BMI Calculator
Your BMI with the WHO bands, the NICE ethnicity-adjusted cut-offs, and the number BMI misses — waist-to-height ratio. Over 50 or 65, it also tells you what the evidence says the bands get wrong.
Your BMI
—
The bands
| Category | WHO | NICE lower cut-offs |
|---|---|---|
| Underweight | < 18.5 | < 18.5 |
| Healthy weight | 18.5–24.9 | 18.5–22.9 |
| Overweight | 25–29.9 | 23–27.4 |
| Obesity class 1 | 30–34.9 | 27.5–32.4 |
| Obesity class 2 | 35–39.9 | 32.5–37.4 |
| Obesity class 3 | 40+ | 37.5+ |
NICE class 2 and 3 cut-offs are the standard ones minus 2.5, as NICE NG246 rec 1.9.11 describes.
Over 50, and over 65
From around 30, adults lose muscle at roughly 3–8% per decade, faster after 60 (Volpi 2004). Weight can stay flat while fat replaces muscle, so BMI flatters you and waist-to-height ratio does not — use both. After 65 the picture flips at the low end: in Winter's 2014 meta-analysis of 32 cohorts (197,940 adults, 12-year average follow-up), the lowest mortality sat around BMI 27–28, overweight carried no extra risk, and risk climbed below 23 — 12% higher at 21–21.9 and 19% higher at 20–20.9 than at 23–23.9. NICE asks clinicians to interpret BMI with caution at 65+. Unplanned weight loss at any age is worth raising with a doctor.
Common questions
What is a healthy BMI?
For adults the WHO bands are: under 18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight, 30 or more obesity (class 1 30–34.9, class 2 35–39.9, class 3 40+). NICE lowers the overweight and obesity cut-offs to 23 and 27.5 for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background.
Is BMI accurate after 50 or 65?
Less so. Muscle mass falls with age, so the same BMI can carry more fat at 70 than at 30. In a meta-analysis of about 198,000 adults aged 65+, overweight BMI was not linked to higher mortality, while risk rose below 23 (Winter 2014). NICE advises interpreting BMI with caution in people 65 and over.
What is waist-to-height ratio and why add it?
Waist divided by height, same units. NICE classes 0.4–0.49 as healthy, 0.5–0.59 as increased and 0.6+ as high central adiposity, for all sexes and ethnicities when BMI is under 35. The rule of thumb: keep your waist under half your height. It catches belly fat that a 'healthy' BMI can miss.
How do I measure my waist?
NICE's method: find the bottom of your ribs and the top of your hips, measure around the middle of those points, breathing out naturally.
How it's computed
BMI = weight (kg) ÷ height (m)². Pounds are divided by 2.2046 and inches multiplied by 2.54 first. The healthy-weight range is 18.5 × m² to 24.9 × m² (22.9 with NICE cut-offs). Waist-to-height ratio = waist ÷ height, both in the same unit. The 65+ note appears from age 65; the 50+ muscle note from 50. Not for children, pregnancy, or diagnosis.
Sources
- WHO. A healthy lifestyle — WHO recommendations (adult BMI categories). www.who.int
- NICE NG246. Overweight and obesity management (2025): identifying and assessing overweight, obesity and central adiposity, recs 1.9.10–1.9.15. www.nice.org.uk
- Winter JE, MacInnis RJ, Wattanapenpaiboon N, Nowson CA. BMI and all-cause mortality in older adults: a meta-analysis. Am J Clin Nutr. 2014;99(4):875-890. pubmed.ncbi.nlm.nih.gov
- Volpi E, Nazemi R, Fujita S. Muscle tissue changes with aging. Curr Opin Clin Nutr Metab Care. 2004;7(4):405-410. pubmed.ncbi.nlm.nih.gov
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