How to Calculate Your BMI (Formula, Categories, and Caveats)

Body Mass Index, or BMI, is a single number that relates your weight to your height. Doctors, gyms, and health forms reach for it because it is quick to work out and easy to compare. This guide explains exactly how BMI is calculated in both metric and imperial units, what the standard categories mean, and where the number falls short. It is a general guide, not medical advice.

What BMI actually measures

BMI is a ratio of weight to height squared. The idea is that taller people naturally weigh more, so dividing by height (twice over) puts people of different sizes on a comparable scale.

The result is a single figure, usually between roughly 15 and 40 for most adults, that sorts into broad bands. It does not measure health directly. It is a rough proxy that flags whether your weight is in a typical range for your height, which is why it shows up so often on intake forms and fitness apps.

The BMI formula in metric units

If you think in kilograms and meters, the formula is clean.

BMI = weight in kilograms divided by (height in meters squared).

Worked example: someone who is 1.75 meters tall and weighs 70 kilograms.

1. Square the height: 1.75 times 1.75 = 3.06.
2. Divide weight by that: 70 divided by 3.06 = about 22.9.

A BMI of 22.9 sits in the middle of the typical range. Note that height goes in as meters, not centimeters, so 175 cm becomes 1.75 m before you square it.

The BMI formula in imperial units

If you measure in pounds and inches, there is an extra factor to keep the units consistent.

BMI = (weight in pounds divided by height in inches squared) times 703.

Worked example: someone who is 5 feet 9 inches (69 inches) and weighs 154 pounds.

1. Square the height: 69 times 69 = 4,761.
2. Divide weight by that: 154 divided by 4,761 = 0.03235.
3. Multiply by 703: 0.03235 times 703 = about 22.7.

That 703 multiplier is just the conversion that makes pounds and inches line up with the metric definition. The two methods describe the same person, so the answers land close together.

What the categories mean

Health bodies group adult BMI into four standard bands. They are the same regardless of which formula you used.

• Below 18.5: underweight
• 18.5 to 24.9: normal or healthy range
• 25.0 to 29.9: overweight
• 30.0 and above: obese

These cutoffs are population-level guides for adults. They are not a diagnosis, and a number near a boundary does not flip a switch about your health. Treat the category as a starting point for a conversation, not a verdict.

The honest caveats

BMI is popular because it is simple, and that simplicity is also its weakness. A few things it cannot see:

• It does not distinguish muscle from fat. A muscular athlete can register as overweight while carrying very little body fat.
• It ignores where weight sits. Two people with the same BMI can have very different fat distribution, which matters for health.
• It was built around general adult populations. It is less reliable for children, older adults, pregnant people, and some ethnic groups.

So use BMI as one rough signal among many, not the whole story. For anything that concerns you, a qualified health professional can interpret it in context.

Get your BMI instantly

You do not need to square heights and juggle conversion factors by hand. The free BMI Calculator takes your height and weight in metric or imperial units and returns your BMI and category right away.

Enter your numbers and read off the result. The BMI Calculator runs entirely in your browser, so there is no sign-up and nothing you enter is uploaded anywhere. It is the quickest way to check the figure without reaching for the formula.

Related calculators

A few nearby tools round out the picture. Since many health benchmarks shift with age, the Age Calculator works out an exact age in years, months, and days. If you want to track readings over time, the Average Calculator turns a series of numbers into a single representative figure. And to express a change, like progress toward a goal, as a proportion, the Percentage Calculator handles it in one step.

The short version

BMI divides weight by height squared, using a 703 multiplier in imperial units, and sorts the result into four broad categories. It is a fast general guide, not a measure of body fat or overall health. When you want the number without the math, the BMI Calculator gives it to you in seconds.