BMI

What is Body Mass Index?

The definition of body mass index (BMI) is the measurement of a person’s weight in relation to their height. Based on this measurement, a person’s weight will be categorised as either underweight, within the healthy BMI range, overweight, or obese, as defined by the World Health Organization (WHO).

BMI however cannot tell the difference between fat, muscle and bone density. For example, if you have low body fat and a lot of muscle, you may be classed as overweight or obese. 

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 for assessing a person's risk of developing weight-related diseases, such as heart disease, high blood pressure, and diabetes.

How does BMI formula work for adults?

A standard BMI formula ( or height weight calculator) is used to calculate an adult's BMI. You just need to divide a person's weight by the square of their height to calculate their average BMI.

For example:

Metric BMI formula

BMI = Weight (kg) / Height (m)²

BMI Range 

The Body Mass Index indicates an ideal BMI for most adults. The BMI ranges are:

Underweight = BMI <18.5

Healthy BMI weight = BMI 18.5–24.9 

Overweight = BMI 25–29.9

Obesity = BMI of 30 or above

How does BMI work for children?

For children, although BMI is still calculated the same as for adults, it also considers the age and gender of the child, as other factors, such as the timing of puberty, can make the classification of a child’s BMI more complex.

A child’s average BMI measurements are compared to a growth reference (usually the UK90 Growth Reference) to more accurately determine if a child’s weight is right for their height. The result is given as a percentile ranking. For example, a healthy weight for a typical child would be between the 3rd and 91st percentile.

The link between BMI thresholds and future morbidity and mortality is weaker for children than 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 the Body Mass Index as a screening tool to identify a person’s weight category. For example, a person with a high BMI indicates they may have high body fat levels, which could lead to an increased risk of developing obesity-related illnesses, such as heart disease and type II diabetes. However, if a person has a very low BMI, this could indicate they have too little body fat, which can lead to other health problems such as anaemia, lowered immunity, and decreased bone density.

As BMI is a screening tool only - doctors would need to carry out further tests, such as a skinfold thickness check, waist measurements, diet and physical activity evaluations, and family history to accurately assess an individual’s overall health as BMI cannot alone be used as a diagnostic tool.

A BMI check is, therefore, most commonly used for monitoring the prevalence of overweight and obesity at a population level rather than an individual level.

Why use BMI?

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

BMI data has been compiled worldwide to give an overview of people’s health and weight across various populations. 

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 consider a person’s fitness and muscle mass, ethnic origin, age, sex and body fat distribution.

It’s possible to have a ‘normal’ BMI and still be unhealthy. Similarly, it’s also possible to have a high BMI and be healthy. For example:

• Athletes and bodybuilders who have a high proportion of muscle (muscle weighs more than fat) are categorised with a high BMI even though they may be a healthy weight.
• There are height and weight ratio differences between races. For example, some ethnic groups may have a higher risk of certain health problems, such as diabetes, even if their weight is within a healthy BMI range for their height.
• BMI for men and women may not always give an accurate presentation of health as women tend to have more body fat than men of an equal BMI.
• Due to loss of muscle mass, older people tend to have more body fat than younger people with the same BMI.
• BMI for women can also be inaccurate as it does not account for pregnant or lactating women.
• BMI cannot be accurately used for children and teenagers who are still growing and are not yet physically mature.

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