Health guide

What is BMI and what does it tell you?

Body mass index is the most widely used way to check whether your weight may be affecting your health. Here is what it measures, where it falls short, and how to use it well.

Published by Well Medicare, a GPhC-registered pharmacy in Marylebone, London · Last updated October 2026

Key points
  • BMI is your weight in kilograms divided by your height in metres squared.
  • It is a quick screening tool, not a diagnosis.
  • It cannot tell fat from muscle or show where fat is stored, so use it alongside your waist measurement.

Where BMI comes from

The formula was devised in the 1830s by the Belgian mathematician Adolphe Quetelet, who was studying the “average man”. It was renamed the body mass index in 1972 and later adopted by the World Health Organization because it is simple, cheap and works well for comparing large groups of people.

The adult ranges

  • Below 18.5 — underweight
  • 18.5 to 24.9 — healthy weight
  • 25 to 29.9 — overweight
  • 30 to 39.9 — obesity
  • 40 or more — severe obesity

For people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background, NICE uses lower cut-offs: 23 for overweight and 27.5 for obesity. Find out why.

Where BMI falls short

  • It ignores body composition. A rugby player and someone with little muscle can have the same BMI and very different health.
  • It ignores fat distribution. Fat around the organs is far riskier than fat on the hips and thighs.
  • It changes with age. Muscle naturally declines with age, so the same BMI can mean more body fat later in life.
  • It is not used in pregnancy or for children, who are assessed with age- and sex-specific growth charts.

Better together: BMI and your waist

NICE recommends measuring waist-to-height ratio alongside BMI for adults with a BMI under 35. Keeping your waist below half your height is a simple target that works for everyone. Learn how to measure your waist correctly.

How clinicians use BMI

Healthcare professionals use BMI as a starting point: to spot people who may benefit from support, and to decide eligibility for treatments. Medical weight-loss programmes, for example, are usually considered from a BMI of 30, or 27 with a weight-related condition. A full assessment always looks at the whole person — health history, medicines, waist size and goals — not just one number.

Not sure if medical weight loss is right for you?

Our free online assessment takes about five minutes. A GPhC-registered prescriber reviews it and only recommends treatment if it is safe and suitable for you.

Start the free assessment

Frequently asked questions

Is BMI accurate?

It is accurate as a calculation and a useful screening tool, but it does not measure body fat directly. Combining it with your waist measurement gives a much better picture of health risk.

Is BMI different for men and women?

The adult ranges are the same. At the same BMI, women tend to carry more body fat than men, which is one reason BMI is best used alongside other measures.

What is a good BMI for my age?

The same adult ranges apply at every age from 18. In older adults, BMI is a less precise guide because of muscle loss, so clinicians tend to interpret it more flexibly and pay more attention to strength and waist size.

This page is general health information. It does not replace advice from your GP or another healthcare professional who knows your history. If you have symptoms that worry you, contact your GP or NHS 111; in an emergency call 999. Prescription treatment is only supplied after an assessment by a registered prescriber.