Kilocalories to joules:
multiply by 4,184

How many joules are in a kilocalorie?

One kilocalorie is 4,184 joules, or 4.184 kilojoules. The kilocalorie, also called a food Calorie, is the unit nutrition labels use for the energy in what we eat, while the joule is the SI unit used in science.

The calorie is defined here with the thermochemical value of 4.184 joules, so one kilocalorie is exactly 4,184 joules. A small calorie is a thousandth of that, which is why a kilocalorie is sometimes written as a capital C Calorie.

The kcal to joules formula

A second example runs in reverse. A science problem gives 1,350,000 J of heat released. Dividing by 4,184 gives 322.6577 kcal, so it is roughly the energy in a small meal.

Chart: kcal to joules

For other values, scale a row. A 150 kcal drink is 100 kcal plus 50 kcal, so 418,400 + 209,200 = 627,600 J, which is 627.6 kJ.

Where kcal to joules is used

• Nutrition labels: countries that list energy in kJ ask you to convert from the kcal figure on older recipes and diet plans.
• Science classes: calorimetry problems may give data in calories and ask for joules.
• Fitness trackers: some devices show kcal while research papers report kJ.
• Cooking and diet planning: a daily target of 2,000 kcal is 8,368 kJ.

Why there is more than one calorie

The word calorie covers several related units. The small calorie is about 4.184 joules, the kilocalorie is 1,000 small calories, and the food Calorie is another name for the kilocalorie. A further version, the international table calorie, is 4.1868 joules, which differs from the thermochemical one by about 0.07 percent.

This is why you may see slightly different factors online. For food labels, the difference is far too small to matter. This site uses 4,184 joules per kilocalorie, so you can reproduce the chart by hand.

A short worked check with a meal

A lunch of 650 kcal is 650 × 4,184 = 2,719,600 J, which is 2,719.6 kJ. As a quick check, 650 × 4 is 2,600 kJ, and adding 5 percent gives about 2,730 kJ, which is close. If the answer you get is near 2,720 or 2,719,600 you are in the right place, depending on whether you wanted kJ or J.

Common mistakes with kcal and joules

• Mixing calories and kilocalories. A food of 250 kcal is 250,000 small calories, so check whether a value is in cal or kcal.
• Using 4.184 for joules instead of kilojoules. Multiplying kcal by 4.184 gives kJ, while multiplying by 4,184 gives J.
• Rounding to 4 or 4.2 too early. On a 2,000 kcal target, using 4.2 gives 8,400 kJ against an exact 8,368 kJ.
• Confusing kcal with kWh. One kWh is about 860.4 kcal, so the units differ by a large factor.