BMI Calculator

What BMI actually measures

Body Mass Index is weight divided by height squared. It's a quick, widely used screening number — not a diagnosis — that sorts into four general bands: underweight, normal, overweight, and obesity.

It doesn't account for muscle mass, bone density, or body composition, so an athlete and a sedentary person can land on the same number for very different reasons. Treat it as a starting point, not a verdict, and talk to a doctor for anything more specific.

The standard BMI ranges

Under 18.5 is classed as underweight, 18.5-24.9 as normal weight, 25-29.9 as overweight, and 30+ as obese, using the WHO's adult ranges. These bands were built from population-level data and don't adjust for age, sex, or ethnicity, all of which genuinely affect how much a given BMI number means for an individual — which is exactly why it's treated as a screening tool rather than a diagnostic one.

If BMI doesn't seem to fit

For anyone with more muscle mass than average, waist-to-hip ratio or a body-fat-percentage measurement will generally give a more accurate picture of health risk than BMI alone. BMI's real value is as a fast, free, first-pass number — not the last word on anyone's health.

A worked example

Someone at 70kg and 175cm: BMI is weight divided by height in meters squared, so 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 ≈ 22.9, which falls in the normal range. Drop the same person to 60kg with no height change and BMI falls to about 19.6 — still normal, but close to the underweight cutoff. Raise them to 90kg instead and BMI climbs to about 29.4, just under the obesity threshold. Small weight changes move the number less than people expect at higher heights, since height is squared in the formula — the same 10kg swing moves BMI by roughly 3 points for someone at 175cm, but by more at a shorter height and less at a taller one.

BMI for children isn't the same scale

This calculator uses the adult BMI bands, which don't apply to children and teenagers — growing bodies have a different, age- and sex-specific expected BMI range that shifts as a child develops, tracked against percentile growth charts rather than the fixed adult cutoffs used here. A pediatrician is the right source for interpreting BMI in anyone still growing; running a child's numbers through this adult-focused tool will give a misleading category.