What Is BMI and How Is It Calculated?
Body Mass Index, or BMI, is a simple screening number that relates your weight to your height. It was developed in the 19th century by the Belgian mathematician Adolphe Quetelet and remains one of the most widely used health indicators because it requires only two measurements and a quick calculation. While it does not measure body fat directly, it gives a useful first estimate of whether your weight sits in a healthy range for your height.
The BMI Formula
In metric units, BMI is your weight in kilograms divided by the square of your height in metres. The formula is BMI = weight (kg) ÷ height (m)². Because most people know their height in centimetres, the calculator above divides your centimetre value by 100 first, then squares it. In imperial units the formula is BMI = 703 × weight (lb) ÷ height (in)², where the factor of 703 converts the result into the same scale as the metric version.
Reading Your Result
Once you have your number, you can place it on the standard adult scale. A BMI below 18.5 is classified as underweight, 18.5 to 24.9 is the normal or healthy range, 25.0 to 29.9 is overweight, and 30.0 or above is obese. The coloured bar in the calculator shows roughly where your value lands. These ranges apply to adults aged 20 and over and are the same for men and women, which is one of the reasons BMI is so convenient as a population-level measure.
What BMI Does Not Tell You
BMI is a starting point, not a complete picture of health. Because it uses only height and weight, it cannot distinguish between muscle and fat. A very muscular athlete may register as overweight despite having low body fat, while an older person who has lost muscle may sit in the normal range yet carry excess fat. BMI also does not account for where fat is stored, your bone density, ethnicity, or sex differences in body composition. For these reasons, doctors use BMI alongside other measures such as waist circumference, blood pressure, and blood tests.
Using BMI Wisely
The most useful way to use BMI is as a quick, repeatable check that you can track over time. A single reading is far less informative than watching the trend across months. If your number sits outside the normal range, treat it as a prompt to look more closely rather than a diagnosis. Small, consistent changes to diet and activity move the figure gradually, and a healthcare professional can interpret it in the context of your full health profile. Children and teenagers need age- and sex-specific BMI percentile charts rather than the adult ranges shown here, so the categories above should not be applied to anyone under 20. Used sensibly, BMI is a fast and genuinely helpful tool for keeping an eye on your general health.
This calculator is for general information only and is not medical advice. Consult a qualified healthcare provider for guidance about your health.