The equation is BMI = body weight in kilograms/height in meters squared.
Body Mass Index Calculator
Research has identified the health risks associated with a wide range of BMIs (both high and low values.)
The National Institutes of Health state these BMI categories:
Underweight: BMI less than 18.5
Normal weight: BMI between 18.5 and 24.9
Overweight: BMI between 25 and 29.9
Obese: BMI of 30 or greater.
BMI can be inaccurate in estimating body fat. Athletes with a very muscular build will have a BMI that overestimates body fat. They are leaner than their BMI indicates. On the opposite end, older people and others who have lost muscle may have higher body fat than their BMI indicates.