BMI Calculator
Enter your height and weight to calculate your Body Mass Index. Results use NHS weight categories. Supports metric and imperial units.
Reviewed by Richard Ross · Last updated April 2026
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.
Is BMI accurate for athletes and muscular people?
BMI is a poor indicator for people with above-average muscle mass — muscle is denser than fat, so a muscular person may have a high BMI but low body fat. Elite athletes and bodybuilders are frequently classified as "overweight" by BMI despite being extremely lean. For these individuals, body fat percentage, waist circumference, or DEXA scans are better indicators of health risk.
Does BMI differ between men and women?
BMI uses the same scale for men and women, but the health implications differ. Women naturally carry more body fat than men at the same BMI. A BMI of 25 in a woman indicates a different body composition than the same BMI in a man. Some researchers argue for sex-specific BMI thresholds, but the standard WHO categories are gender-neutral. Body fat percentage provides a more sex-appropriate measure of adiposity.
Is BMI different for different ethnic groups?
Yes. WHO and NICE guidelines recognise that people of South Asian, Chinese, and other East Asian backgrounds have a higher risk of type 2 diabetes and cardiovascular disease at a lower BMI. Public Health England recommends lower action thresholds for these groups: overweight is from BMI 23 (not 25) and obese from BMI 27.5 (not 30). This reflects differences in where body fat is stored and its metabolic effects at different weights.
How was the BMI scale developed?
BMI was developed by Belgian mathematician Adolphe Quetelet in the 1830s as a statistical measure of population weight distributions — not as a clinical tool. It was not widely adopted in medicine until the 1970s when Ancel Keys' study validated it as a proxy for body fat in population studies. Its simplicity made it popular, but its limitations for individuals are well-documented. Most health professionals use it as one of several screening tools rather than a definitive measure.
What BMI is associated with the lowest health risk?
Research generally shows the lowest mortality risk at BMI 20–25 for most populations. The "obesity paradox" is a finding in some studies that slightly overweight people (BMI 25–30) have lower all-cause mortality than normal-weight people — though this may reflect confounding factors. Below BMI 18.5, risks of malnutrition, osteoporosis, and cardiac complications increase. The relationship is a J-curve: both very low and very high BMI are associated with higher mortality.
How does BMI relate to waist circumference?
Waist circumference is a complementary measure that captures abdominal (visceral) fat, which is more metabolically harmful than subcutaneous fat. NICE recommends that people with a BMI of 25–35 who have a waist circumference above 88cm (women) or 102cm (men) are at higher cardiometabolic risk. For South Asian men, a waist above 90cm is the action threshold. Used together, BMI and waist circumference provide a better health risk picture than either alone.
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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.