BMI Calculator

This calculator computes Body Mass Index — weight divided by height squared — and bands it into the World Health Organization's standard classification: under 18.5 underweight, 18.5-25 normal, 25-30 overweight, then three obesity classes at five-point steps (30-35 Class I, 35-40 Class II, 40+ Class III). These are the same cutoffs used internationally by public health agencies to compare population weight data, not a scale this calculator invented. The Unit System selector decides which fields are read: Imperial uses Weight (lbs) with Height in feet and inches (applying the 703 conversion factor), while Metric reads the kilogram and centimeter fields instead — the two sets do not mix, so editing pounds while in Metric mode changes nothing. Whichever mode is active, height is the stronger lever in practice because it enters the formula squared and, in imperial mode, each foot is twelve inches: moving the feet field by one step shifts BMI far more than a single pound.

The healthy weight range outputs invert the formula — multiplying your squared height by 18.5 and 24.9 — so they are pure functions of height and never move when weight changes. One structural caveat the number cannot show: BMI conflates muscle with fat and says nothing about distribution, which is why muscular athletes routinely land in the overweight band. The category edges are hard cutoffs, so a BMI of 24.9 and 25.1 are numerically trivial but carry different labels.