BMI Calculator

The formula

Body mass index is weight in kilograms divided by height in metres squared. In imperial units it is weight in pounds divided by height in inches squared, multiplied by 703. The result is a single number intended to place body weight relative to height on a common scale.

The standard categories

The World Health Organization bands for adults are: below 18.5 underweight, 18.5 to 24.9 normal, 25 to 29.9 overweight, and 30 and above obese, subdivided into classes I, II and III. These are population-level classifications drawn from mortality and morbidity data, not individual diagnoses.

What BMI does not measure

It cannot distinguish muscle from fat. A trained athlete frequently registers as overweight or obese on BMI while carrying low body fat, and someone sedentary with little muscle can sit in the normal band while carrying substantial visceral fat. BMI also says nothing about where fat is distributed, and abdominal fat carries meaningfully higher metabolic risk than the same mass elsewhere.

It was designed in the nineteenth century as a population statistic, not a clinical tool for individuals. Its continued use reflects how cheap it is to compute, not how well it describes any one person.

Where the standard bands do not apply

Children and adolescents are assessed on age-and-sex percentile charts, not adult thresholds. Pregnancy makes BMI meaningless. Older adults may have better outcomes slightly above the normal band. Several health bodies use lower thresholds for people of South Asian, Chinese and some other ancestries, where cardiometabolic risk rises at a lower BMI: an overweight cutoff of 23 rather than 25 is commonly applied.

More informative measures

Waist circumference and waist-to-height ratio track abdominal fat directly and predict cardiometabolic risk better than BMI. Keeping your waist under half your height is a simple, well-supported rule. Body fat percentage, from a DEXA scan or good bioimpedance, is more accurate again.

This tool provides general information and is not medical advice. Discuss any concern about weight with a clinician who can consider your full picture.