What is the BMI formula?

BMI equals weight in kilograms divided by height in metres squared. In imperial units, it is weight in pounds times 703, divided by height in inches squared. Both produce the same result for the same person.

Where does the 703 come from in the imperial formula?

It is a unit-conversion constant, not a separate formula — it converts pounds and inches into the metric equivalent before the same division happens. There is no such thing as a distinct "imperial BMI"; it is the identical calculation with different starting units.

Why does BMI use height squared rather than height alone?

Body mass scales roughly with the square of height across a normal adult range, which is an empirical observation from the population data BMI was built on, not a law of geometry. Squaring height is what keeps BMI from unfairly favoring or penalizing people purely for being taller or shorter.

Does the BMI formula use age or sex?

No. The adult BMI formula uses only height and weight. Age and sex affect how a given BMI number should be interpreted — through tools like estimated body fat percentage — but they are not inputs to the BMI calculation itself.

BMI Formula

The BMI formula divides weight by the square of height. It is the same calculation regardless of unit system — metric and imperial versions always agree, because the imperial version is the metric formula with a unit-conversion constant built in.

The formula, both ways

Metric: BMI = weight (kg) ÷ height (m)²

Imperial: BMI = 703 × weight (lb) ÷ height (in)²

Metric example

175 cm, 70 kg

22.9

Imperial example (same person)

68.9″, 154.3 lb

22.9

Both examples describe the same person and produce the same BMI, as they must — the unit system changes the inputs, never the result.

How to calculate it step by step

Measure height and weight. Measure height in metres (or feet and inches) and weight in kilograms (or pounds), without shoes and in light clothing for the most accurate result.
Square the height. Multiply your height in metres by itself. If using feet and inches, first convert to inches or centimetres.
Divide weight by height squared. Divide your weight in kilograms by the squared height in metres. In imperial units, multiply weight in pounds by 703 first, then divide by height in inches squared.
Compare against the category thresholds. The result is your BMI. Compare it against the WHO adult thresholds — under 18.5 is underweight, 18.5 to 24.9 is a healthy weight, 25 to 29.9 is overweight, and 30 or above is obesity.

Prefer not to do the arithmetic by hand? Use the BMI calculator — it applies exactly this formula and shows the category alongside the number.

In this section

Why height is squared

Body mass tends to scale roughly with the square of height across the normal adult range — an empirical pattern in population data, not a law of physics. Squaring height is what keeps two people of different heights but proportionally similar builds landing at a similar BMI, rather than the taller person being penalized simply for being tall.

This is also exactly where BMI's limitations begin: the formula assumes a population-average relationship between height, weight and body composition, so it says nothing about whether an individual's weight is muscle or fat.

Common questions

What is the BMI formula?: BMI equals weight in kilograms divided by height in metres squared. In imperial units, it is weight in pounds times 703, divided by height in inches squared. Both produce the same result for the same person.
Where does the 703 come from in the imperial formula?: It is a unit-conversion constant, not a separate formula — it converts pounds and inches into the metric equivalent before the same division happens. There is no such thing as a distinct "imperial BMI"; it is the identical calculation with different starting units.
Why does BMI use height squared rather than height alone?: Body mass scales roughly with the square of height across a normal adult range, which is an empirical observation from the population data BMI was built on, not a law of geometry. Squaring height is what keeps BMI from unfairly favoring or penalizing people purely for being taller or shorter.
Does the BMI formula use age or sex?: No. The adult BMI formula uses only height and weight. Age and sex affect how a given BMI number should be interpreted — through tools like estimated body fat percentage — but they are not inputs to the BMI calculation itself.