What is the body mass index (BMI)?

BMI is a measure that adults can use to see if they are a healthy weight for their height.

What is a healthy BMI?

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

If your BMI is 25 or more, you weigh more than is ideal for your height:

  • 25-29.9 is overweight
  • 30-39.9 is obese
  • 40 or more is very obese

If your BMI is less than 18.5, you weigh less than is ideal for your height.

If you want to calculate your BMI, see How can I work out my BMI? or try our healthy weight calculator.

These ranges are only for adults. BMI is interpreted differently for children. If you're concerned about your child's weight, seek advice from your GP.

Overweight BMI

If your BMI is 25 or more, you should think about losing weight. Being overweight or obese can increase your risk of health problems, such as:

Healthcare professionals use the words ‘obese’ and ‘obesity’ as clinical terms to describe your increased risk of health problems. They do not use these terms to describe what you look like. Read more information about obesity.

Talk to your GP before starting a weight loss programme if you have a long-term health condition, such as type 2 diabetes or heart failure.

If you intend to go on a low-fat or low-calorie diet to achieve gradual weight loss, you should seek advice from your GP beforehand. They can offer you help, support and advice before you start and during your diet.

Underweight BMI

If your BMI is less than 18.5, you may want to talk to your GP about gaining weight. Being underweight can also increase your risk of health problems, such as:

For more information, see How can I gain weight safely?

Accuracy of BMI

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 GP if you're concerned about your weight.

Read the answers to questions about exercise.

Further information:


Video: understanding BMI results in adults

An expert explains how to calculate your BMI (body mass index) and interpret the results correctly. He also discusses the health risks associated with a BMI that is too low or too high.

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Page last reviewed: 21/07/2014

Next review due: 20/07/2016