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). If you've never heard of kilojoules, you won't need to worry about them.
A scientific explanation of calories and kilojoules is given in the Technical Notes, below. But for practical purposes, they are a measure of the energy contained in both the foods we eat and our body fat. Our bodies use the energy found in the food we eat to keep us running, and store any excess as body fat for future use.
When we expend energy it is said that we are burning calories, and when we burn more calories than we eat our bodies turn to our fat stores to find the additional energy they require. Thus when we eat more calories than we burn we gain weight, and when we burn more calories than we eat we lose weight.
We have received many letters from people grateful to learn this single point. Confused by the countless rules of countless diets, they entirely missed the single most important fundamental of weight loss. It bears repeating:
Eat more calories than you burn and you will gain weight; eat fewer calories than you burn and you will lose weight.
Calories are also used to measure the energy required to perform different activities, including the energy required just to keep our bodies running. It is therefore possible to calculate how many calories you burn in a day, which equates to the number you would eat to maintain your current weight.
If you want to lose weight, you can use the Weight Loss Calculator to calculate how many more calories you need to burn each day than you eat. This figure is called a calorie deficit.
calories eaten - calories burned = calorie deficit
Once you determine the calorie deficit you'd like to achieve, you can use the Food Calculator to calculate how much less you need to eat, or the Activity Calculator to calculate how much more you need to exercise. Or you can achieve the deficit you desire with a combination of eating less and exercising more.
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 a 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)
In the scientific and educational communities, it is also common practice to refer to kilocalories as Calories (with an uppercase "c"). However, outside these communities, it has become common practice to simply 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.