Free BMI Calculator — Adult Body Mass Index Online

Body Mass Index, or BMI, is a number calculated from your height and weight. The formula is your weight in kilograms divided by your height in metres squared, giving a single value that places you on a standardised scale.

BMI is a measure of weight relative to height. It does not directly measure body fat, muscle mass, or fat distribution, and it cannot tell you anything specific about an individual's body composition.

It is widely used because it is cheap, fast, and gives a reasonable population-level signal of weight-related health risk. For most adults of average build it correlates well with body fat percentage, which is why public health bodies still rely on it as a first-line screening tool.

Athletes and people with a lot of muscle can land in the overweight or obese range despite carrying very little body fat, because muscle is denser than fat. Older adults can show a normal BMI while having reduced muscle mass and increased body fat, which BMI does not detect.

BMI also ignores fat distribution. Two people with the same BMI can have very different health profiles depending on where fat is stored, which is why waist circumference and waist-to-hip ratio are often used alongside it. Different ethnic groups may face higher health risks at lower BMI thresholds, particularly people of South and East Asian descent. For a more detailed body composition estimate, try the Body Fat Calculator.