BMI Calculator

Calculate Body Mass Index from your height and weight, in metric or imperial units, and see which standard weight category — underweight, normal, overweight, obese — that score falls into.

How it works

  1. Enter your height and weight in your preferred units — metric or imperial.
  2. The tool calculates your BMI and shows which standard category it falls into.
  3. A short note explains what BMI measures and why the result should be considered alongside other health indicators.

How BMI is calculated

Body mass index is weight divided by height squared: BMI = kg / m^2. In imperial units the calculator uses BMI = 703 x pounds / inches^2, where 703 is the conversion factor between the two systems. Feet and inches are combined into total inches first. The result is shown to one decimal place and placed in one of four adult categories: under 18.5 is underweight, 18.5 to 24.9 is normal, 25 to 29.9 is overweight, and 30 and above is obese. These cut-offs follow the World Health Organization's adult classification.

As a worked example, a person who is 175 cm tall and weighs 70 kg has a BMI of 70 / (1.75 x 1.75) = 22.9, which is in the normal range. Someone who is 5 ft 9 in (69 inches) and weighs 160 lb has a BMI of 703 x 160 / 69^2 = 23.6.

Where the number comes from and what it is for

The formula goes back to the Belgian mathematician Adolphe Quetelet in the 19th century, and the name Body Mass Index was popularised in 1972. Its strength is that it is simple, needs no equipment and is consistent, which makes it useful for comparing groups of people and spotting broad trends. For one person it is a rough screening measure, not a diagnosis of health.

What BMI cannot tell you

BMI does not distinguish muscle from fat, so very muscular people can score as overweight, and it does not show where fat is carried. Fat around the waist carries more health risk than fat elsewhere, so waist measurement is often used alongside BMI. The categories were developed for adults, and they are not suitable for children and teenagers, whose healthy range depends on age and sex, or for pregnancy. In older adults a slightly higher BMI may not carry the same risk. Some health organisations also use lower thresholds for people of South and East Asian descent, because health risks can rise at lower BMI values in those groups.

If your result is outside the normal range, treat it as a prompt to look at the bigger picture, such as waist size, fitness, blood pressure, diet and family history, and talk to a doctor or nurse who can do so. A result in the normal range does not guarantee good health either.

Frequently asked questions

What is the BMI formula?

BMI equals weight in kilograms divided by height in metres squared, or weight in pounds divided by height in inches squared, times 703, for imperial units.

Is BMI an accurate measure of health?

BMI is a quick population-level screening tool, not a diagnosis — it doesn't distinguish muscle from fat, so athletes and very muscular people are often misclassified. Talk to a doctor for a full assessment.

Sources and further reading

Formulas checked against the sources below. Last reviewed . General information only, not professional advice.