Body Mass Index (BMI) is the most widely used weight-screening metric in the world. It's simple, free and needs only your height and weight — but knowing what it does and doesn't tell you is key to using it well.
What is BMI?
BMI divides your weight in kilograms by the square of your height in metres. The result places you into one of four categories from underweight to obese.
The formula
BMI = weight(kg) ÷ height(m)². For imperial units: BMI = 703 × weight(lb) ÷ height(in)².
Limitations
BMI doesn't measure body fat directly and can't account for muscle mass, bone density, or fat distribution. Athletes, older adults and different ethnic groups may need adjusted interpretations.
Frequently asked questions
For most adults, a BMI between 18.5 and 24.9 is considered the healthy range. Below 18.5 is underweight and 25 or above is overweight.
Not always. BMI can't tell muscle from fat, so very muscular people often score as 'overweight' despite being lean and healthy. Use body-fat percentage for a better read.
The BMI formula and categories are the same for adult men and women. Body-composition differences are one reason BMI is best treated as a rough screening tool.