The body mass index (BMI) is a measure that uses your height and weight to work out if your weight is healthy.

Who can use the calculator

The BMI calculator is suitable for adults and children aged 2 years old and over.

The BMI calculator may not be suitable if you think you have an eating disorder or are being treated for one. Ask a GP for advice.

If you are pregnant, use your pre-pregnancy weight when calculating your BMI.

BMI weight ranges

Adults

A BMI calculation in the healthy weight range is between 18.5 to 24.9.

For Black, Asian and some other minority ethnic groups, the healthy weight range is 18.5 to 23.

For people of White heritage, a BMI:

below 18.5 is underweight
between 18.5 and 24.9 is healthy
between 25 and 29.9 is overweight
of 30 or over is obese

Black, Asian and some other minority ethnic groups have a higher risk of developing some long-term conditions such as type 2 diabetes with a lower BMI. People from these groups with a BMI of:

23 or more are at increased risk (overweight)
27.5 or more are at high risk (obese)

Children's BMI

For children and young people aged between 2 to 18 years old, the BMI calculator takes into account age and gender as well as height and weight.

A child's BMI is given as a "centile". The centile number shows how their BMI compares with other children of the same age and sex as a percentage.

For example, a girl on the 75th centile is heavier than 75 out of 100 other girls her age.

The BMI calculator works out if a child or young person is:

underweight – on the 2nd centile or below
a healthy weight – between the 2nd and 91st centiles
overweight – 91st centile or above
very overweight – 98th centile or above

See a GP if you're concerned about your child's weight. They may be able to refer you to your local healthy lifestyle programme for children, young people and families.

Limitations of the BMI

Muscle mass

The BMI calculation does not include muscle mass, which weighs more than fat.

This means muscular adults and athletes may be classed as overweight or obese even though their body fat is low.

BMI results

Underweight

Being underweight could be a sign you're not eating enough or you may be ill. If you're underweight, a GP can help.

Find out more in underweight adults

If you have an eating disorder, the BMI calculation results do not apply. Please get further advice from a GP.

Healthy weight

Find out more about having a balanced diet and find fitness videos you can try at home to maintain a healthy weight.

Overweight

If you want to lose weight, you can find more information at NHS Better Health.

Obese

Read more information about obesity. You may want to see a GP for help and advice.

Waist to height ratio

Measuring your waist to height ratio can tell you if you have excess tummy fat, even if you have a healthy BMI.

To calculate your waist to height ratio, measure your waist and divide it by your height. Use measurements in the same units (for example, centimetres or inches).

A waist to height ratio of 0.5 or higher means you may have increased health risks such as heart disease, type 2 diabetes and stroke.

To measure your waist:

Find the bottom of your ribs and the top of your hips (just above your belly button).
Wrap a tape measure around your waist midway between these points.
Breathe out naturally before taking the measurement.

How the BMI is calculated

The BMI calculation divides an adult's weight in kilograms (kg) by their height in metres (m) squared.

For example, if you weigh 70kg (around 11 stone) and are 1.73m (around 5 feet 8 inches) tall, you work out your BMI by:

squaring your height: 1.73x1.73 = 2.99
dividing 70 by 2.99 = 23.41

Your result will be displayed to 1 decimal place, for example 23.4.

A child's BMI is expressed as a "centile" to show how their BMI compares with other children of the same age and sex as a percentage.