BMI: What the Numbers Actually Mean (and What They Don't)

What BMI is — and what it isn't

Body Mass Index (BMI) is weight (kg) divided by height squared (m²). It's a fast screening number — cheap, quick, and used by the NHS as the first filter for overweight and obesity-related health risk in adults.

What it isn't is a measure of body composition. BMI cannot tell muscle from fat. A prop rugby player can be "obese" on BMI and have 10% body fat. An older adult losing muscle can be "healthy" on BMI and sarcopenic. Run your own number through the BMI calculadora before reading the categories below.

NHS adult BMI categories

Ethnicity-adjusted thresholds

The NHS and NICE use lower cut-offs for adults of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background — because cardiovascular-disease risk kicks in at a lower BMI in these groups. Specifically:

• Overweight from 23
• Obese from 27.5
• Severely obese from 32.5

Where BMI misleads

Muscular people. Trained rugby players, strength athletes and some builders carry so much muscle that BMI flags them as overweight or obese. A DEXA scan or tape test (our body-fat percentage calculadora) reveals the truth.

Older adults. After 65, loss of muscle (sarcopenia) can keep BMI in the "healthy" range even while body-fat percentage climbs. A GP will often add hand-grip strength, waist circumference or frailty index.

Very tall or very short people. The squared-height denominator under-estimates risk at extreme tall heights and over-estimates it at short ones. A 2m professional rower and a 1.5m petite office worker with identical BMI are not running the same risk profile.

Pregnancy. BMI is not a valid measure during pregnancy. Antenatal care uses booking weight and gestation instead.

Better measures — and when to use them

• Waist circumference — a single tape measure at the navel. NHS says risk starts at 94 cm (37") for men, 80 cm (31.5") for women; high risk at 102 cm and 88 cm.
• Waist-to-hip ratio (WHR) — captures fat distribution. WHO cut-offs: high risk at > 0.90 (men) and > 0.85 (women). Use our WHR calculadora.
• Body-fat percentage — from skinfolds, DEXA or tape-based formulas (US Navy method). More useful for athletes.
• Waist-to-height ratio — one of the simplest and most robust: keep your waist under half your height.

What BMI is still very good for

Despite its limits, BMI remains the best first-pass screening tool for population-level risk of type-2 diabetes, cardiovascular disease and some cancers. It's free, reproducible, and needs no equipment beyond scales and a height stick.

For an individual adult with a red flag on BMI, the next step is a GP appointment — not panic. The conversation with a clinician will usually bring in blood pressure, fasting glucose, HbA1c, cholesterol and waist circumference to build a fuller picture.

Children and teenagers — different rules

Under 18s use age- and sex-specific BMI percentiles from the UK-WHO growth charts (infants) and the UK 1990 reference (2–18 years). The raw BMI value is not interpreted the same way as adults — a healthy child's BMI can be 14 or 22 depending on age.

Clinical thresholds typically use the 91st centile (overweight) and 98th centile (clinically obese).

How to use the BMI result constructively

• If you're in the healthy range — do nothing about weight, but still check waist if you sit all day.
• If you're overweight — the NHS recommends the NHS Weight Loss Plan, 12 weeks, free.
• If you're obese — book a GP appointment. The NHS Digital Weight Management Programme is free to eligible adults.
• If you're underweight — don't starve further, and book a GP appointment if it's unintentional or accompanied by fatigue.
• Whatever the number says — sleep, exercise, not smoking and moderate alcohol matter more to mortality than any specific BMI reading.