What is the body mass index (BMI)?

The BMI calculation divides an adult's weight in kilograms by their height in metres squared. For example, A BMI of 25 means 25kg/m2.

BMI ranges

For most adults, an ideal BMI is in the 18.5 to 24.9 range.

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

If your BMI is:

• below 18.5 – you're in the underweight range
• between 18.5 and 24.9 – you're in the healthy weight range
• between 25 and 29.9 – you're in the overweight range
• between 30 and 39.9 – you're in the obese range

How can I work out my body mass index (BMI)?

To work out your BMI:

• divide your weight in kilograms (kg) by your height in metres (m)
• then divide the answer by your height again to get your BMI

For example:

• if you weigh 70kg and you're 1.75m tall, divide 70 by 1.75 – the answer is 40
• then divide 40 by 1.75 – the answer is 22.9
• your BMI is 22.9kg/m2

Accuracy of BMI

BMI takes into account natural variations in body shape, giving a healthy weight range for a particular height.

As well as measuring your BMI, healthcare professionals may take other factors into account when assessing if you're a healthy weight.

Muscle is much denser than fat, so very muscular people, such as heavyweight boxers, weight trainers and athletes, may be a healthy weight even though their BMI is classed as obese.

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.

You should not use BMI as a measure if you're pregnant. Get advice from your midwife or doctor if you're concerned about your weight.