About this calculator
Body Mass Index (BMI) is weight divided by height squared, a quick proxy for whether someone’s weight is in a healthy range relative to their height. Developed by Adolphe Quetelet in 1832 as a population statistic, it was repurposed by the WHO in the 1990s as a clinical screening tool. It’s simple, free, and reasonably useful — but it’s also famously imperfect for individuals.
The formula
Metric: BMI = kg / m². Imperial: BMI = (lb / in²) × 703. WHO categories: under 18.5 (underweight), 18.5–24.9 (normal), 25–29.9 (overweight), 30–34.9 (obese class I), 35–39.9 (class II), 40+ (class III).
Where BMI is genuinely useful
As a population-level screening number, BMI correlates well with cardiovascular disease risk, diabetes risk, and overall mortality across large studies. For a typical sedentary adult who’s not exceptionally muscular, BMI ≥ 30 is a strong signal to engage with a clinician about weight-related health risks.
Where BMI fails individuals
- Muscular athletes — A 6’2", 220 lb NFL running back has a BMI of 28.3 ("overweight") with under 10% body fat. The category is meaningless for them.
- Skinny-fat — Someone with low muscle mass and high fat mass can have "normal" BMI while having unhealthy body composition.
- Elderly — Lean muscle mass declines with age; "normal BMI" can mask sarcopenia (muscle wasting).
- Children — Pediatric BMI uses age-and-sex-specific percentiles, not adult categories.
- Different ethnic groups — South Asian populations show metabolic risk at lower BMI thresholds (overweight from 23, obese from 27). African-American populations have higher BMI-to-body-fat ratios.
Better alternatives for individuals
Waist-to-height ratio (waist circumference ÷ height; target <0.5) tracks visceral fat better than BMI. Body fat percentage via DEXA scan, BIA, or US Navy tape method is more precise. For most people, the simplest improvement is: BMI as a screening number, body fat % as a follow-up if BMI is borderline.