Convert kilojoules into calories
Works in both directions — kilojoules to calories, or calories to kilojoules. Type a value in either box; the conversion updates as you type, and the arithmetic behind it is shown so you can check it yourself.
What is a kilojoule?
A joule is the SI unit of energy. One joule is the energy used when a force of one newton moves something one metre. It is a very small quantity in food terms, so nutrition uses the kilojoule (kJ): one thousand joules.
Kilojoules measure energy of any kind. The same unit describes the electricity a kettle draws, the kinetic energy of a moving car, and the chemical energy your body can extract from a slice of bread. That universality is exactly why the metric system chose it: one unit, every context, no conversion needed between disciplines.
For scale: a medium banana holds about 440 kJ. An average adult needs somewhere around 8,700 kJ per day, the reference figure printed on Australian and New Zealand packaging.
What is a calorie?
A calorie is an older, non-SI energy unit defined by heat: it is
the energy needed to raise the temperature of one gram of water by one degree Celsius.
That original unit is now called the small calorie and written
cal.
Here is the part that confuses almost everyone. The small calorie is far too tiny
to be useful for food, so nutrition uses the kilocalorie
(kcal): one thousand small calories, the energy that heats one kilogram
of water by one degree.
The everyday "calorie" is really a kilocalorie. When a label says a chocolate bar has 250 calories, it means 250 kcal, which is 250,000 small calories. Nutritionists sometimes write it as Calorie with a capital C to mark the difference, but almost nobody outside textbooks bothers. Throughout this site, "calorie" means kcal unless stated otherwise.
This is also why the converter above shows a "small calories" figure alongside the main result: it is the only place the distinction ever visibly bites.
The relationship between kilojoules and calories
The two units measure exactly the same thing. Converting between them is not an approximation or a model — it is a fixed, exact ratio, the same way inches and centimetres are:
The 4.184 figure is the thermochemical calorie, defined as exactly 4.184 joules. It is the definition used in EU food-labelling law and, in practice, by every food regulator worldwide. You may occasionally meet the International Steam Table calorie at 4.1868 J in older engineering texts; the difference is about 0.04 percent and never matters for food.
Because the ratio is fixed, a food's energy content in kJ and in kcal are two spellings of one number. A 2,000 kcal daily target and an 8,368 kJ daily target are the same target.
How to convert kilojoules and calories yourself
Kilojoules to calories
Divide by 4.184.
Calories to kilojoules
Multiply by 4.184.
Doing it in your head
Standing in a supermarket aisle you rarely need three decimal places. Two shortcuts work well:
- Divide by 4 to go from kJ to calories. This overestimates by about 4.6 percent — close enough to compare two packets.
- Multiply by 4 and add 5 percent to go from calories to kJ. 400 kcal → 1,600 → add 80 → 1,680 kJ. The exact answer is 1,673.6 kJ, so the error is about 0.4 percent.
A useful anchor: 100 kcal is almost exactly 420 kJ (418.4 to be precise). Once you know that, most label comparisons become mental arithmetic.
Why do both units exist?
Purely history. The calorie arrived first, in early-1800s French thermochemistry, and it became entrenched in nutrition through Wilbur Atwater's food-energy research at the end of that century. By the time the modern SI system was formalised in 1960 and named the joule as its energy unit, calories had already been on food labels and in dietetics textbooks for decades.
What followed was a slow, incomplete migration:
- Australia and New Zealand label food in kilojoules only. Calories are essentially absent from packaging there.
- The European Union requires both. Regulation (EU) No 1169/2011 mandates that the energy value appear in kJ and kcal, per 100 g or 100 ml.
- The United States and Canada use calories only.
- The United Kingdom follows the EU dual-unit convention, though consumers there think and talk almost exclusively in calories.
Scientific literature has largely moved to kJ. Consumers have largely not. Neither unit is more accurate than the other — the choice is about which number a population already has an intuition for, and intuitions are slow to rebuild. This is why regulators that switched, notably Australia in the 1980s, faced years of public confusion, and why the EU compromise of printing both has proven durable.
