Calories to kJ Converter

Calories and kilojoules (kJ) are two units for the same thing: the energy in food and drink. US food labels count in Calories, while Australia and New Zealand label food in kilojoules. The math is one multiplication. The main risk is starting from the wrong kind of calorie.

What Calories and Kilojoules Measure

The Calorie on a food label or in a diet app is a kilocalorie (kcal). Australia's government health service puts it simply: kilocalories are "also known as 'calories'"¹. A kilojoule is 1,000 joules, and the joule is the metric (SI) unit of energy. International convention recommends joules for food energy, but calories are still widely used³. This tool changes the unit only. 500 kcal and 2,092 kJ are the same amount of energy.

The Formula

kJ = kcal x 4.184

kJ = small calories (cal) x 0.004184

MJ (megajoules) = kJ / 1,000

The US National Institute of Standards and Technology (NIST) lists the thermochemical kilocalorie as exactly 4.184 kJ², and the UN Food and Agriculture Organization (FAO) uses 1 kcal = 4.184 kJ for food energy³.

How This Calculator Computes Your Result

Type your number in Calories (kcal), the default, as 2000 or 2,000. The tool multiplies by 4.184 and shows kilojoules to the nearest whole number (one decimal under 100 kJ), the exact product, and megajoules to two decimals. For example, 1,850 kcal x 4.184 = 7,740.4, shown as 7,740 kJ (7.74 MJ).

A small calories (cal) switch is there for physics and chemistry values. It is off by default, because a small calorie is 1,000 times smaller than a food Calorie. When it is on, the tool shows a warning, gives the result in kJ to three decimals and in joules, and shows the answer you would get in kcal. The swap button opens our kJ to Calories Converter with your result filled in, and the copy button copies a line such as "2,000 kcal = 8,368 kJ".

What Your Result Means

Your result is the same energy written in kilojoules. These values were computed with the 4.184 factor.

Sanity check: the kJ number is always a little more than 4 times the Calorie number. If your answer is smaller than the number you typed, the small calories switch is probably on.

Calories vs Small Calories: The 1,000x Mistake

Physics and chemistry use a small calorie (cal). NIST lists it as 4.184 joules, exactly 1,000 times smaller than the kilocalorie². Food labels do not always print "kcal", though. The US Nutrition Facts label just says "Calories"⁵. A number that came from food, a recipe or a diet app is in kcal, whatever word sits beside it¹.

Mixing the two gives an answer that is 1,000 times too small. Some online "calorie to kilojoule" converters use the small calorie, so 500 becomes 2.092 kJ instead of 2,092 kJ. That is why this tool starts in kcal and warns you when you switch.

Converting a Calorie Target for Australian and New Zealand Labels

Australia and New Zealand share one food standards code. It requires the energy on a nutrition panel in kilojoules, with kilocalories optional⁴. Percent daily intake figures on those labels are based on an average adult diet of 8,700 kJ⁴, which is about 2,079 kcal. US labels use 2,000 Calories as a general guide⁵, which is 8,368 kJ. So 10% of daily intake on an Australian label is about 208 kcal, not 200.

If you track in Calories, convert your daily target once instead of converting every food. A target of 1,800 kcal is 7,531 kJ, so a 2,100 kJ serve uses about 28% of it. The same works for an app goal or a dietitian's plan. Treat 8,700 kJ as a labeling reference, not your own target, because calorie needs depend on age, sex, height, weight and activity⁵.

Restaurant meals use the same unit. A policy guideline approved by food ministers says large chains should show kilojoules per item where you order, with the statement that the average adult daily energy intake is 8700kJ⁶. For a recipe written in Calories, convert the per-serving figure: a 450 kcal serving is 1,883 kJ.

Why 4.184 and Not 4.2

Some guides round the factor to 4.2, including Healthdirect¹. That shortcut is about 0.4% high, so 2,000 kcal becomes 8,400 kJ instead of 8,368 kJ. NIST also lists an International Table kilocalorie of exactly 4.1868 kJ², which gives results about 0.07% higher (8,374 kJ for 2,000 kcal). This tool uses 4.184, the factor the FAO uses for food³.