BMI (Body Mass Index) is calculated by dividing weight in kilograms by height in metres squared (kg/m²). It was developed by Belgian statistician Adolphe Quetelet in the 1830s and is used worldwide by GPs and health organisations as a quick screening tool.
The NHS uses four BMI categories: Underweight (below 18.5), Healthy weight (18.5–24.9), Overweight (25–29.9), and Obese (30 and above). These thresholds apply to most adults aged 18–65. Different thresholds apply to children, pregnant women and certain ethnic groups — South Asian adults, for example, are advised to aim for a BMI below 23.
BMI has limitations. It does not distinguish muscle from fat, so trained athletes often show elevated BMIs despite low body fat. For a more complete picture, consider waist circumference alongside BMI.
Tap Calculate — see your BMI, category and healthy range
BMI categories (adults)
The standard adult BMI ranges used by the NHS and WHO. BMI is a general screening measure and the same number can mean different things depending on age, sex, ethnicity and muscle mass.
Category
BMI range
Underweight
Below 18.5
Healthy weight
18.5 – 24.9
Overweight
25.0 – 29.9
Obese
30.0 and above
Frequently Asked Questions
ℹ️ Accuracy note: BMI is a screening tool, not a diagnostic measure. It does not account for muscle mass, age, or ethnicity. Consult your GP for a full health assessment.
18.5–24.9. Under 18.5 is underweight, 25–29.9 is overweight, 30+ is obese per NHS and WHO guidelines.
No — BMI does not distinguish muscle from fat. Muscular people may show a high BMI despite low body fat. Body fat % is more precise.
Speak to your GP. BMI is one indicator — they will consider age, lifestyle and medical history together.
A BMI of 30 or above is classified as obese by the NHS. 30–34.9 is Class 1 (moderate) obesity, 35–39.9 is Class 2 (severe), and 40 or above is Class 3 (very severe). Different lower thresholds apply to people of South Asian or Chinese background.
BMI is calculated the same way for children but interpreted using age and sex-specific centile charts rather than fixed thresholds. A result in the 85th percentile or above for a child's age and sex indicates overweight. Use a paediatric BMI calculator or consult your child's GP.
The NHS uses the same BMI thresholds (18.5–24.9 is healthy) for both men and women. However, women naturally carry a higher proportion of body fat than men at the same BMI. For a complete picture, waist circumference is also a useful measure — above 80cm for women or 94cm for men indicates increased risk.