What is the body mass index (BMI)?

Information written and reviewed by Certified Doctors.

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

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.

Content supplied by NHS Choices