What BMI Tells You (and What It Misses)

Updated: 2026-11-09

Body mass index (BMI) is the most widely used measure of weight status. It is quick, free and useful across large groups of people. But it is only a starting point. Here is what it shows and what it does not.

What BMI measures

BMI compares your weight with your height: weight in kilograms divided by height in metres squared. The World Health Organization uses it to classify adults as underweight (below 18.5), healthy weight (18.5–24.9), overweight (25–29.9) or living with obesity (30 and above). Try it with our BMI calculator.

Why doctors use it

Across populations, higher BMI is linked with higher risk of type 2 diabetes, high blood pressure, heart disease and some cancers. BMI is easy to measure and track, which makes it a good first screening tool.

Its main limits

• Muscle versus fat. BMI cannot tell them apart. A muscular athlete can have a BMI of 28 with little body fat.
• Fat distribution. Fat around the abdomen carries more risk than fat on the hips, but BMI ignores where fat is stored.
• Age. Older adults lose muscle, so a "normal" BMI can hide extra fat.
• Ethnicity. People of South Asian, Chinese and some other Asian backgrounds have higher risk at lower BMIs. The WHO and the UK's NICE recommend lower action points (23 and 27.5).
• Sex. At the same BMI, women generally have more body fat than men.

Better together: measures to add

• Waist-to-height ratio: keep your waist under half your height. NICE recommends this alongside BMI.
• Waist circumference: higher risk above about 94 cm for men and 80 cm for women (lower thresholds for some Asian groups).
• Body fat percentage: estimate it with the body fat calculator.
• Blood tests and blood pressure from your doctor tell you far more about your actual health.

The bottom line

Use BMI as a quick check, not a verdict. If your BMI is outside the healthy range, or your waist measurement is high, talk to your doctor about what it means for you.

Sources: WHO, "Obesity and overweight"; WHO Expert Consultation, The Lancet, 2004; NICE guidance on overweight and obesity management; CDC, "About adult BMI".

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These tools give general information, not medical advice. Talk to a doctor about your health. See our medical disclaimer.