BMI: What It Measures and Its Limits

ficklestudio26 · Facts checked October 25, 2026

BMI compares weight with height. It is quick and free, which makes it a useful first look, and it also has well-known limits.

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The formula

Metric: BMI = weight in kilograms ÷ (height in meters)². A person who is 1.70 m tall and weighs 70 kg has 70 ÷ 2.89 = 24.2.
US units: BMI = 703 × weight in pounds ÷ (height in inches)².

Adult categories

For adults 20 and older the standard ranges are: below 18.5 underweight, 18.5 to 24.9 healthy weight, 25.0 to 29.9 overweight, and 30.0 and above obesity. Children and teens are assessed differently, using percentiles for their age and sex, so adult cut-offs do not apply to them.

Limits

BMI is a screening tool, not a diagnosis of body fatness or health. It does not tell muscle from fat, so a very muscular person can score high. It does not show where fat is carried, and it can read differently for older adults or for different body types. Doctors use it together with other measures, such as waist size, blood pressure, blood tests and family history.

How to use the number

Treat it as a starting point for a conversation with a health professional.
Look at the trend over time more than a single reading.
Do not use it to judge children, pregnant women or competitive athletes without professional advice.

This page is general information and not medical advice.