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Body Mass Index (BMI) is a calculation that determines whether an adult is of a healthy weight.


Your BMI is calculated by dividing your weight in kilograms by your height in metres squared. If you’re aged between 2 to 18, your BMI calculation will also include your age and gender.

For most adults, a healthy BMI is between 18.5 to 24.9 kg/m2. Numbers that are fewer or more than this may mean you’re under- or overweight.

Our BMI calculator tool has been designed for adults and is not suitable for those under 18.


Different BMI numbers may mean different things. Let’s go through them:

  • A BMI below 18.5
    This is classed as the underweight range. Being underweight is not good for your health, as it could mean that you have nutritional deficiencies, a weakened immune system or fertility problems. You may be underweight because you're not eating enough, or you may be ill. But being underweight could also mean that you have a fast metabolism. If you're underweight, it may be advisable to seek advice from your GP.

  • A BMI between 18.5 and 25
    This means that you're in the healthy weight range. Being a healthy weight means that you are at a lower risk of certain diseases. In adulthood, people typically gain around 1kg per year, which could make your BMI fluctuate, so it is important to keep an eye on this and try to maintain a healthy weight.

  • A BMI between 25 and 30
    This means that you're in the overweight range. Being overweight puts you at greater risk of disease such as heart disease, high blood pressure and type 2 diabetes. Weight gain is generally caused by an energy imbalance where you consume more calories than you burn off through physical activity. The excess energy is stored by the body as fat. However, there can be many factors that contribute to  this including biological, psychological and social factors. If you’re overweight, we’d recommend assessing the amount of exercise you do per week compared to your calorie intake or eating habits. This could give you an indication of how to reduce your BMI to a healthy weight by either eating healthier or doing more physical exercise.

  • A BMI over 30
    This is classed as obese. Obesity is excessive weight with a lot of body fat. Obesity is a common problem in the UK, which is estimated to affect around 1 in every 4 adults. Obesity further increases the risk of diseases such as heart disease, high blood pressure, type 2 diabetes and some cancers. Obesity reduces life expectancy by an average of 3 to 10 years, depending on how severe it is. Obesity can cause difficulties with daily activities including breathlessness and joint pain and it can also impact mental health. The best way to treat obesity and maintain a healthy weight is to eat a healthy balanced diet and exercise regularly – this helps to achieve a favourable energy balance.

Keep in mind, these are just indications. There may be a few different reasons why your BMI isn’t what you thought it would be. If you’re worried or want to find out more, we’d recommend chatting to your GP.


For the vast majority of people BMI is a very good measure of weight status as it takes into account natural variations in body shape, giving a healthy weight range for a particular height. However, it does have its limitations. Muscle is much denser than fat, so very muscular people, such as elite heavyweight boxers and athletes, may be a healthy weight even though their BMI is classed as overweight.

Your ethnic group can also affect your risk of some health conditions. For example, adults of Asian origin may have a higher risk of health problems at BMI levels below 25.

BMI should not be used as a guide to weight if you're pregnant. Get advice from your midwife or GP if you are pregnant and you're concerned about your weight.