BMI Calculator

How BMI is calculated and interpreted

Body mass index, or BMI, compares an adult’s weight with the square of their height. In metric units, the formula is weight in kilograms divided by height in meters squared. For U.S. units, BMI equals weight in pounds divided by height in inches squared, multiplied by 703. The same inputs should produce nearly identical results apart from rounding.

Worked example

An adult who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.86. The displayed result rounds to about 22.9.

For many adults, commonly used screening ranges classify BMI below 18.5 as underweight, 18.5–24.9 as healthy weight, 25.0–29.9 as overweight and 30 or above as obesity. These categories are screening labels, not a diagnosis. Clinical interpretation may consider health history, body composition, age, medications and other measurements.

What BMI can and cannot tell you

BMI is quick and inexpensive, which makes it useful for population research and an initial health conversation. It does not directly measure body fat, muscle mass, bone density or fat distribution. A muscular athlete and a person with lower muscle mass can have the same BMI while having different health profiles.

This adult calculator should not interpret children or teenagers. Their results are evaluated using age- and sex-specific growth charts. Pregnancy and certain medical conditions also require professional interpretation rather than a standard adult category.

Improve input accuracy

Use a recent height measured without shoes and weight from a level, calibrated scale.
Make sure feet/inches or centimeters match the selected system.
Avoid rounding height too early because it is squared in the formula.
Discuss concerning results with a qualified healthcare professional.

Use the result as a starting point

A single BMI value does not describe a person’s overall health or explain why weight changed. Trends may be more informative when measurements are taken consistently, but even a trend needs context. Sleep, nutrition, activity, waist measurement, blood pressure, laboratory results and medical history can all matter. Avoid using the category to label or judge yourself or another person. If you are planning a major diet or exercise change, are pregnant, have an eating disorder history or notice an unexplained change in weight, seek individualized medical advice.

Frequently asked questions

Is BMI a medical diagnosis? No. It is a screening calculation considered alongside other health information.

Why can a muscular person have a high BMI? BMI uses total weight and cannot distinguish muscle from fat.