BMI Calculator

Body mass index, its category and your healthy weight range.

Technique moves the reading more than most people expect: a waist taped at the navel and one taped at the narrowest point can differ by two inches, and impedance scales shift with hydration. Consistency of method matters more than the absolute number on any single day.

BMI was devised in the 1830s and adopted by the US Public Health Service in the 1980s; the CDC puts overweight at 25 to 29.9 and obesity from 30. It was built on European populations and reads high for muscular builds and Pacific Islander groups, which is why waist measurement and body-fat estimates sit alongside it.

Body measurements in the United States are read against published reference tables: BMI categories from the CDC, body-fat ranges from the American Council on Exercise, and circumference standards used by the Army and Navy. BMI divides weight by height squared and cannot separate muscle from fat, so a lean, heavily built person can read as overweight.

Waist measurement and body-fat estimates fill in what the index alone misses.

These scales were built on population averages, so a single reading says more about where you sit against a table than about your own health.

Fat-free mass, hydration and time of day all move a reading, and a measurement taken after a meal or a workout reads differently from one taken fasted. Trends over weeks, taken the same way, describe a body; a single figure describes a moment.

BMI Calculator sits with the rest of the library's health work, and it rests on the same body-measurement arithmetic the way a clinical reference table does. Its answer is only as good as the assumptions standing behind it.

The quantities it works from — age, gender, weight, height and waist — are the ones a real body-measurement question actually turns on, and each carries a convention of its own, set out below.

Health figures in the United States are read against reference tables published by federal agencies and professional bodies, and those tables describe populations rather than individuals. One measurement is a data point; a series taken the same way over months is information.

Clinical guidance also shifts as evidence accumulates, which is why a range that was standard a decade ago may no longer be quoted.