Calories to Kilojoules
Convert calorie to kilojoule instantly — type a value and read the result, with the exact formula shown.
Example
1 cal = 0.004184 kJ, 5 cal = 0.02092 kJ, 10 cal = 0.04184 kJ.
How it works
Kilojoule = Calorie × 0.004184. Every value is converted through a single joule base unit using internationally defined conversion factors, so any from/to pair stays consistent.
Good to know
Converting calories to kilojoules matters most when you are reading energy values that use different regional conventions for the same food, fuel, or chemical reaction. Australia, New Zealand and much of Europe print kilojoules on nutrition labels, while older charts, US recipes, and laboratory data often stick with calories. Translating between the two lets you compare a snack labelled in kJ against a diet plan written in calories without guessing.
The calorie is the older, non-SI unit, originally defined as the energy needed to warm one gram of water by one degree Celsius. The kilojoule belongs to the metric SI system, where the joule is the base unit of energy and one kilojoule equals 1,000 joules. Because the modern thermochemical calorie is fixed at exactly 4.184 joules, the small calorie used here converts at exactly 0.004184 kJ — an exact definition, not an approximation.
For mental math, remember that roughly 239 calories make up a single kilojoule, or flip it the easy way: every calorie is about four-thousandths of a kilojoule. A quick shortcut is to take your calorie figure, multiply by four, and shift the decimal — 1,000 cal lands near 4.18 kJ, close enough for a sanity check.
- The biggest trap is the small calorie (cal) versus the food Calorie (kcal): nutrition labels use kilocalories, so 1 food Calorie is actually 4.184 kJ, a thousand times larger than the 1 cal = 0.004184 kJ used on this page.
Frequently asked questions
How do you convert calorie to kilojoule?
Multiply the number of calories by 0.004184 to get kilojoules. For example, 1 cal = 0.004184 kJ.
What is 1 calorie in kilojoules?
1 calorie equals 0.004184 kilojoules (1 cal = 0.004184 kJ).
How many calories are in 1 kilojoule?
There are 239.0057 calories in 1 kilojoule.
Is this converter free and private?
Yes. It runs entirely in your browser, so your inputs never leave your device, there is no sign-up, and it works offline once loaded.
Are the conversions exact?
Conversions use internationally defined factors and are exact where the definitions are exact (for example, 1 inch = 2.54 cm). Displayed results are rounded for readability.
People also ask
Is a calorie the same as a kilocalorie when converting to kilojoules?
No. This page converts the small calorie (cal), where 1 cal = 0.004184 kJ. The food Calorie on nutrition labels is a kilocalorie (kcal), and 1 kcal = 4.184 kJ — a thousand times more energy.
How many kilojoules are in 100 calories?
100 small calories equal 0.4184 kJ (100 × 0.004184). If you mean 100 food Calories (kcal), that is 418.4 kJ.
Why is the conversion factor exactly 0.004184?
Because the thermochemical calorie is defined as exactly 4.184 joules, and a kilojoule is 1,000 joules. Dividing 4.184 by 1,000 gives the exact factor of 0.004184 kJ per calorie.
Which countries use kilojoules instead of calories on food labels?
Australia and New Zealand require kilojoules as the primary energy unit, and the EU lists kilojoules alongside kilocalories. The United States and many older charts use calories or food Calories.
How do I quickly estimate calories to kilojoules in my head?
Multiply the calorie figure by about 0.004, or roughly four-thousandths. For a rough check, 250 cal is close to 1 kJ since about 239 calories make one kilojoule.
How do I convert kilojoules back to calories?
Divide the kilojoule value by 0.004184, or multiply by 239.0057. So 1 kJ equals about 239 small calories, and 10 kJ equals roughly 2,390 calories.
Is 4.184 the same as 4.1868 for calories?
They are slightly different definitions. The thermochemical calorie used here is exactly 4.184 J, while the International Table (IT) calorie is 4.1868 J. The difference is under 0.07 percent and rarely matters outside precise lab work.
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