Where the number on the label actually comes from
No one puts a chocolate bar in a bomb calorimeter. Label energy is calculated: the manufacturer measures how many grams of each energy-yielding component the food contains, then multiplies by a fixed factor. Annex XIV of EU Regulation 1169/2011 sets those factors:
| Component | kJ per g | kcal per g |
|---|---|---|
| Fat | 37 | 9 |
| Carbohydrate (except polyols) | 17 | 4 |
| Protein | 17 | 4 |
| Alcohol (ethanol) | 29 | 7 |
| Polyols | 10 | 2.4 |
| Organic acids | 13 | 3 |
| Fibre | 8 | 2 |
| Erythritol | 0 | 0 |
These are the Atwater factors, and they explain several things at once: why fat is the most calorie-dense macronutrient, why sugar alcohols in "reduced-calorie" products still carry some energy, and why fibre — which humans cannot digest but gut bacteria partly ferment — is credited with 2 kcal per gram rather than zero.
The consequence worth remembering: label calories are a standardised estimate, not a measurement of what your body extracted. Whole almonds are the classic case, delivering roughly 20 percent less usable energy than their label states because much of the fat stays locked inside intact plant cell walls.
Reference tables
| kJ | kcal |
|---|---|
| 50 | 12 |
| 100 | 24 |
| 200 | 48 |
| 250 | 60 |
| 500 | 120 |
| 750 | 179 |
| 1,000 | 239 |
| 1,500 | 359 |
| 2,000 | 478 |
| 2,500 | 598 |
| 3,000 | 717 |
| 4,000 | 956 |
| 5,000 | 1,195 |
| 6,000 | 1,434 |
| 7,000 | 1,673 |
| 8,000 | 1,912 |
| 8,700 | 2,079 |
| 9,000 | 2,151 |
| 10,000 | 2,390 |
| kcal | kJ |
|---|---|
| 10 | 42 |
| 25 | 105 |
| 50 | 209 |
| 100 | 418 |
| 150 | 628 |
| 200 | 837 |
| 250 | 1,046 |
| 300 | 1,255 |
| 400 | 1,674 |
| 500 | 2,092 |
| 750 | 3,138 |
| 1,000 | 4,184 |
| 1,200 | 5,021 |
| 1,500 | 6,276 |
| 1,800 | 7,531 |
| 2,000 | 8,368 |
| 2,500 | 10,460 |
| 3,000 | 12,552 |
Common questions
How many calories are in 1000 kJ?
1000 kJ is 239 kcal. Divide the kilojoule figure by 4.184 to get calories: 1000 ÷ 4.184 = 239.0 kcal.
How many kJ are in 1 calorie?
One food calorie (1 kcal) is 4.184 kJ. One small calorie (1 cal), the physics unit, is 4.184 joules — a thousand times smaller.
Is a calorie the same as a kilocalorie?
On food packaging, yes. The 'calorie' on a nutrition label is really a kilocalorie: 1000 small calories. Scientists write kcal; food labels and everyday speech usually just say calories. A 200-calorie snack contains 200 kcal, or 200,000 small calories.
Why do some countries use kJ and others use calories?
The kilojoule is the SI unit for energy, so countries that adopted the metric system fully — Australia, New Zealand, and much of the EU — label food in kJ. The United States and the United Kingdom kept the calorie because it was already familiar to consumers. EU law requires both units on the label.
What is 8700 kJ in calories?
8700 kJ is 2079 kcal. 8700 kJ is the average adult daily energy intake used as the reference value on Australian and New Zealand food labels, roughly equivalent to the 2000 kcal reference used in the US and EU.
Can I just divide kilojoules by 4?
For a rough mental estimate, yes. Dividing by 4 instead of 4.184 overestimates calories by about 4.6 percent. On a 2000 kcal day that is roughly 90 kcal, which is fine for a quick sanity check but not for careful tracking.
Does the body actually absorb all the calories on a label?
Not exactly. Label figures come from fixed conversion factors applied to the grams of protein, fat, carbohydrate, and fibre in the food. Real absorption varies: whole nuts deliver noticeably less energy than their label states because some fat stays trapped in intact cell walls, and cooking or processing can raise the usable energy of starches.
Look up a food
Every food page shows calories and kilojoules side by side, adjustable to any portion size, with the full macronutrient and micronutrient breakdown.
Work out what you need
Converting units tells you what is in the food. The basal metabolic rate page covers the other half: how much energy your body uses at rest, what raises or lowers it, and how large a deficit produces a given rate of weight loss.