How the BMI calculator works
The formula, demystified
BMI = weight (kg) ÷ height² (m) — Adolphe Quetelet's 1830s population statistic, repurposed in 1972 as an individual screen. It divides weight by height squared because human weight scales roughly with the square of height. The categories (18.5 / 25 / 30) came from insurance-era mortality curves, which is why they are ranges rather than laws.
What it measures, and what it cannot
BMI correlates with body fat at population scale — useful for studying millions and reasonable as a personal screen. It cannot distinguish muscle from fat (athletes read "obese"), cannot see fat distribution (waist circumference adds that), and shifts meaning with age and ethnicity (risk arrives at lower BMIs in many Asian populations). The waist-to-height and body-fat calculators on this site exist to add exactly those missing dimensions.
Using the range instead of the point
The most useful output is not your BMI but the range: at 5'10", everything from ~129 to ~175 lb reads "healthy." A weight inside the range with good markers beats a number chasing. People 5–10 lb outside the range with normal bloodwork, waist, and energy are a rounding error from the category line — the math rewards perspective, not obsession.