BMI Calculator UK
Check your adult BMI instantly in kg and cm, stone and pounds, or US pounds, then add waist and body composition context. Results update as you type and never leave your browser.
Shown to one decimal place: BMI is a screening estimate, and extra precision would be false precision.
Body composition: context beyond BMI
BMI cannot see where weight sits. Waist and body fat estimates add the context the NHS and NICE increasingly emphasise.
Body fat % (US Navy method)
Waist circumference risk
Thresholds: raised from 94 cm (men) and 80 cm (women); very high from 102 cm and 88 cm.
Waist-to-hip ratio
Skinfold body fat (3-site)
Jackson-Pollock equation; needs callipers, used by UK sports scientists.
Use the number as a guide, not a diagnosis
BMI is quick but it cannot see muscle mass, pregnancy, age-related changes, ethnicity, symptoms, medication or history. It cannot tell you whether any treatment is suitable: that is a clinical decision involving far more than one number. Waist measurement and clinical judgement matter too.
How the estimates are worked out
BMI is weight in kilograms divided by height in metres squared, shown to one decimal place. Body fat uses the US Navy circumference method; skinfold uses the Jackson-Pollock 3-site equation; waist thresholds follow NHS guidance. All figures are educational estimates, not medical advice.
BMI questions
What is BMI?
BMI means body mass index. It compares your weight with your height and gives a quick adult screening estimate. It is useful, but it does not measure body fat directly.
Should I use BMI or waist measurement?
Use both if you can. BMI gives a quick height-and-weight estimate, while waist measurement adds context about central body fat and possible health risk.
Can this calculator tell me if I qualify for treatment?
No. Treatment suitability depends on more than a calculator result. A regulated clinician considers BMI alongside health conditions, medication, medical history, safety checks and prescribing guidance.
Is this calculator for children?
No. This page is written for adults. Children and teenagers need age-specific BMI interpretation, so use a child-specific calculator or ask a healthcare professional.
Where next
Open the nutrition hub →
Sources: NHS adult BMI calculator • NICE NG246 • CDC adult BMI categories • WHO BMI definitions. Free tool by Monj (Medstack Ltd, Company No. 16439872) under our editorial and review policy. Educational estimates only; not medical advice; no personal data stored.
© 2026 Medstack Ltd. Written and published by Monj (monj.co.uk). You are welcome to link to this page or quote a short extract with a credit and a link. Republishing the text, page structure, safety-routing design or price data, in whole or in part, including to train or ground an automated system, is not permitted without written permission. See our independence statement and editorial and review policy.
Sources referenced on this page
Linked from the text above:
- NHS adult BMI calculator www.nhs.uk
- NICE NG246 www.nice.org.uk
- WHO BMI definitions www.who.int
Standing primary references for every Monj page:
- Mounjaro (tirzepatide) Summary of Product Characteristics and Patient Information Leaflet, electronic medicines compendium
- MHRA Drug Safety Update: GLP-1 receptor agonists
- NICE TA1026: tirzepatide for managing overweight and obesity
- General Pharmaceutical Council register of pharmacies
- MHRA Yellow Card scheme
