BMI Calculator — educational

Body Mass Index is a height-to-weight ratio. It's a population statistic from 1835, often misused as an individual health verdict. Use it to learn the math, not to diagnose yourself.

Units
BMI
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kg/m²
WHO classification (adults)
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WHO adult BMI classification

Below 18.5 Underweight
18.5 – 24.9 Normal weight
25.0 – 29.9 Overweight
30.0 + Obese

These are population-level ranges. They do not account for muscle mass, bone density, body composition, age, sex, or ethnicity — all of which matter for individual health. The marker on the bar shows where the current input falls (if any).

How BMI is calculated, and what it doesn't measure

BMI = weight (kg) ÷ height² (m²). For example, 70 kg and 170 cm: 70 ÷ 1.70² = 24.22 kg/m², in the WHO "normal weight" band for adults.

What BMI doesn't measure: muscle mass (athletes can register "overweight" or "obese" despite low body fat), body composition (where fat is distributed), bone density, metabolic health, fitness, or any individual health outcome. Different ethnicities have different BMI risk thresholds — the WHO cut-points were calibrated on European populations.

Who shouldn't use BMI as a guide: children and adolescents (use age- and sex-specific percentiles), pregnant people, competitive athletes, the elderly, and people with high muscle mass.

For a fuller picture, talk to a doctor about body composition, waist circumference, blood pressure, cholesterol, blood glucose, and family history.

About this tool

BMI Calculator computes the Body Mass Index from height and weight using the standard formula: weight in kilograms divided by height in metres squared. It supports both metric and US customary units, and shows the WHO classification band the result falls into (underweight, normal weight, overweight, obese class I-III). The page also reports the healthy BMI range for the given height and notes that BMI is a population-level screening tool, not a diagnosis — it does not distinguish muscle from fat, and it can misclassify athletes and people with unusual body composition.

When to use it

  • A quick screening check between doctor visits, in metric or US customary units
  • Teaching the BMI formula and its classification bands in a health class
  • Tracking BMI over time as one of several inputs to a fitness plan

When not to use it

  • You need a body composition assessment — BMI does not measure muscle vs fat. Use a DEXA scan or a bioimpedance scale for that
  • You are a serious athlete or a bodybuilder — BMI misclassifies high-muscle individuals as overweight. Use a body-fat percentage instead
  • You are pregnant or have a medical condition that affects fluid balance — BMI is not validated for these cases

Frequently asked questions

Is BMI a good measure of health?

As a population-level screening tool, yes — large studies show correlation between high BMI and metabolic disease risk. As a per-person diagnosis, no — it does not distinguish muscle from fat, and it ignores fat distribution, which matters more than total mass for cardiovascular risk. Use BMI as one signal among many, not as a verdict.

What is a healthy BMI?

The WHO defines 18.5 to 24.9 as normal weight for adults. Below 18.5 is underweight; 25 to 29.9 is overweight; 30 and above is obese, with sub-classes I, II, and III. The thresholds are the same for men and women, and for all adult ages, which is a known limitation.

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