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What are calories and kilojoules?

Depending on what country you live in, you will find nutrition data listed in calories, kilojoules, or both. Our calculators give you the option of calculating in either calories (kcal) or kilojoules (kJ).

A calorie is a unit of measure of the energy required to raise the temperature of 1 gram of water by 1?C. A joule is a unit of electrical energy, commonly used in the physical sciences, equal to the work done when a current of one ampere is passed through a resistance of one ohm for one second. Of course we don't use energy to raise the temperature of water or pass current through resistance, but similar processes maintain our body temperature and perform other bodily functions.

Because calories and joules are so small, when referring to food and energy expenditure it has become common practice to refer to them in multiples of 1,000. The term for 1,000 calories is kilocalories or kcal, and the term for 1,000 joules is kilojoules or kJ.

1 calorie = 4.184 joules

1 kilocalorie (kcal) = 4.184 kilojoules (kJ)

However, outside of scientific literature, it has also become common practice to refer to kilocalories as calories. Therefore when you read 500 calories on a food label it actually means 500 kilocalories, and the same holds true when you calculate an activity that burns 500 calories.

How many calories are there in a pound of body fat?

If you were to burn 500 calories more than you consume each day, you would lose approximately one pound each week:

500 x 7 = 3,500

While the number 3,500 is commonly used to represent the number of calories in a pound of body fat, keep in mind that it is only approximate. Weight loss will vary with each individual.

Also, depending on your body composition (percent muscle vs. fat), when you lose weight it may not be all fat. The more overweight you are the greater the likelihood that you will be losing mostly fat; the more lean you are the greater the likelihood that you will be losing some muscle, too.

How many calories are there in a gram of food?

Yes, that means that each gram of fat you consume provides more than twice as many calories as a gram of protein or carbohydrate!

As an example of how these numbers are used, imagine a food containing 10 grams of protein, 10 grams of fat, and 10 grams of carbohydrates. That would total 170 calories:

(10 g protein x 4) + (10 g fat x 9) + (10 g carbs x 4) = 170

In this imaginary food 40 calories come from protein; 90 calories come from fat; and 40 calories come from carbohydrates.

Calories burned calculations include the total number of calories burned during the period of time calculated. Therefore when calculating how many calories you need in a day, do not add your BMR or RMR.

In theory:

If you used your current weight and consume the number of calories indicated, your weight will stay the same.

If you used your goal weight and consume the number of calories indicated, in time your weight will match your goal.

Because additional calories are burned by your body processing the food you eat, you should be able to eat a little more than indicated by the calories burned calculation. See What is the thermic effect of food? for further information.

Remember that the above statements are only theoretical and that the figures are only estimates. Through trial and error you will find what works well for you.