Children and teenagers are still growing, so the same BMI number means different things at different ages. A BMI of 19 is high at age 6 and low-normal at age 18. The CDC therefore uses BMI-for-age percentiles against the 2000 growth reference cohort for the same age and sex. The four bands: underweight below the 5th percentile, healthy weight 5th to less than 85th, overweight 85th to less than 95th, and obesity at or above the 95th percentile. Severe obesity is sometimes defined as 120% of the 95th percentile.
BMI for adults age 20+ (WHO cut-offs)
For adults, the WHO cut-offs are the international standard: underweight below 18.5, healthy 18.5–24.9, overweight 25.0–29.9, obese 30 and above (with sub-classes I, II, III at 35 and 40). The same numeric cut-offs apply to males and females. Body composition at a given BMI does differ by sex, but the cut-offs are sex-neutral.
BMI for older adults
After about age 60–65, BMI starts to lose some of its predictive power. Older adults often have sarcopenia (age-related muscle loss) and a higher proportion of visceral fat at the same BMI. Some geriatric guidelines suggest a slightly higher healthy range (23–29) for adults over 65, but the WHO cut-offs are still widely used. A high BMI in an older adult is not by itself a reason to lose weight — the trend and the waist-to-height ratio matter more than the number.
BMI for pregnant people
BMI is not validated during pregnancy. Obstetric care uses pre-pregnancy BMI to set personalised weight-gain targets (typically 11.5–16 kg for a normal pre-pregnancy BMI, 7–11.5 kg for overweight, 5–9 kg for obese). The calculator suppresses the body-fat estimate and several follow-on numbers during pregnancy because the underlying formulas were not derived for that population.
How to use the calculator at any age
- Switch to Child / Teen mode for ages 2–19.
- For adults 20+, the default mode is Adult — use it as is.
- For older adults, the calculator works normally; just interpret the result with the sarcopenia caveat above.
Next steps
- Child BMI calculator for ages 2–19
- Teen BMI calculator for ages 13–19
- Adult BMI calculator for 20+
- CDC growth charts and WHO adult tables