Body Composition
As of 2026-04-24
How BMI Calculator works
Methodology for the BMI Calculator: formulas, coefficients, data sources, assumptions, and known limitations.
Scope
Computes Body Mass Index and returns the WHO range classification. BMI is a population-level screening tool, not a diagnosis.
Formula
BMI = weight_kg / (height_m)^2
Coefficients
| Parameter | Value | Note |
|---|---|---|
| WHO underweight | < 18.5 | |
| WHO normal | 18.5 – 24.9 | |
| WHO overweight | 25.0 – 29.9 | |
| WHO obese I | 30.0 – 34.9 | |
| WHO obese II | 35.0 – 39.9 | |
| WHO obese III | 40+ |
Data sources
- World Health Organization — BMI classification.
- Keys A, Fidanza F, Karvonen MJ, Kimura N, Taylor HL. Indices of relative weight and obesity. J Chronic Dis. 1972;25(6):329-343. — PMID 4650929. Origin of the modern BMI formula (adapted from Quetelet).
Assumptions
- Subject is an adult (>=18).
Approximation range
Population-level relationships with morbidity are well established; individual risk varies substantially within each BMI band.
Limitations
- BMI does not distinguish fat from lean mass — muscular athletes often fall in the overweight or obese I band.
- Not validated for children, adolescents, pregnant women, or competitive bodybuilders.
Reproducibility
85 kg, 1.80 m. BMI = 85 / 1.8^2 = 26.2. Range: overweight.
Change log
- 2026-04-24: methodology page first published.
Related tools
- Body Fat Percentage Calculator — Estimate body fat percentage using the U.S. Navy circumference method.
- Ideal Weight Calculator — Compare Devine, Robinson, Miller, and Hamwi formulas as a realistic range.
- Waist-to-Hip Ratio Calculator — Calculate waist-to-hip ratio and assess body composition using WHO guidelines.
- FFMI Calculator — Calculate Fat-Free Mass Index to gauge muscularity and compare against natural benchmarks.