Published on August 08, 2026 • Last updated August 08, 2026
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Body Mass Index (BMI) is a simple numerical measure, calculated from a person's height and weight, used to broadly categorize whether their body weight falls within a healthy range.
Body Mass Index (BMI) is one of the most widely used health screening tools in the world. It takes just two numbers — your height and your weight — and combines them into a single score that places you into one of four broad categories: underweight, normal weight, overweight, or obese. The idea behind BMI is simple: taller people naturally weigh more than shorter people, even at the same level of body fat. BMI accounts for this by dividing weight by the square of height, making it possible to compare body weight across people of different heights on a level playing field.
For most adults, a BMI between 18.5 and 24.9 is considered a healthy range. A score under 18.5 suggests you may be underweight, 25 to 29.9 falls into the overweight range, and 30 or above is classified as obese. Doctors, insurers, and public health bodies use these categories as a quick, low-cost way to flag people who might benefit from a closer look at their diet, activity levels, or overall health. It's worth understanding what BMI does not measure.
It cannot tell the difference between muscle and fat, so a muscular athlete can register a "high" BMI despite having very low body fat. It also doesn't account for where fat is stored, bone density, age, or sex — all of which affect how much weight is truly healthy for a given person. Because of this, BMI is best treated as a starting point for a conversation about health, not a diagnosis on its own. BMI is closely related to other body-composition metrics: BMR tells you how many calories your body burns at rest, and Body Fat Percentage estimates how much of your weight is fat versus lean mass — both paint a fuller picture than BMI alone.
Learn how BMI is calculated
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