How it works
BMI is weight in kilograms divided by height in metres, squared. It gives one number that is easy to compare, which is why doctors use it as a first screen.
The bands used here are the standard adult ones: under 18.5 underweight, 18.5 to 24.9 healthy, 25 to 29.9 overweight, 30 and above obese.
Some guidelines for Asian populations use lower cut-offs, because health risks can begin at a lower BMI. Ask your doctor which applies to you.
The formula
BMI = weight (kg) ÷ height (m) ^ 2
In pounds and inches: BMI = 703 × weight (lb) ÷ height (in) ^ 2.
A worked example
A person who weighs 70 kg and is 170 cm tall has a BMI of 70 ÷ 1.7² = 24.2, which is in the healthy band.
Frequently asked questions
Is BMI accurate?
It is a rough guide only. It does not separate muscle from fat or show where fat is carried, so an athlete can have a high BMI and be healthy.
Does it work for children?
No. Children and teenagers are assessed against growth charts for their age. This tool is for adults.
What should I do with the result?
Treat it as a prompt for a conversation, not a diagnosis. If you are concerned about your weight, talk to a doctor.