What is BMI and how is it calculated?
Body Mass Index (BMI) is a measure of body fat based on your height and weight. It is calculated by dividing your weight in kilograms by your height in metres squared (kg/m²). BMI is used by health professionals as a quick screening tool to identify potential weight-related health issues.
While BMI is a useful starting point, it does not directly measure body fat and does not account for muscle mass, bone density, or where fat is distributed in the body. Athletes with high muscle mass may have a high BMI despite being very healthy.
BMI categories for adults in the UK
The NHS uses the following BMI categories for adults aged 18 and over:
Underweight: BMI below 18.5 — may indicate malnutrition or other health issues.
Healthy weight: BMI 18.5 to 24.9 — associated with the lowest health risks.
Overweight: BMI 25 to 29.9 — increased risk of health conditions.
Obese: BMI 30 or above — significantly increased risk of serious health conditions.
Frequently asked questions
Is BMI accurate for everyone? ▾
BMI is a useful general guide but has limitations. It does not distinguish between muscle and fat, so athletes or very muscular people may have a high BMI without excess body fat. It also does not account for age, ethnicity, or where fat is stored in the body. Always consult your GP for a full health assessment.
What is a healthy BMI for women in the UK? ▾
For adult women in the UK, the NHS considers a BMI between 18.5 and 24.9 to be a healthy weight. However, some research suggests that women naturally carry slightly more body fat than men at the same BMI, so this range is considered appropriate for both sexes.
What is a healthy BMI for men in the UK? ▾
For adult men in the UK, a healthy BMI is also considered to be between 18.5 and 24.9. Men typically have more muscle mass than women, which can affect BMI readings. If you are very muscular, your BMI may overestimate your body fat levels.
Can I use this BMI calculator for children? ▾
BMI for children and teenagers is calculated differently to adults, as it takes age and sex into account. This calculator is designed for adults aged 18 and over. For children, please use the NHS BMI healthy weight calculator which uses age-specific charts.
How can I reach a healthy BMI? ▾
Reaching a healthy BMI involves a combination of balanced diet and regular physical activity. The NHS recommends at least 150 minutes of moderate exercise per week. If you are concerned about your weight, speak to your GP who can provide personalised advice and refer you to support services if needed.
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