Body mass index (BMI)

BMI is a measurement used to work out if you are a healthy weight for your height. This is something you can’t tell from just weighing yourself, because whether you’re a healthy weight depends on how tall you are too.

BMI is an indicator of how much body fat you have, which is important for working out if your weight is likely to affect your health.

BMI is only a guide to healthy weight. It’s aimed at healthy adults, and doesn’t give a useful measure for children, older people, muscular athletes or people belonging to some ethnic groups.

How to work out your BMI and BMI calculator

To work out your BMI, you first need to know your height in metres (m) and your weight in kilograms (kg). You can then calculate your BMI by dividing your weight by your height, then dividing the answer you get by your height again. You can also try our BMI calculator below

So BMI is:

• weight (kg)
• height (m) x height (m)

What does the BMI measurement mean?

BMI values are used to put healthy adults into the following categories: underweight, healthy weight, overweight, obese or morbidly obese. The BMI values corresponding to each of these categories are shown in the table below.

These BMI categories are the same for both men and women.

Underweight

If your BMI is under 18.5, you’re classed as underweight. Being underweight for a long period of time can lead to long-term health problems, such as osteoporosis and damage to your heart.

Being underweight can also affect fertility. Women who are underweight often have no menstrual bleeding (this is known as amenorrhoea) and may have difficulty getting pregnant. Men who are underweight are likely to have poor sperm quality.

Healthy weight

According to the World Health Organization (WHO), you are a healthy weight for your height if your BMI is between 18.5 and 24.9.

Overweight

If your BMI is between 25 and 29.9, you’re classed as being overweight. Being overweight increases your risk of health problems. If you’re overweight, you’re more likely to develop problems such as high blood pressure, heart disease, stroke, diabetes, and some types of cancer. Excess weight also makes you more likely to get arthritis and can make breathing and sleeping difficult.

Women in the overweight category are more likely to have difficulty becoming pregnant and having problems (such as high blood pressure and a condition called pre-eclampsia) during pregnancy.

Obese and morbidly obese

If your BMI is over 30, you’re classed as obese. Being ‘obese’ means that you have put on enough weight to seriously harm your health.

If your BMI is greater than 40, you’re classed as ‘morbidly obese’. This means that the amount you’re overweight is harmful to your health. The more overweight you are, the more likely you are to have health problems.

If you are overweight or obese, it’s important that you try to lose excess weight by eating a healthy balanced diet and being physically active. Don’t go on a “crash diet” as this can be harmful to your health and won’t help you keep the excess weight off in the long term. Your GP or dietitian can give you advice about maintaining a healthy weight.

Limitations of BMI

In certain people, BMI won’t give an accurate measure of healthy weight. This is because BMI only uses your height and weight, so it doesn’t take into account how the weight is distributed around your body, how much muscle you have or how heavy your bones are.

BMI isn’t appropriate for the following groups of people.

Pregnant women

Your BMI is important if you’re trying to get pregnant. Low and high BMI are both associated with fertility problems, so it’s important that you maintain a healthy weight if you’re planning to start a family. But once you’re pregnant you shouldn’t use BMI to work out if you’re a healthy weight.

Older people – aged over 60

BMI may not be accurate if you’re over 60, because as you get older your bones start to get less dense and so weigh less. Because of this, the BMI measurement may underestimate your body fat (eg it could put you in the ‘underweight’ range if you’re a healthy weight, or put you in the ‘healthy weight’ range if you’re overweight).

Younger people – children

BMI alone isn’t useful for monitoring the weight of children. This is because children’s bodies change as they grow – for example, babies naturally have much more body fat than toddlers. When your child’s BMI is calculated, it will be plotted on a special graph that also takes into account gender and age. The location of your child’s BMI on this graph will show if he or she is a healthy weight.

Athletes/bodybuilders

If you have well-developed muscles from training for a sport, your BMI measurement may overestimate your body fat. This is because BMI doesn’t take into account your increased muscle mass. You may find that your BMI measurement puts you in the ‘overweight’ category, even though you may have little body fat and a healthy body shape.

Some ethnic groups, eg Asian populations

Populations around the world have varying amounts and distributions of body fat. For example, it’s been shown that the BMI classification scale isn’t completely accurate for people of Asian origin, as the link between BMI and health risks differ to that of the European populations. For example a BMI of 22 for an Asian person may have a health risk the same as a BMI of 25 for a Caucasian person.