BMI: what it tells you and what it doesn't
BMI was developed by Belgian mathematician Adolphe Quetelet in the 1830s. It wasn't designed as a clinical tool. It was designed as a population-level statistical measure. That context matters when you're interpreting your own number.
The formula
BMI = weight (kg) / height (m)². For a 75kg person who is 1.75m tall: 75 / (1.75 × 1.75) = 75 / 3.0625 = 24.5. In imperial: BMI = (weight in lb / height in inches²) × 703.
WHO classification
Below 18.5 is underweight. 18.5 to 24.9 is normal. 25 to 29.9 is overweight. 30 to 34.9 is Class I obesity. 35 to 39.9 is Class II obesity. 40 and above is Class III (severe) obesity. These thresholds were defined using predominantly European populations. For people of Asian descent, health risks increase at lower BMI values — some guidelines use 23 as the overweight threshold.
Where BMI fails
BMI can't distinguish fat from muscle. A rugby player or bodybuilder may have a BMI of 28 with very low body fat. An elderly person may have a "normal" BMI but high fat mass because muscle mass declines with age. BMI also tells you nothing about fat distribution — visceral (abdominal) fat carries a much higher cardiovascular and metabolic risk than subcutaneous fat at the same BMI. Waist circumference is a better predictor of metabolic risk than BMI alone.
BMI in drug dosing
BMI matters clinically when selecting dosing weights for obese patients. For drugs that don't distribute into fat tissue, ideal body weight is used instead of actual weight. For intermediate situations, adjusted body weight applies. Use our Ideal Body Weight Calculator and Weight-Based Dosage Calculator for those calculations. BMI is also used to determine bariatric surgery eligibility (generally BMI ≥ 40, or ≥ 35 with significant comorbidities).