What's my body mass index (BMI)?

  • Body Mass Index (BMI) is a good way to tell if you are a healthy weight for your height
  • You can use the calculator on this page to work out yours
  • Measuring your waist is another way to see if you are a healthy weight

What is BMI?

BMI stands for Body Mass Index. It tells you if you are a healthy weight for your height. BMI is worked out by dividing a person’s weight by their height squared. This gives a number which tells us whether someone is in a healthy weight range, underweight, overweight or obese. There’s a BMI calculator further down the page that work this out for you.

For most adults a BMI of

  • Under 18.5 means they are underweight
  • 18.5 to 24.9 means they are a healthy weight
  • 25 to 29.9 means they are overweight
  • 30 or over means they are obese

Having a BMI higher than 25 increases the risk of 13 types of cancer.


    Is BMI reliable?

    Yes, BMI works for most people, and is a good way of telling if someone has a healthy or unhealthy weight. But, because BMI isn’t a direct measure of body fat, there are a few exceptions to this, such as:

    • Professional athletes and bodybuilders - people with lots of muscle and low body fat could be classed as overweight or obese
    • Children and young people (up to age 18) – a different calculation is used for a child’s BMI. You can select ‘child’ on the calculator below


    BMI Calculator

    Calculate your BMI below and find out if you have a healthy weight for your height.


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    Is waist size important?

    Measuring your waist is another useful way to see if you are a healthy weight.

    Put the tape measure about an inch above your belly button. If you don’t have a tape measure, try using a piece of string and a ruler.

    A healthy waist measurement is:

    • Less than 31.5 inches (80cm) for most women
    • Less than 37 inches (94cm) for most men

    Your ethnic background can influence what a healthy weight is for you. The cut off point for being considered overweight or obese may be a little lower for people of Black, Asian, some Middle Eastern or mixed ethnicities. Talk to a doctor or nurse for more information and support.

    If you have any questions or concerns about your BMI or waist measurement, talk to your GP or nurse who’ll be able to help.


    Adab, P., Pallan, M. & Whincup, P. H. Is BMI the best measure of obesity? BMJ 360, 15–16 (2018).

    NICE. Obesity: identifying, assessing and managing obesity in adults, young people and children. 1–14 (2014).

    Freisling, H. et al. Comparison of general obesity and measures of body fat distribution in older adults in relation to cancer risk: Meta-analysis of individual participant data of seven prospective cohorts in Europe. Br. J. Cancer 116, 1486–1497 (2017).

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    My BMI is more than 25 – what can I do?


    A BMI higher than 25 doesn’t mean you’ll definitely develop cancer. There are lots of steps you can take to reduce your risk.

    If you have any questions or concerns about your BMI or waist measurement, talk to your GP or nurse.