720 kJ.

That is mechanical work, not food energy. Your muscles turn only part of the food energy they use into work, and the rest becomes heat. This share is called gross efficiency. In a study of 26 trained cyclists, average gross efficiency was about 20 to 22.5 percent at 150 to 180 watts⁷.

Here is the coincidence. At an efficiency of 23.9 percent (1 divided by 4.184), each kJ of work costs exactly 1 Calorie of food energy. Real efficiency is close to that, so kJ of work roughly equals Calories burned.

This is approximate. Efficiency differs between riders and depends strongly on how hard you pedal⁸. Still, a review found that work rate explains about 91 percent of the variation in energy use during cycling⁸, so kJ of work is a sound starting point.

Do not put power meter kJ into this converter. It would turn 720 kJ into 172 kcal, which is only the energy that reached the pedals. For an estimate from your weight, time and speed, use the Calories Burned Biking Calculator.