https://www.nhs.uk/live-well/healthy-weight/managing-your-weight/understanding-calories/

The amount of energy in food or drink is measured in calories.

Why calories are important

You need energy from calories for your body to work properly. Your body uses this energy to function properly.

To stay at around the same weight, the calories your body uses should be the same as the amount of calories you eat and drink.

If you do not use the same amount of calories as you eat and drink, your body weight may change. For example:

• you’re likely to put on weight if you eat and drink more calories than you use. This is because your body stores the extra energy as fat
• you’re likely to lose weight if you eat and drink fewer calories than you use. This is because your body uses its stored fat for energy

Daily calories

Calorie information is often given in kcals, which is short for kilocalories. It may also be given in kJ, which is short for kilojoules.

As a guide:

• an average man needs 2,500kcal a day
• an average woman needs 2,000kcal a day

This could be different based on your:

• age
• weight
• height
• how much exercise you do