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We measure the amount of energy contained in an item of food in calories, just as we measure the weight of that item of food in kilograms.
If you’re trying to lose weight, it’s a good idea to eat less and be more active. Eating less is important when you're trying to lose weight, even if you already have a balanced diet.
You can check whether you’re a healthy weight by using our BMI healthy weight calculator.
Calories and energy balance
When we eat and drink, we’re putting energy (calories) into our bodies. Our bodies then use up that energy, and the more physical activity we do, the more energy (calories) we use.
To maintain a stable weight, the energy we put into our bodies must be the same as the energy we use by normal bodily functions and physical activity. If there are some days where we put in more energy than we use, then there should also be days where the opposite is true, so that overall the energy in and energy used remain balanced.
Weight gain occurs when we regularly put more energy into our bodies than we use. Over time, that excess energy is stored by the body as fat. Research shows that most adults eat and drink more than they need, and think that they are more physically active than they are.
Checking calories in food
Knowing the calorie content of foods can be a useful tool when it comes to achieving or maintaining a healthy weight. It can help us to keep track of the amount of energy we are eating and drinking, and ensure we're not consuming too much.
The calorie content of many foods is stated on the packaging in the nutrition label, which you will often find on the back or side of the packaging. This information will appear under the "Energy" heading. The calorie content is often given in kcals, which is short for "kilocalories", and also in kJ, which is short for "kilojoules".
A "kilocalorie" is another word for what is commonly called a "calorie", so 1,000 calories will be written as 1,000kcals.
Kilojoules are the metric measurement of calories. To find the energy content in kilojoules, multiply the calorie figure by 4.2.
The label will usually tell you how many calories are contained in 100 grams or 100 millilitres of the food or drink, so you can compare the calorie content of different products. Many labels will also state the number of calories in "one portion" of the food. But remember that the manufacturer’s idea of "one portion" may not be the same as yours, so there could be more calories in the portion you serve yourself.
You can use the calorie information to assess how a particular food fits into your daily calorie intake. As a guide, the average mans needs 2,500kcal (10,500kJ) to maintain his weight, and the average woman needs 2,000kcal (8,400kJ).
Some restuarants put calorie information on their menus, so you can also check the calorie content of foods when eating out. Calories should be given per portion or per meal.
You can learn more on our page on food labels.
The amount of calories people use by doing a certain physical activity varies, depending on a range of factors, including size and age.
The more vigorously you do an activity, the more calories you will use. For example, fast walking will use more calories than walking at a moderate pace.
If you’re gaining weight, it usually means you’ve been regularly eating and drinking more calories than you've been using through normal bodily functions and physical activity.
To lose weight, you have to tip that balance in the other direction. You must start to use more energy than you consume, and do this over a sustained period of time.
You can do this by making healthy changes to your diet so that you eat and drink fewer calories. Learn more on our pages on food and diet.
The best approach is to combine these changes with increased physical activity. Find out what the physical activity guidelines for adults are.
You can learn more about making healthy changes to your diet in our lose weight section.
You can also talk to your GP or practice nurse to get more advice on achieving the right energy balance and losing weight.