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FAQs about body mass index (BMI)

Key points about body mass index

Your body mass index (BMI) gives you an idea of whether you're underweight, overweight or a healthy weight for your height.
It's calculated using your weight and your height and can be worked out using a BMI calculator (see below).
It should be used as an indicator as it's not a perfect measure and is not suitable for all people.
BMI is less accurate for adults with high muscle mass and for people in ethnic groups with smaller body size.
Talk to your healthcare provider if you have any concerns about your weight.

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Your BMI is calculated using your weight and your height and is best done using a calculator (see BMI calculator). BMI is a useful measurement of growth for most people. However, it is only an estimate and it can't tell how much of your weight is body fat and how much is muscle mass.

BMI result	What does the result mean?
Less than 18.5	Underweight This result means that you may be underweight. Being underweight can be associated with a range of health issues. If you're concerned about your weight, talk about your result with your GP, practice nurse or dietitian.
18.5–24.9	Normal weight This result means that you are a healthy body weight, which is generally good for your health. Keep up the great work!
25–29.9	Overweight This result means that you may be overweight. Carrying extra weight is associated with a range of health issues, including being at increased risk of heart disease. If you're concerned about your weight, talk about your result with your GP, practice nurse or dietitian
30 or more	Obese This result means that you may be obese. Obesity is associated with a range of health issues, including being at an increased risk of heart disease. If you're concerned about your weight, talk about your result with your GP, practice nurse or dietitian.

The BMI measurement is not suitable for some people:

women who are more than 10 weeks pregnant
children under 2 years of age
people, particularly children, with certain health conditions such as some endocrine or genetic conditions.

Talk to your healthcare provider about your weight if you have any questions or concerns.

Although BMI is not a perfect measure, it's generally agreed to be the most useful and valid measure for most people. However, there are some limitations.

Adults with higher than normal levels of muscle mass
BMI is less accurate as an indicator of being overweight in adults with higher than normal levels of lean body tissue (muscle mass). This is because muscle weighs more than fat and the BMI does not take this into consideration.

Ethnic groups with smaller body stature
BMI is less accurate as an indicator of being overweight for people in ethnic groups with smaller body stature (eg, Asian ethnic groups). Along with BMI, measuring your waist can help you work out whether you are carrying excess weight around your middle and therefore at risk of heart disease and type 2 diabetes.

Being underweight can increase your risk of, or be a sign of, ill health. Talk to your doctor or practice nurse for further advice.

'Overweight' and 'obese' are medical terms used to describe a person who has body fat at a level above what is considered healthy. BMI provides an indication of excess weight for height, but can't tell the difference between fat and muscle mass.

Being obese is not good for your health. It can increase your risk of developing high blood pressure and serious health conditions, including type 2 diabetes, arthritis, heart disease and cancer.

Measuring your waist is a good way to check that you don't have too much fat around your stomach (tummy). It is a better estimate of visceral fat, the dangerous internal fat that coats your organs. It's therefore a more accurate predictor of your risk of heart disease, type 2 diabetes and stroke.

To measure your waist:

find the top of your hip bone and the bottom of your ribs (this is usually at the level of your belly button)
wrap a tape measure around your waist midway between these points
breathe out normally before taking the measurement.

No matter what your BMI is, or how tall you are, you should try to lose weight if your waist size is:
Men	94 cm (37 in)
Women	80 cm (31.5 in)

You're at very high risk of serious health conditions and should talk to your GP or practice nurse if your waist size is:
Men	102 cm (40 in)
Women	88 cm (34 in)