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What is BMI and how does it work?

BMI is an acronym for the term body mass index. It’s a measurement of a person’s body weight in relation to their height for all adults, irrespective of age and sex.

In 1985 the National Institutes of Health (NIH) recommended that physicians adopt the BMI as an index of obesity. It has now become the standard formula to determine what is a healthy weight, underweight, overweight and obese. These weight categories are used to help assess a person’s risk of obesity related diseases.

How does it work for adults?

You need to know your height and weight and input them into the standard BMI formula. BMI is calculated by dividing a person’s weight in kilograms by the square of their height in metres (kg/m2).

Metric BMI formula

BMI = Weight (kg) / Height (m)²

Imperial BMI formula

BMI = [Weight (lbs) / Height (inches)²] x 703

The imperial BMI formula uses a conversion factor of 703.

How does BMI work for children?

For children the relationship between weight classification and BMI varies with age and sex. Children’s BMI measures are usually compared to a growth reference in order to determine a child’s weight status.

Some factors, for example timing of puberty or ethnicity, can make the classification of children’s BMI more complex. There are a number of different child growth references and associated thresholds. In the UK, we most often use the UK90 Growth Reference.

The link between BMI thresholds and future morbidity and mortality is weaker for children than for adults but evidence shows that children with a high BMI are more likely to have a high BMI in adulthood, and therefore a raised risk of future health problems.

How is BMI used?

BMI is a proxy measure of excess body fat. Doctors use BMI as a screening tool to signpost a person’s weight category. A person with a high BMI indicates they may have high body fatness and an overweight or obese weight status with an increased risk of obesity related illnesses. BMI is not a diagnostic tool of an individual’s body fatness or health so further tests are required.

BMI is the most commonly used measure for monitoring the prevalence of overweight and obesity at population level.

How are BMI levels interpreted?

BMI provides an indication of your health status. The BMI categories are:

Underweight = BMI<18.5

Normal weight = BMI 18.5–24.9

Overweight = BMI 25–29.9

Obesity = BMI of 30 or above

Why use BMI?

BMI is an easy and non-invasive method of accurately and consistently assessing excess body fat at a population level. Excess body fat is linked to a person’s current and future morbidity. True measures of BMI are expensive or impractical and other proxy measures are more difficult to measure precisely.

BMI population data has been compiled on a worldwide scale.

Limitations of BMI

BMI has limitations. It may not be an accurate tool for assessing weight status at an individual level as it does not take into account a person’s fitness and muscle mass, their ethnic origin, body shape, age and sex. It also doesn’t provide an indication of body fat distribution.

At a population level these limitations are not seen as particularly important as they even out across large numbers of people.

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