The BMI Formula — and Why It Was Never Meant to Be Used Like This
Where Body Mass Index comes from, how to calculate it in metric or imperial, what the categories actually mean, and where the formula breaks down.
The formula
Metric: BMI = weight (kg) ÷ [height (m)]²
Imperial: BMI = [weight (lb) ÷ height (in)²] × 703
The unit conversion factor 703 makes the imperial form yield the same number as the metric form.
A short history
The formula was published in 1832 by the Belgian polymath Adolphe Quetelet, who called it the “Quetelet Index.” He was not a doctor — he was an astronomer and statistician studying “the average man.” It was rediscovered for medical use by the physiologist Ancel Keys in 1972, who proposed renaming it Body Mass Index after testing it against skinfold and densitometry data.
Keys explicitly wrote that BMI was “not entirely satisfactory” but useful as a population screening tool. It was never designed to diagnose an individual.
WHO categories (adults)
| BMI (kg/m²) | Category |
|---|---|
| Below 16.0 | Severe thinness |
| 16.0–16.9 | Moderate thinness |
| 17.0–18.4 | Mild thinness |
| 18.5–24.9 | Healthy weight |
| 25.0–29.9 | Overweight |
| 30.0–34.9 | Obesity class I |
| 35.0–39.9 | Obesity class II |
| 40.0+ | Obesity class III |
Worked example
A person 1.75 m tall, 80 kg:
BMI = 80 ÷ (1.75 × 1.75) = 80 ÷ 3.0625 ≈ 26.1
That’s just into the “overweight” range — but for a regular lifter, 26.1 could represent excellent health.
Limitations
- Muscle reads as fat. An NFL running back at 6’0” and 220 lb is “obese” (BMI 29.8) at 8% body fat.
- Body fat distribution is invisible. Two people with BMI 24 can have wildly different visceral fat.
- Age and sex confound it. Women carry ~10% more essential fat than men at the same BMI; older adults have less muscle than the BMI norms assume.
- It’s height-biased. Taller people get systematically higher BMIs even at the same body fat percentage, because area scales with height² but mass scales with height³.
Better individual metrics
- Waist-to-height ratio (keep waist below half your height) — the single best simple risk indicator for visceral fat.
- Waist circumference — <94 cm (37 in) for men, <80 cm (31.5 in) for women.
- DEXA or BodPod scan — gold-standard body composition.
Try it
Use the BMI calculator for a quick screening, then pair it with waist-to-height ratio for a more complete picture.
Sources
- WHO. Obesity: preventing and managing the global epidemic. WHO Technical Report Series 894, 2000.
- Keys A, Fidanza F, Karvonen MJ, Kimura N, Taylor HL. Indices of relative weight and obesity. J Chronic Dis. 1972;25(6):329–43.