What Your BMI Number Actually Means

Body Mass Index, or BMI, is one of the most commonly used health screening numbers in the world. It's calculated from just two measurements — height and weight — yet it's used by doctors, fitness apps and insurance companies as a quick indicator of whether someone's weight falls within a typical range for their height.

The four categories

BMI results are generally grouped into four bands. A BMI below 18.5 is considered underweight, 18.5 to 24.9 is considered a normal or healthy weight range, 25 to 29.9 is considered overweight, and 30 or above is considered obese. These thresholds are based on population-level studies linking BMI to health outcomes.

Why it's a starting point, not a verdict

BMI doesn't distinguish between muscle and fat. A very muscular person — an athlete, for example — might have a high BMI despite having low body fat, simply because muscle is denser than fat. Similarly, BMI doesn't account for where fat is distributed on the body, which can matter more for certain health risks than the overall number.

Age, sex, and ethnicity can also affect how meaningful a given BMI number is for an individual. For these reasons, BMI is best used as a quick screening tool — a starting point for a conversation about health, rather than a complete picture on its own.

Using the calculator

Our BMI calculator takes your height and weight and instantly shows your BMI along with which category it falls into, so you have a quick reference point whenever you need one.