About this calculator
Body Mass Index (BMI) is a simple screening number, widely used by health organizations, that estimates whether your weight falls into a range typically associated with underweight, healthy weight, overweight, or obesity categories, based purely on height and weight. This calculator accepts either metric or imperial units and returns your BMI along with the standard World Health Organization category it falls into.
How it works
BMI is calculated as weight in kilograms divided by height in meters squared (BMI = kg / m²). For imperial units, the equivalent formula is 703 × pounds / inches². The calculator converts your inputs to metric internally, runs this formula, and compares the result against the standard WHO adult BMI categories: under 18.5 (underweight), 18.5-24.9 (healthy weight), 25-29.9 (overweight), and 30+ (obesity).
Worked example
A person who is 175cm tall and weighs 70kg has a BMI of 70 / (1.75²) = 22.9, which falls in the WHO 'healthy weight' range of 18.5-24.9.
Formula cross-checked against: World Health Organization — A Healthy Lifestyle: BMI.
Frequently asked questions
Is BMI an accurate measure of health?
BMI is a population-level screening tool, not a diagnosis. It doesn't distinguish muscle from fat, so athletes and very muscular people are often classified as 'overweight' despite low body fat, and it doesn't account for age, sex, or where fat is distributed on the body.
Should I make health decisions based on this number alone?
No — treat BMI as one data point among many. For personal health guidance, a doctor or qualified health professional who can consider your full history will give far more useful advice than a single formula.
Does BMI apply the same way to children?
No, child and teen BMI is interpreted using age- and sex-specific percentile charts rather than the flat adult categories used here, so this calculator is intended for adults.
Last reviewed: September 2026.