Health workers use body mass index (BMI) to assess obesity. They measure the height and mass of a person and then use the equation below to calculate their BMI:

BMI = mass (kg) ÷ height^² (m^²)

For example, if a person has a mass of 60kg and a height of 1.65m:

BMI = 60kg ÷ 1.65^²

BMI = 22 kg/m^²

A person is overweight if their BMI is between 25kg/m^² and 30kg/m^².

A person with a BMI over 30 kg/m^² is said to be obese. Therefore, the person in our example is not overweight or obese.

Issues related to BMI

While BMI can give a general guide to whether someone is a overweight or obese it is not always accurate:

• BMI cannot tell the difference between different types of body tissue.
• It does not take account of age, gender or muscle mass.
• A very muscular individual could be classed as overweight because of the extra mass of their muscle compared to height.
• Older adults who have lost muscle mass may be classed as of a healthy weight even if carrying excess fat.
• The BMI of pregnant women can't be measured accurately. Their pre-pregnancy BMI should be used.