Calculate your Body Mass Index. Supports metric and imperial units.
Body Mass Index (BMI) is a numerical value calculated from a person's height and weight. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and has since become one of the most widely used screening tools in healthcare. BMI provides a simple way to categorise individuals into weight ranges that may be associated with health risks, making it a useful starting point for conversations about weight and health.
The metric BMI formula is: BMI = weight in kilograms ÷ (height in metres)². For imperial measurements, the formula is: BMI = 703 × weight in pounds ÷ (height in inches)². For example, a person who is 175cm tall and weighs 70kg has a BMI of 70 ÷ (1.75)² = 70 ÷ 3.0625 = 22.9, which falls in the normal weight range.
The World Health Organisation defines four main BMI categories for adults. Underweight is a BMI below 18.5, which may indicate malnutrition or other health concerns and warrants medical attention. Normal weight is a BMI of 18.5 to 24.9, generally associated with the lowest health risks for most adults. Overweight is a BMI of 25 to 29.9, which is associated with increased risk of certain health conditions. Obese is a BMI of 30 or above, which is further divided into Class 1 (30–34.9), Class 2 (35–39.9), and Class 3 (40+), with progressively higher health risks at each level.
BMI is a useful population-level screening tool but has important limitations when applied to individuals. It does not distinguish between fat mass and muscle mass, so athletes and bodybuilders often have a "overweight" BMI despite having very low body fat. It also does not account for where fat is distributed on the body — abdominal fat carries higher health risks than fat stored elsewhere. BMI is less accurate for the elderly, who may have lost muscle mass, and children require age and sex-specific BMI charts. People of different ethnic backgrounds may also face different health risks at the same BMI value.
For most adults aged 20 and over, a BMI between 18.5 and 24.9 is considered healthy. However, health is multidimensional — blood pressure, cholesterol levels, blood sugar, fitness level, and mental health all matter alongside BMI. Two people with identical BMIs can have very different health profiles. A BMI in the normal range is a positive sign but does not guarantee good health, and a BMI slightly outside the normal range does not automatically mean poor health. Always use BMI as one data point among many, and consult a healthcare professional for a complete assessment.
BMI calculations for children and teenagers (ages 2–19) use the same formula but are interpreted differently using age and sex-specific growth charts. Rather than fixed cut-off points, children's BMI is expressed as a percentile compared to other children of the same age and sex. A BMI percentile below the 5th is considered underweight, 5th–84th is healthy weight, 85th–94th is overweight, and 95th or above is considered obese. Our calculator is designed for adults — please use a paediatric BMI calculator for children.