How to Calculate BMI

A complete guide to Body Mass Index — what it is, how it is calculated, what the categories mean, and its limitations as a health measure.

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What adult BMI measures

Body Mass Index compares weight with height. It is a screening measure used for adults and does not directly measure body fat, muscle, bone, fluid or overall health.

How to calculate BMI

In metric units, BMI equals weight in kilograms divided by height in metres squared: BMI = kg ÷ m². A person weighing 70 kg at 1.75 m has a BMI of about 22.9.

Imperial measurements can be converted to kilograms and metres before the same formula is used. ToolBullet performs those conversions using 1 pound = 0.45359237 kilograms and 1 inch = 0.0254 metres.

Standard adult screening categories

Common adult categories are below 18.5, 18.5–24.9, 25.0–29.9 and 30 or above. These are population screening references rather than diagnoses or personalised weight targets.

Important limitations

BMI does not distinguish fat from muscle or show fat distribution.
Adult categories are not used in the same way for children or during pregnancy.
Age, ethnicity, muscularity, health history and other clinical information can affect interpretation.
A single BMI result cannot diagnose a condition or determine an individual treatment plan.

Using the result

Treat BMI as one screening data point. A qualified healthcare professional can interpret it alongside symptoms, waist measurement, blood pressure, laboratory results, medical history and other relevant information.

Sources and further reading

Sources reviewed 23 June 2026.

This guide is general educational information, not medical advice or diagnosis. Adult BMI categories are screening references and are not personalised weight targets.

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Frequently Asked Questions

How is adult BMI calculated?

For metric measurements, divide weight in kilograms by height in metres squared. Imperial inputs are converted to kilograms and metres before the same formula is applied.

What do standard adult BMI categories represent?

They are screening references for adults, not diagnoses or personalised weight targets. BMI 18.5–24.9 is the standard reference range, with higher and lower categories outside it.

What are the limitations of BMI?

BMI does not directly measure body fat, muscle, fat distribution or overall health. It is not interpreted the same way for children or pregnancy, and individual context matters.