BMI Calculator
Body Mass Index calculator with WHO adult bands and CDC LMS pediatric percentiles (ages 2-19). Imperial default; one-click metric.
How this is calculated
Adult BMI uses the standard formula BMI = kg / m² (or
703 × lb / in² for imperial units — identical result up to rounding).
The WHO labels: Underweight < 18.5, Normal 18.5–24.9,
Overweight 25.0–29.9, Obese class I 30.0–34.9, II
35.0–39.9, III ≥ 40.0. Half-open intervals on the lower edge — a BMI of
exactly 25.0 lands in Overweight.
The healthy weight range for adults uses BMI 18.5–24.9 multiplied by your height in metres squared, then rounded for display.
Pediatric BMI (ages 2–19) is the same number, but interpreted via the CDC
growth charts. We look up the LMS triple (L, M, S) for the user's
sex × age in months from CDC's bmiagerev.csv, compute a Z-score
Z = ((BMI/M)^L − 1) / (L·S), and convert to a percentile via the
standard-normal Φ approximation (Abramowitz & Stegun 7.1.26). The 2022 CDC
bands: < 5th = Underweight; 5–< 85th = Healthy; 85–<
95th = Overweight; ≥ 95th splits into class 1 (< 120 % of P95), class
2 (120–< 140 %), class 3 (≥ 140 %).
Sources: WHO BMI classification; CDC Defining Childhood Weight Status; CDC 2022 extended BMI charts; NHLBI clinical guidelines.