Body mass index (BMI) is widely used as a simple and reliable way of finding out if you are overweight or obese.

BMI measures whether you're a healthy weight for your height.

For most adults, having a BMI of 18.5 to 24.9 means you are considered to be a healthy weight. A person with a BMI of 25 to 29.9 is considered to be overweight, and someone with a BMI over 30 is considered to be obese.

However, while BMI is a useful measurement for most people, it is not accurate for everyone.

For example, the normal BMI scores may not be accurate if you are very muscular, because muscle can add extra pounds and may mean you have a high BMI when you are not an unhealthy weight. In such cases, it may be more helpful to use your waist circumference as a better guide (see below).

What is considered a healthy BMI is also influenced by your ethnic background. The scores mentioned above generally apply to people with a white Caucasian background. If you are from an ethnic minority background, the threshold for being considered overweight or obese may be lower.

BMI should not be used to work out if a child is a healthy weight, as their bodies are still developing. If you want to find out whether your child is overweight, speak to your GP.

Visiting your GP

If you are overweight or obese, visit your GP for advice about losing weight safely and to find out if you have an increased risk of health problems.

Your GP may ask about:

• your lifestyle – particularly your diet and how much physical activity you do, and also whether you smoke and how much alcohol you drink
• any possible underlying causes for your obesity – for example, if you are taking medication or have a medical condition that may contribute to weight gain
• how you feel about being overweight – for example, if you are feeling depressed
• how motivated you are to lose weight
• your family history – as obesity and other health conditions, such as diabetes, are often more common in families

As well as calculating your BMI, your GP may perform tests to determine if you are at increased risk of health complications because of your weight. These could include:

• measuring your blood pressure
• measuring your glucose (sugar) and cholesterol levels in a blood sample
• measuring your waist circumference (the distance around your waist)

People with very large waists (generally, 94cm or more in men and 80cm or more in women) are more likely to develop obesity-related health problems.

Your GP may also take your ethnicity into account, as these can affect your risk of certain conditions. For example, some people of Asian, African or Afro-Caribbean ethnicity may be at increased risk of high blood pressure (hypertension). Healthy waist measurements can also be different for people from different ethnic backgrounds.