How is BMI worked out?
BMI is your weight in kilograms divided by your height in metres squared. You do not need to do the maths yourself — a calculator does it for you.
The same number is then compared against standard categories to give a general idea of weight status.
What do the categories mean?
For most adults: under 18.5 is 'underweight', 18.5–24.9 is 'healthy weight', 25–29.9 is 'overweight', and 30 or above is in the 'obese' range.
These cut-offs are lower for some groups. In the UK, people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds are advised to use lower thresholds, because health risks can appear at a lower BMI.
What BMI does not tell you
BMI is only a rough guide. It does not measure body fat directly and cannot tell the difference between fat and muscle, so very muscular people may have a high BMI while being healthy.
It also says nothing about where fat is stored. Waist measurement is often used alongside BMI, because fat around the middle carries more health risk. BMI is a starting point for a conversation, not a diagnosis.
Common questions
Is BMI accurate for everyone?
No. It is less reliable for very muscular people, older adults, pregnant women, and children (who use different charts). It is a general screening guide, not a precise measure of health.
What should I do if my BMI is high?
Use it as a prompt to talk to your doctor or practice nurse, who can look at the bigger picture — your waist, blood pressure, blood sugar and overall health — and suggest realistic next steps.
Related tools
These calculators are designed for healthcare professionals.