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Calculation and rounding
- Formula: weight (kg) ÷ height (m)², or 703 × weight (lb) ÷ height (in)².
- Round to one decimal place before classifying. A BMI of 24.96 displays as 25.0 and should be classified as overweight.
- Measure, don't ask. Self-reported height tends to be overestimated and weight underestimated, which lowers BMI.1 Measuring technique.
Classification
| Population | Classification |
|---|---|
| Adults (20+) | <18.5 underweight · 18.5–24.9 healthy · 25–29.9 overweight · 30–34.9 class 1 · 35–39.9 class 2 · 40+ class 32 |
| Adults of Asian and some other ethnic backgrounds | Lower thresholds may apply: 23 overweight, 27.5 obesity. Asian BMI |
| Children and teens (2–19) | CDC BMI-for-age percentiles; ≥120% of the 95th percentile for severe obesity. Percentile method |
| Pregnancy | Use pre-pregnancy BMI for IOM weight-gain targets. Pregnancy |
Special populations
- Limb loss: adjust weight for the missing segment before calculating BMI. Adjusted BMI for amputees.
- Patients who can't stand: estimate height from knee height, ulna length or demi-span. Height estimation.
- Older adults: height loss inflates BMI if an old height is used; consider muscle loss and recent weight change. Older adults.
- Athletes: BMI overestimates adiposity in muscular patients; add waist or body-composition measures. Athletes.
BMI in malnutrition screening (MUST)
The Malnutrition Universal Screening Tool, widely used in the UK, scores BMI as step 1:3
| BMI | MUST score |
|---|---|
| Over 20 | 0 |
| 18.5–20 | 1 |
| Under 18.5 | 2 |
It's combined with scores for unplanned weight loss and acute illness. See BAPEN's toolkit for the full tool.
Talking with patients about BMI
- Ask permission before discussing weight, and use neutral language ("your BMI is…", "weight" rather than labels).
- Pair BMI with health measures the patient cares about: blood pressure, blood sugar, mobility.
- Explain what BMI can't show. It's a screening number, not a verdict.
Tools for many patients
- Batch BMI calculator: paste or upload a CSV and get BMI and categories for every row, processed in the browser.
- BMI in Excel and in code for records systems and audits.
Sources
Further reading
Kept up to dateLast updated October 9, 2026
This page is for education and is not medical advice. Talk with a healthcare professional about your weight and health.
