BMI CALCULATOR
One useful number. Not the whole verdict.
Check your BMI, understand what it can — and cannot — tell you, then choose one useful next step.
How this works, limitations & evidence
Interpretation, limitations & evidence
Understand the result — then decide what matters
BMI gives a quick relationship between height and weight. It is widely used by NHS and NICE services, but it is a screening measure, not a diagnosis.
How BMI is calculated
BMI = weight in kilograms ÷ height in metres².
Example: 90 kg at 1.80 m gives 90 ÷ (1.80 × 1.80) = BMI 27.8.
How to interpret an adult BMI
| BMI | Standard adult classification |
|---|---|
| Under 18.5 | Below the standard healthy range |
| 18.5–24.9 | Healthy weight range |
| 25–29.9 | Overweight |
| 30+ | Obesity, with classes increasing as BMI rises |
For people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds, NHS/NICE use lower thresholds for increased cardiometabolic risk: overweight starts at 23 and obesity at 27.5.
Where BMI is useful
Screening
It helps identify people who may benefit from a wider health assessment.
Treatment pathways
BMI is still used in NHS medication and bariatric-surgery criteria, usually alongside health conditions.
Tracking
For one person over time, falling BMI generally reflects falling body weight at the same height.
Where BMI falls short
BMI cannot distinguish fat from muscle, cannot show where fat is stored and does not directly measure fitness, blood pressure, glucose, cholesterol or wellbeing. A muscular person can have a high BMI without high body fat; another person can have a lower BMI but high central adiposity.
Use waist-to-height ratio alongside it
NICE recommends waist-to-height ratio as an additional measure of central adiposity in adults with BMI under 35. A practical public-health message is to keep your waist to less than half your height. Ratios of 0.5–0.59 indicate increased risk and 0.6+ further increased risk.
Calculate waist-to-height ratio →
What should you do with the number?
If BMI is above the healthy reference range, look at the wider picture: waist, blood pressure, sleep, mobility, family history, diabetes risk and how weight is affecting daily life. That is much more useful than treating BMI as a score you have passed or failed.
Common questions
Is BMI accurate?
It is useful at population and screening level but imperfect for individuals.
Can athletes use BMI?
Yes, but high muscle mass can make BMI overestimate body-fat-related risk.
Does BMI decide whether I can have Mounjaro or surgery?
BMI is one part of eligibility; health conditions, local pathways and clinical assessment also matter.
What should I use next?
Sources & methodology
We use UK guidance and primary research where appropriate, and disclose when a result is an estimate rather than a clinical threshold.
Use this page as information, not judgement. The best next step is the one that actually fits your health, your life and what you are ready to change.
Written and editorially reviewed by the Shift Some Timber. Editorial review is not clinical review. Health content is checked against recognised UK guidance and primary sources where appropriate.
Last reviewed: 8 August 2026. This page is informational and does not replace individual medical advice.