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Obesity diagnosis (including BMI)

Body mass index (BMI)

Body mass index (BMI) is often used to assess if you are overweight or not.

How to measure your BMI

To measure your BMI you will need to know your height and waist measurement in centimetres and weight in kilograms. It is easiest to use a BMI calculator to measure your BMI.

For most adults:

  • between 25 and 29, you would be considered overweight
  • between 30 and 39, you would be considered obese
  • 40 or more, you would be considered very obese .

The BMI calculation cannot take into account very muscular figures. Muscle can add extra weight, and this may give you an overweight or obese BMI when you are not an unhealthy weight. For example, a heavyweight boxer would be classed as obese using the BMI, when he is in fact a healthy weight for him.

However, for most people, BMI remains an accurate method of assessing weight.

Children and young people should not use BMI to calculate if they are a healthy weight, as their bodies are still developing. Instead, children and young people should visit their doctor, who will be able to tell them if they are overweight or obese for their height and sex.

Waist circumference

Another useful method to assess your weight is to measure your waist circumference. Men whose waist measurement is 94 cm or more and women whose waist measurment is 80 cm or more are more likely to develop obesity-related health problems, such as type 2 diabetes, heart disease and some types of cancer.

How to measure your waist circumference

It is important to measure your waist circumference accurately.

  1. Place a tape measure directly on your skin, or on no more than one layer of light clothing. Make sure the tape is snug, without squeezing the skin.
  2. Make sure to place the tape measure horizontally halfway between your lowest rib and the top of your hipbone. This is roughly in line with your belly button.
  3. Breathe out normally and take the measure.

Visiting your doctor

If you are overweight or obese, visit your doctor to find if you are at increased risk of health problems, and how you can safely lose weight. Talk to your doctor about:

  • any underlying causes your might have for your obesity - for example, if you are on certain medication or have a medical condition that causes weight gain
  • your lifestyle - particularly your diet and how much physical activity you do, and also whether you smoke, and how much alcohol you drink
  • how you feel about being overweight - for example, if you are feeling depressed about it
  • how motivated you are to lose weight
  • your family history of obesity and other health conditions, such as diabetes (a condition where there is too much glucose (sugar) in the blood).

Further tests

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

  • measuring your blood pressure
  • measuring the glucose (sugar) and lipid (fat) levels in your blood.

Last reviewed: August 2016

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