BMI Calculator

The NHLBI classification puts a BMI below 18.5 in the underweight band, 18.5 to 24.9 in the healthy band, 25.0 to 29.9 in the overweight band, and 30.0 and above in one of three obesity classes. The World Health Organization uses the same 25 and 30 cut-offs. They are population screening thresholds, not a diagnosis of anything.

Most US calculators use the shortcut weight in pounds divided by height in inches squared, times 703. That 703 is a rounded constant; the exact figure is 703.0696, because a pound is exactly 0.45359237 kg and an inch exactly 0.0254 m. This calculator converts with the exact factors and then applies the kilograms-per-metre-squared definition, so the answer is right to full precision and identical whichever units you enter.

Poorly. BMI knows your height and your weight and nothing else — not how much of that weight is muscle, bone or fat, and not where the fat sits. A heavily trained athlete can land in the overweight or obese band with very little body fat. That is a real limitation of the measure, not a rounding problem, which is why clinicians pair it with waist circumference and other risk factors.

Not these numbers. The bands here are for adults aged 20 and over. Children and teenagers are assessed against age- and sex-specific growth percentiles instead, because healthy body composition changes a great deal through growth. A 12-year-old with a BMI of 22 is not in the same position as an adult with a BMI of 22.

The formula and the cut-offs are the same. Women carry more essential body fat than men at the same BMI, so the same number can mean somewhat different body composition, but the NHLBI and WHO thresholds are not split by sex. Some health authorities do use lower thresholds for people of South and East Asian descent, where risk rises at a lower BMI.

BMI Prime is your BMI divided by 25, the upper limit of the healthy band. It turns the number into a ratio: 1.0 sits exactly on the overweight threshold, 0.9 is ten percent below it, and 1.2 is twenty percent above. It is the same information as BMI, scaled so the distance from the threshold is readable at a glance.

Because the bands are ranges, not points. NHLBI prints the healthy band as 18.5 to 24.9 and overweight as 25.0 to 29.9, but those printed endpoints are a display convention for "below 25" — no real BMI lands between 24.9 and 25.0. This calculator uses 25 as the boundary itself, so every weight belongs to exactly one band and none falls through a gap.