BMI Calculator
Instantly compute your Body Mass Index from your weight and height. Use it to screen whether you fall into underweight, normal, overweight, or obese ranges as defined by the WHO.
About this calculator
Body Mass Index (BMI) is a simple screening metric that relates body weight to height squared. The formula is BMI = weight (kg) / (height (m))². Because height is entered in centimetres, it is first divided by 100 to convert to metres before squaring. The result is expressed in kg/m². The WHO classifies BMI below 18.5 as underweight, 18.5–24.9 as normal weight, 25–29.9 as overweight, and 30 or above as obese. BMI is widely used in clinical settings and population studies as a quick, cost-free proxy for body-fat levels, though it does not distinguish muscle from fat and should be interpreted alongside other health indicators.
How to use
Suppose you weigh 75 kg and are 175 cm tall. Step 1 — Convert height to metres: 175 ÷ 100 = 1.75 m. Step 2 — Square the height: 1.75² = 3.0625 m². Step 3 — Divide weight by that value: 75 ÷ 3.0625 ≈ 24.5 kg/m². A BMI of 24.5 falls in the 'Normal weight' range (18.5–24.9). Enter your own weight and height above and the calculator performs these steps instantly.
Frequently asked questions
What is a healthy BMI range for adults?
For adults aged 18 and over, the World Health Organization defines a healthy BMI as 18.5 to 24.9 kg/m². Values below 18.5 indicate underweight, 25–29.9 indicate overweight, and 30 or above indicate obesity. These thresholds are population-level guidelines and may not apply equally to all ethnic groups — for example, some Asian health authorities use a lower obesity cut-off of 27.5. Always discuss your BMI result with a healthcare provider for personalised advice.
Why does BMI use height in metres squared rather than just height?
Squaring the height accounts for the fact that body volume — and therefore mass — scales in three dimensions while height is one-dimensional. Dividing weight by height alone would systematically make taller people appear heavier relative to their actual body composition. Using height² produces a dimensionally consistent ratio (kg/m²) that is roughly comparable across people of different statures. This mathematical relationship was first formalised by Adolphe Quetelet in the 19th century, which is why BMI is sometimes called the Quetelet Index.
When is BMI not an accurate indicator of body fat?
BMI cannot distinguish between fat mass and lean mass, so it often misclassifies highly muscular individuals — such as athletes — as overweight or obese. Conversely, older adults who have lost muscle (sarcopenia) may have a 'normal' BMI while carrying an unhealthy proportion of body fat. It is also less reliable for children, pregnant women, and certain ethnic populations. In these cases, clinicians complement BMI with measures such as waist circumference, body-fat percentage via DEXA scan, or waist-to-hip ratio.