What Is BMI? Definition, Formula and Limits

BMI stands for body mass index: your weight divided by your height squared, reported in kg/m². It exists to sort adults quickly into four weight categories using two measurements anybody can take. The NHLBI is careful about what that buys you — it calls BMI a screening tool that does not necessarily diagnose body fatness.

The formula, both ways

In metric units BMI is the definition itself: kg ÷ m². Someone 68 kg and 1.68 m computes 68 ÷ 2.82 = 24.1.

In US units the same person is 150 lb and 5 ft 6 in, and the formula picks up a conversion factor: 703 × lb ÷ in², giving 703 × 150 ÷ 66² = 24.2. The small gap is rounding in the height conversion, not two different indexes. There is no American BMI and international BMI — there is one index and two ways to feed it.

Why an index instead of a weight

Weight alone says nothing without height. Two adults at 150 lb, one 5 ft 2 in and one 6 ft 0 in, are in different situations, and any table of “healthy weight” would need a row per height to say so. Dividing by height squared collapses that into a single number that can be compared across people — which is exactly what a public health agency needs when it is describing a country rather than a patient.

That origin matters. The index was designed in the 19th century to describe populations, and it still behaves like a population instrument: excellent at summarizing millions of people, coarse at describing any one of them.

The four adult categories

BMI	Category
Below 18.5	Underweight
18.5 – 24.9	Healthy weight
25.0 – 29.9	Overweight
30.0 and above	Obesity

Those are the NHLBI ranges, and the NIDDK publishes the same ones for adults aged 20 and over. Note that healthy weight is a band, not a target — there is no ideal value inside it, and nothing special about its middle. See underweight BMI and overweight BMI for what the bands on either side imply.

Three things BMI cannot see

• What the weight is made of. MedlinePlus states that because muscle weighs more than fat, very muscular people may have a high BMI. The scale reports pounds, not tissue.
• Where the fat is. Abdominal fat carries more cardiometabolic risk than the same amount elsewhere, and the formula has no input for location.
• Everything else about you. Blood pressure, glucose, lipids, fitness, and age all shape risk and none of them appear in the calculation.

Reading the result honestly

A BMI inside the healthy band is mildly reassuring and proves nothing. A BMI outside it is a reason to measure more — waist, blood pressure, labs — and not a diagnosis of anything. Used that way, two numbers and thirty seconds of arithmetic earn their place.

For the calculation in pounds and inches, use the BMI calculator; to look up a range rather than compute one, use the BMI chart.

This content is informational and does not replace evaluation by a health professional.