How BMI is calculated

Body Mass Index (BMI) is your weight in kilograms divided by your height in metres, squared. In formula form: BMI = weight (kg) ÷ height (m) × height (m). The idea dates back to the 1830s and 1840s, when the Belgian mathematician Adolphe Quetelet noticed that, across a population, weight tends to scale with the square of height. The name "Body Mass Index" only arrived in 1972, and it has been used as a quick screening number ever since.

Here is a worked example. Someone who is 175 cm tall and weighs 70 kg first converts the height to 1.75 metres. Squaring that gives 3.0625. Dividing 70 by 3.0625 gives a BMI of about 22.9. Another person at 170 cm and 85 kg has a squared height of 2.89, and 85 divided by 2.89 gives roughly 29.4. This calculator does the same arithmetic as soon as you enter your height in centimetres and your weight in kilograms. If you usually think in feet, inches, or pounds, convert first: 1 inch is 2.54 cm and 1 kg is about 2.2 lb.

What the BMI ranges mean

The World Health Organization groups adult BMI into broad bands. Under 18.5 is classed as underweight, 18.5 to 24.9 is the normal range, 25 to 29.9 is overweight, and 30 or above is obese, which is further split into three classes (30 to 34.9, 35 to 39.9, and 40 and over). These bands were drawn from large population studies that linked higher and lower BMI values to different rates of illness.

The same cut-offs do not fit everyone equally well. Because health risks can rise at lower BMI values in some groups, guidelines for people of South and East Asian background often use lower action points, commonly 23 for increased risk and 27.5 for high risk. Children and teenagers are not assessed with these adult bands at all. They use age- and sex-specific growth charts, so this calculator is meant for adults.

When BMI is useful, and when it is not

BMI is useful because it needs only two measurements and no equipment. For large groups of people it is a reasonable way to spot general trends, and for many adults it gives a quick first impression of whether their weight is far from a typical range for their height.

Its main weakness is that it cannot tell muscle from fat. Muscle is denser than fat, so a strength athlete can land in the overweight range while carrying very little body fat. The opposite also happens: someone with a normal BMI can have low muscle and a high share of body fat. BMI also says nothing about where fat is stored, and fat around the waist is generally linked to greater health risk than fat elsewhere. Age, sex, and ethnicity all affect how the number should be read.

How to use your result

Treat your BMI as one data point rather than a verdict. If your result is well outside the normal range, it is a sensible prompt to look at other measures, such as waist size and body fat percentage, and to talk to a doctor or dietitian. If it is only slightly outside the range and you are active and feeling well, the number alone is rarely a reason to worry. For a fuller picture, try the Body Fat % Calculator, check a reference weight range with the Ideal Weight Calculator, or estimate your daily energy needs with the Calorie / BMR Calculator. This tool gives general information only and is not medical advice.