Quick answer
Kilojoules (kJ) and calories (kcal) both measure energy. 1 kcal = 4.184 kJ. The kJ number is always about four times larger than the calorie number for the same food.
The three units side by side
| Unit | Symbol | Size | Where you see it |
|---|---|---|---|
| Joule | J | SI unit of energy | Science, physics, engineering |
| Kilojoule | kJ | 1,000 J | Food labels in Australia and New Zealand; EU and UK labels alongside kcal |
| Calorie (small) | cal | 4.184 J | Chemistry and physics |
| Kilocalorie (food Calorie) | kcal or Cal | 1,000 cal = 4.184 kJ | Food labels, diet plans, fitness apps |
Conversion between kJ, cal and kcal
| From | To kJ | To kcal | To cal |
|---|---|---|---|
| 1 kJ | 1 | 0.239006 | 239.006 |
| 1 kcal | 4.184 | 1 | 1,000 |
| 1 cal | 0.004184 | 0.001 | 1 |
To convert between them, use the KJ to Cal converter or read how to convert kJ to calories.
Why does the word "calorie" mean two things?
The original calorie was a small unit, the heat needed to warm one gram of water by one degree Celsius. It was too small to describe food, so nutrition adopted the kilocalorie (1,000 calories) and shortened the name back to "calorie", sometimes marked with a capital C. That is why a snack labelled "200 calories" holds 200 kcal, not 200 small calories.
Why do labels use kJ in some countries and kcal in others?
- Australia and New Zealand: energy is shown in kilojoules. Kilocalories may also appear but are optional.
- European Union and United Kingdom: labels show both kJ and kcal.
- United States and Canada: labels show Calories (kcal).
The joule is the official SI unit for energy, and most countries have moved toward it in science and regulation. Diet culture, fitness apps and everyday language have kept the calorie alive. If you travel, shop online or use overseas recipes, you will meet both.
Are kJ or calories more accurate?
Neither is more accurate. They are two scales for the same physical quantity, like centimetres and inches. A number in kJ is simply bigger, because the unit is smaller. Choose whichever your labels, doctor or app uses, and convert only when you must compare across sources.
How much energy per gram?
Food labels calculate energy from the amount of each nutrient using standard factors:
| Nutrient | kJ per gram | kcal per gram |
|---|---|---|
| Carbohydrate | 17 | 4 |
| Protein | 17 | 4 |
| Fat | 37 | 9 |
| Alcohol | 29 | 7 |
| Dietary fibre | 8 | 2 |
Because these factors are rounded separately in each unit, the kJ and kcal printed on a label may not follow 4.184 exactly. For example, fat at 37 kJ and 9 kcal per gram gives a ratio of 4.11, not 4.184. That is normal and not a mistake.
Other calorie definitions
Scientists have defined the calorie in slightly different ways. The thermochemical calorie is exactly 4.184 J and is used here. The International Table calorie is 4.1868 J, which is about 0.07% larger. For food and everyday work the difference is too small to matter. Our methodology page explains which definition we use and why.
Which should I use?
Use the unit your source uses. If you are following an Australian plan, count kJ. If you are following a US diet, count kcal. If a plan and a label use different units, convert the label once using the calories to kJ converter or the KJ to Cal tool and stay consistent.
Frequently asked questions
Is a kilojoule bigger than a calorie?
A kilojoule is smaller than a kilocalorie. It takes 4.184 kJ to equal 1 kcal. It is bigger than a small calorie, since 1 kJ is 239.006 cal.
Do kJ and calories measure the same thing?
Yes. Both measure energy, so you can convert freely between them using 1 kcal = 4.184 kJ.
Is cal the same as kcal?
No. A small cal is 1,000 times smaller than a kcal. On food labels, though, "calories" almost always means kcal.
Which is better for weight loss, kJ or calories?
Neither. What matters is total energy intake compared with what your body uses. Pick one unit and track it consistently.
Last updated 11 Oct 2026. How we calculate.