BMI Calculator

Body mass index is a single number that relates your weight to your height. It was designed as a quick screening tool for large populations, and that is still the best way to read it: BMI tells you which weight band you fall into, not whether you as an individual are healthy. This calculator returns your BMI, the standard World Health Organization category, the healthy weight range for your height, and the gap between your current weight and that range.

The arithmetic is simple. Metric BMI is weight in kilograms divided by height in metres squared. Imperial BMI takes weight in pounds, multiplies by 703, and divides by height in inches squared. The two formulas agree exactly; the 703 is just the unit conversion constant. Because height is squared, small differences in measured height move the result noticeably, which is why a clinician measures height carefully rather than asking.

BMI is deliberately blunt. Muscle is denser than fat, so athletes often land in the overweight band with very low body fat. Pregnancy, older age, and some medical conditions shift the meaning of the number too. Conversely, BMI can miss elevated risk in people with a normal weight but high waist circumference. The number is a starting point for a conversation, never a diagnosis.

If you are tracking change over time, weigh yourself under the same conditions — same time of day, same scale, after using the bathroom, before eating — and look at the trend across weeks rather than any single reading. A shift of one or two pounds is usually water, not fat. For children and teenagers, BMI is interpreted with age- and sex-specific percentile charts instead, and requires a paediatric reference.

This BMI calculator is an educational screening tool, not a medical diagnosis. BMI does not measure body fat directly and misclassifies muscular, pregnant, older, and very tall or short people. Discuss your weight and health with a qualified clinician.