BMI Calculator
Enter your height and weight to calculate your Body Mass Index. Results use NHS weight categories. Supports metric and imperial units.
How BMI Calculator works
What is BMI?
Body Mass Index (BMI) is a measure of body fat based on height and weight, calculated by dividing weight in kilograms by the square of height in metres. It was developed by Belgian statistician Adolphe Quetelet in the 19th century and is widely used by the NHS and clinicians as an initial screening tool for weight-related health risks.
NHS BMI categories
The NHS defines the following categories: Underweight (BMI below 18.5), Healthy weight (18.5–24.9), Overweight (25–29.9), Obese class I (30–34.9), Obese class II (35–39.9), and Obese class III or severely obese (40 and above). Being underweight can indicate malnutrition or other health issues; being overweight or obese increases the risk of type 2 diabetes, heart disease, and some cancers.
Limitations of BMI
BMI does not distinguish between muscle and fat, so athletes and highly muscular individuals may be classified as overweight despite having low body fat. It is also less accurate for older people, who tend to have more body fat than BMI suggests, and for children and young people (who use age-related BMI charts). The NHS and NICE acknowledge these limitations and use BMI as one indicator among many.
Ethnicity and adjusted thresholds
NICE and the World Health Organisation recommend lower BMI thresholds for some ethnic groups. For adults of South Asian, Chinese, Black African, African-Caribbean, and Arab descent, a BMI of 23 may indicate increased risk (equivalent to 25 for white European adults), and a BMI of 27.5 may indicate high risk (equivalent to 30). Your GP can advise on appropriate thresholds for your background.
Frequently asked questions
What is a healthy BMI?
For most adults, the NHS defines a healthy BMI as 18.5–24.9. A BMI below 18.5 is considered underweight; 25–29.9 is overweight; 30 or above is obese. These thresholds may be lower for some ethnic groups.
Is BMI accurate for athletes?
BMI can overestimate fatness in people with high muscle mass, such as athletes and bodybuilders. Since muscle weighs more than fat, a muscular person can have a high BMI while having a low body fat percentage. For athletes, body composition measurements (DEXA scan, bioelectrical impedance) are more informative.
My BMI says I'm overweight but I feel healthy — should I be worried?
BMI is a screening tool, not a diagnostic. If your BMI is slightly above 25, consider factors like waist circumference (a waist over 94cm for men or 80cm for women is associated with higher metabolic risk), body fat percentage, blood pressure, and cholesterol. Speak to your GP for a complete health assessment.
How do I convert my weight to kilograms?
To convert stones to kg: multiply stones by 6.35029 and add pounds × 0.453592. For example, 11 stone 8 lb = (11 × 6.35029) + (8 × 0.453592) = 69.85 + 3.63 = 73.5 kg. Use the "st + lb" option in this calculator to enter imperial measurements directly.
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This calculator provides estimates for informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider.