BMI calculator.
Your body mass index and category — in imperial or metric.
What BMI measures
Body mass index is weight divided by height squared — in metric it's kilograms ÷ metres², and in imperial it's pounds ÷ inches² × 703, which is just the same formula with a unit-conversion factor folded in. The standard adult categories are: under 18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, and 30 or above obesity. Because BMI uses only height and weight, it can over-state body fat for very muscular people and under-state it for others, and it isn't meant for children, who use age-and-sex percentile charts instead. It's best read as a population-level screening number, not a verdict on an individual's health. To convert between units yourself, the unit converter handles weight and length.
Working it through with real numbers
In metric, an adult who is 1.78 m tall and weighs 77 kg has a BMI of 77 ÷ (1.78 × 1.78) = 77 ÷ 3.1684 = 24.3, which sits at the top of the normal band. In imperial the same calculation uses the 703 factor: someone 5 ft 10 in (70 inches) weighing 170 lb works out as 170 ÷ (70 × 70) × 703 = 24.4. The two systems agree because 703 exists purely to reconcile pounds-per-square-inch with kilograms-per-square-metre.
Note that height is squared, so height errors matter far more than weight errors. Getting your height wrong by an inch shifts the result more than getting your weight wrong by a pound.
What the number does not capture
BMI treats all weight as equivalent, which is its central limitation. It cannot distinguish muscle from fat, so athletes and heavily muscled people are routinely classed as overweight while carrying very little body fat. It also says nothing about where fat sits, and abdominal fat is more strongly associated with metabolic risk than fat carried elsewhere — which is why waist circumference, or waist-to-height ratio, is often used alongside it rather than instead of it.
The standard thresholds were derived largely from European-descent populations, and several health bodies recommend lower cut-offs for people of South and East Asian descent, where metabolic risk appears at a lower BMI. Age and sex also shift what a given number implies. Treat the categories as a starting point for a conversation with a clinician, not a diagnosis.
Children and teenagers
The adult categories do not apply under 18. Children are assessed with age-and-sex-specific percentile charts, because healthy body composition changes substantially through growth — a BMI that is entirely normal for a 16-year-old could be well outside the healthy range for a 6-year-old. If you need a figure for a child, use a paediatric percentile chart rather than this calculator.