How is bmi calculated?

The formula, a worked example and the assumptions are shown on this page. Results are estimates for planning, not financial, medical or legal advice.

Is this bmi calculator free to use?

Yes. It runs entirely in your browser, needs no account and stores nothing on a server.

BMI Calculator

Q: How this is calculated

BMI — how this is calculated. This page shows the formula, a worked example and the assumptions behind the result.

Q: BMI categories (adults, WHO)

BMI — bmi categories (adults, who). This page shows the formula, a worked example and the assumptions behind the result.

Body mass index from your height and weight.

Units

Height (cm)

Weight (kg)

Your BMI

Category–

How this is calculated

BMI = weight (kg) ÷ height (m)². It's the standard World Health Organization formula. BMI is a quick screening measure — it doesn't distinguish muscle from fat, and it isn't a diagnosis. Athletes, older adults, and pregnant women in particular can fall outside the general categories without that reflecting their actual health.

Worked example

A person 170 cm tall weighing 70 kg gives BMI = 70 ÷ (1.70 × 1.70) = 70 ÷ 2.89 = 24.2, which falls in the healthy weight range. The same person at 180 cm and 70 kg gives 70 ÷ 3.24 = 21.6. Height is squared, so a small change in height moves the result more than the same change in weight.

BMI categories (adults, WHO)

Below 18.5: Underweight
18.5 – 24.9: Healthy weight
25.0 – 29.9: Overweight
30.0 and above: Obesity

If you have concerns about your weight or health, a doctor or registered dietitian can give guidance tailored to you — this tool is for a quick estimate only.