The formula for calculating body mass index

A healthy weight

BMI is one way to define a healthy weight

Doctors usually rate people as underweight, normal, overweight or obese by their body mass index (BMI). A higher BMI normally means a higher percentage of body fat; however, this varies according to age, ethnic group and sex. There are also differences between, say, a body-builder who eats piles of protein to add muscle mass and a couch potato who weighs the same but lives on snack foods and, hence, has more body fat.

The BMI is not a perfect measure. It applies best to people who are mainly sedentary, have roughly average body composition and are not too short or tall. Some researchers think that waist size and/or waist-to-hip ratio – or combining these with the BMI – are more useful indicators of a healthy weight.


About this resource

This resource was first published in ‘Food and Diet’ in June 2011 and reviewed and updated in August 2016.

Statistics and maths, Physiology, Health, infection and disease
Food and Diet
Education levels:
16–19, Continuing professional development