Calculate your Body Mass Index using metric or imperial units. Get your BMI value and category instantly.
BMI stands for Body Mass Index. It is a number calculated from your height and weight that gives a rough indication of whether you are in a healthy weight range. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and has been used by doctors and health organisations worldwide as a quick screening tool ever since.
The formula is simple. Divide your weight in kilograms by your height in metres squared. For example someone who is 175cm tall and weighs 70kg has a BMI of 70 divided by 1.75 squared which equals 22.9. This falls in the normal healthy range.
The World Health Organisation defines the following BMI categories for adults aged 18 and over. A BMI below 18.5 is classified as underweight. Between 18.5 and 24.9 is the normal or healthy weight range. Between 25 and 29.9 is classified as overweight. A BMI of 30 or above is classified as obese, with further subdivisions into class 1 obese (30 to 34.9), class 2 obese (35 to 39.9), and class 3 or severely obese (40 and above).
In metric units the formula is weight in kilograms divided by height in metres squared. In imperial units the formula is weight in pounds divided by height in inches squared, multiplied by 703. Both give the same result. This tool accepts both unit systems and converts automatically.
BMI is a useful screening tool at the population level but it has well-known limitations when applied to individuals. It does not measure body fat directly and cannot distinguish between fat mass and muscle mass. A professional athlete with a large amount of muscle may have a BMI in the overweight range despite having very low body fat. Conversely an older person may have a normal BMI but carry a high proportion of body fat due to muscle loss with age.
BMI also does not account for the distribution of fat in the body. Abdominal fat carries a higher health risk than fat stored in other areas, but BMI cannot detect this difference. For a complete assessment of health risk a doctor will consider waist circumference, blood pressure, blood glucose, and cholesterol levels alongside BMI.
BMI categories were originally developed from studies of European populations and may not be equally accurate for people of other ethnic backgrounds. Some health organisations recommend lower BMI thresholds for people of Asian descent where health risks increase at lower BMI values than the standard thresholds suggest.
The standard adult BMI categories do not apply to children and teenagers. For people under 18 BMI is interpreted using age and sex specific percentile charts. A BMI at or above the 95th percentile for age and sex is considered obese for children. A BMI between the 85th and 95th percentile is considered overweight. Consult a paediatrician for BMI assessment in under 18s.
BMI is a useful first screening tool but it is not a complete picture of health. It works reasonably well at the population level for identifying trends but has significant limitations for individuals. It should be one of several factors considered when assessing health rather than the only measure. Always discuss your health with a qualified doctor rather than relying on BMI alone.
For most adults a BMI between 18.5 and 24.9 is considered healthy by the World Health Organisation. However healthy weight ranges can vary by age, sex, and ethnic background. Some organisations use slightly different thresholds. The target range your doctor recommends may differ from the standard WHO categories based on your individual health profile.
BMI decreases when you lose body weight. The most effective approaches combine a calorie-controlled diet with regular physical activity. A reduction of 500 to 1000 calories per day from your maintenance level typically produces a weight loss of 0.5 to 1 kilogram per week. Consult a doctor or registered dietitian before starting a weight loss programme especially if you have existing health conditions.
No. All calculations happen entirely in your browser. Your height, weight, and results are never sent to any server and are not stored anywhere.