The BMI calculation divides an adult's weight in kilograms by their height in metres squared. For example, A BMI of 25 means 25kg/m2.
For most adults, an ideal BMI is in the 18.5 to 24.9 range.
For children and young people aged 2 to 18, the BMI calculation takes into account age and gender as well as height and weight.
If your BMI is:
To work out your BMI:
BMI takes into account natural variations in body shape, giving a healthy weight range for a particular height.
As well as measuring your BMI, healthcare professionals may take other factors into account when assessing if you're a healthy weight.
Muscle is much denser than fat, so very muscular people, such as heavyweight boxers, weight trainers and athletes, may be a healthy weight even though their BMI is classed as obese.
Your ethnic group can also affect your risk of some health conditions. For example, adults of Asian origin may have a higher risk of health problems at BMI levels below 25.
You should not use BMI as a measure if you're pregnant. Get advice from your midwife or doctor if you're concerned about your weight.
Important: Our website provides useful information but is not a substitute for medical advice. You should always seek the advice of your doctor when making decisions about your health.