Guide · Updated November 2026 · How we check our numbers

Understanding BMI: What Your Number Means and Its Limits

Body mass index (BMI) is a quick screening number that compares weight with height. It is useful, but it is not a diagnosis.

How it is calculated

BMI = weight in kg ÷ (height in m)². In US units: 703 × weight in lb ÷ (height in inches)². A person who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ 3.0625 = 22.9.

Adult categories (WHO)

BMI	Category
Below 18.5	Underweight
18.5–24.9	Normal weight
25–29.9	Overweight
30 and above	Obesity

Where BMI falls short

• It cannot tell muscle from fat, so muscular people may score "overweight".
• It does not show where fat is stored.
• Healthy ranges can differ by ethnicity and age, and BMI for children uses growth charts.

Add a waist measurement

Fat around the middle is linked with more health risk. A simple check is to keep your waist under half your height (waist-to-height ratio under 0.5). The WHO also uses waist-to-hip ratio cut-offs of 0.90 for men and 0.85 for women.

If any of your numbers worry you, talk to a doctor — they can look at the whole picture.

Try the calculators

This guide is general information, not professional advice.