BMI and waist-to-height calculator

Body mass index is the number most people meet first, and the one clinicians are least satisfied with. It is quick, free and useful as a starting point — and it cannot tell muscle from fat, or say where that fat sits. Work out your BMI below, add a waist measurement for the number current UK guidance cares about more, and read what both actually mean.

Before you use these numbers

The calculator on this page is a general information tool. It is not a medical assessment, it does not create a doctor–patient relationship, and the result it gives you is not a diagnosis.

BMI and waist-to-height ratio are screening measures. Both are interpreted differently depending on your age, your family background, how much muscle you carry and what else is happening in your health. A number on its own cannot tell you whether treatment is appropriate for you, and it is not a basis for starting, stopping or changing any medication.

Do not use this page as an alternative to advice from your GP or another qualified clinician, and do not delay seeking advice because of something you have read here. If you are unwell, contact your GP or NHS 111. In an emergency, call 999.

This tool is not intended for anyone under 18, or for use during pregnancy.

The classifications used here are taken from NICE guideline NG246, published January 2025. Your measurements are not stored and are not sent to us — the calculation happens entirely in your own browser.

The ranges in full

BMI is your weight in kilograms divided by your height in metres squared. Below 18.5 is classed as underweight, 18.5 to 24.9 as a healthy weight, 25 to 29.9 as overweight, 30 to 34.9 as obesity class 1, 35 to 39.9 as obesity class 2, and 40 or above as obesity class 3.

Those thresholds are lower for some people. NICE advises that people with a South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background are prone to central adiposity and face increased cardiometabolic risk at a lower BMI: overweight from 23, and obesity from 27.5.

Waist-to-height ratio is your waist measurement divided by your height, both in the same units. NICE classifies 0.4 to 0.49 as healthy with no increased health risks, 0.5 to 0.59 as increased central adiposity indicating increased health risks, and 0.6 or more as high central adiposity indicating further increased health risks. Which comes down to one sentence: keep your waist to less than half your height.

What to do with the number

If your BMI or your waist measurement has told you something you want to act on, the next step is an assessment rather than a second calculator. You can read why excess weight is treated as a medical condition, see what an assessment involves and what it costs, or book a free introductory appointment.