Calculate your Body Mass Index and weight category
Body Mass Index (BMI) is a simple numerical measure that uses your height and weight to estimate whether you are within a healthy weight range. It is one of the most widely used screening tools in healthcare, used by the NHS and the World Health Organisation (WHO) to categorise adults as underweight, normal weight, overweight, or obese. Your GP will often calculate your BMI as part of a routine health check. While BMI is a useful population-level screening tool, it has well-known limitations. It does not distinguish between muscle mass and fat mass, which means that very muscular individuals (such as athletes and bodybuilders) may be classified as overweight or obese despite having a low body fat percentage. Similarly, BMI does not account for fat distribution -- carrying excess fat around the waist (visceral fat) is associated with higher health risks than carrying it on the hips and thighs, regardless of overall BMI. Despite these limitations, BMI remains a practical starting point for understanding your weight relative to your height. A BMI outside the normal range can prompt further investigation, such as waist circumference measurement, body fat percentage testing, or blood work, to get a more complete picture of your health.
To calculate your BMI: 1. Enter your weight in kilograms. If you know your weight in stones, multiply by 6.35 to convert to kilograms. For example, 12 stone = 76.2 kg. 2. Enter your height in metres. If you know your height in feet and inches, convert to metres: for example, 5 foot 8 inches = 1.73 metres. Use the decimal format (e.g. 1.75, not 175). 3. View your results. The calculator displays your BMI value (rounded to one decimal place) and your weight category according to the WHO classification: - Below 18.5: Underweight - 18.5 to 24.9: Normal weight - 25.0 to 29.9: Overweight - 30.0 and above: Obese 4. Use the result as a screening tool. If your BMI falls outside the normal range, consider discussing it with your GP for personalised health advice and further assessments.
BMI is calculated using a straightforward formula: BMI = weight (kg) / height (m) squared Or in mathematical notation: BMI = kg / m^2 For example, a person weighing 75 kg with a height of 1.75 m has a BMI of 75 / (1.75 x 1.75) = 75 / 3.0625 = 24.5. The result is a dimensionless number that places you into one of the standard WHO categories. The boundaries were established based on epidemiological data linking BMI ranges to health outcomes across large populations. A BMI of 18.5 to 24.9 is considered the "normal" or "healthy" range because this range is associated with the lowest risk of weight-related health conditions, including type 2 diabetes, cardiovascular disease, and certain cancers. A BMI of 25 to 29.9 (overweight) and 30+ (obese) are associated with progressively higher risks. However, the relationship between BMI and health is not perfectly linear -- other factors such as age, sex, ethnicity, muscle mass, waist circumference, and overall fitness level all play important roles. For individuals of South Asian, Chinese, and other Asian ethnic backgrounds, the NHS notes that health risks may increase at a lower BMI threshold. A BMI of 23 or above may indicate increased risk in these populations, and 27.5 or above may indicate high risk.
BMI is intended for adults aged 18 and over. For children and teenagers, BMI is calculated differently using age- and sex-specific percentile charts. The NHS provides a healthy weight calculator that accounts for these factors. If you are concerned about your weight or health, the NHS recommends speaking to your GP, who can provide a holistic assessment beyond BMI alone.