How BMI Is Calculated

Body Mass Index (BMI) is one of the most widely used numbers in public health, and the maths behind it is refreshingly simple. This guide walks through the exact formula, a worked example you can follow along with, and why your result is identical whether you measure in kilograms or pounds.

The BMI formula

BMI is your weight in kilograms divided by the square of your height in metres:

BMI = weight (kg) ÷ height (m)²

The result is expressed in units of kg/m², though those units are almost always left off in everyday use. That single ratio is the whole calculation — there are no age, sex, or activity adjustments built into the number itself. That simplicity is exactly why BMI became a public-health standard: it needs only two measurements that are quick, cheap, and easy to take accurately.

This is the formula published by the World Health Organization and used in clinical and public-health settings worldwide. Our BMI Calculator applies it directly.

A worked example

Suppose someone is 170 cm tall and weighs 68 kg. First, convert the height to metres — 170 cm is 1.70 m — then square it:

1.70 × 1.70 = 2.89

Now divide the weight by that figure:

68 ÷ 2.89 = 23.5

So the BMI is 23.5, rounded to one decimal place. Under the standard WHO scale, that falls in the normal weight range. (Interestingly, the same 23.5 lands in a different category under the WHO cutoffs used for Asian populations — a difference we unpack in Why India uses different BMI cutoffs.)

Metric or imperial — same answer

You do not have to work in metric. If you enter your height in feet and inches and your weight in pounds, the calculator converts those to metric first and then applies the same formula, so the result is identical either way.

If you ever want to compute an imperial BMI by hand, there is an equivalent form that folds the unit conversion into a constant:

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

The 703 factor simply converts pounds-and-inches into the same scale as kilograms-and-metres. It is the same measurement expressed in different units — not a different formula, and not a different result.

Why the number is squared

A natural question is why height is squared rather than used directly. The short answer is that body weight tends to scale roughly with the square of height across a typical adult population, so dividing by height squared produces a figure that is largely independent of how tall someone is. That is what lets a single set of cutoffs apply to short and tall adults alike. It is an approximation rather than a law of nature — which is part of why BMI is a screening tool rather than a precise measure of body composition — but it holds well enough across large groups to be useful.

What the number does and doesn't tell you

The calculation gives you a single figure; interpreting it is a separate step. A BMI value on its own is just a ratio — its meaning comes from the category bands it falls into, and those bands are where the evidence and the population differences live. The next guide, What is a healthy BMI?, covers how to read your result against the standard ranges.

It is also worth being clear about what the formula cannot see. Because it uses only height and weight, BMI cannot distinguish muscle from fat, and it does not know where weight is carried on the body. For most people that is fine for a screening number, but for very muscular or athletic builds it can be misleading — a limitation we cover in BMI, muscle, and why athletes read high.

The bottom line

BMI is weight in kilograms divided by height in metres squared. It takes two easy measurements, produces one number, and gives the same answer in any unit system. That simplicity is its strength as a population screen and, as the rest of this series explains, also the source of its limitations.

Ready to see yours? Use the BMI Calculator — it shows your value, your category, and the confidence level behind it, and it applies the appropriate regional cutoffs automatically.