The Body Mass Index, referred to more commonly as BMI, is a method used to calculate a person’s weight-related health risk.

Primarily, it takes into account a person’s height and weight.

Those who fall into higher BMI categories are likely to be at risk of developing diet-related diseases such as type-2 diabetes and heart disease.

How it is calculated

There are two ways of calculating BMI: one formula uses metric units while the other uses imperial.

  • mass in kilograms (kg) / height in metres (m) squared
  • (mass in pounds (lbs) / height in inches (in) squared) x 703

For instance, a person who is 72kg and 1.75m tall will have a BMI of:

  • 72/(1.75x1.75) = 23.5

How BMI is categorised

The majority of people will fall into one of three BMI categories:

  • healthy weight
  • overweight
  • obese

However there are in fact eight categories as defined by the World Health Organisation in total, which are as follows:

Very severely underweight<15
Severely underweight15-16
Healthy weight18.5-25
Obese class 235-40
Obese class 340>

These categories are sometimes interpreted differently outside the UK. Until 1998 in the US, the upper integer for healthy weight was actually 27.8, until it was brought in line with WHO guidelines and lowered to 25.

In Japan, there is no overweight category at all, and a BMI of 25 or above is considered obese.

What these categories mean

Those who are above the healthy weight category are considered to be at increased risk of diet-related illness; and those who are obese are considered to be high risk.

Examples of diet-related illness include:

Persons who are of Black or Asian ethnicity may be at increased risk of type-2 diabetes and other conditions at a lower BMI.

Within these groups, those who:

  • have a BMI of 23 or above are considered to be at increased risk
  • have a BMI of 27.5 or above are considered to be high risk

A person who is underweight may also be at risk of certain illnesses. Someone who isn’t eating enough may not be getting the full range of nutrients their body needs, and have a weaker immune system making them more susceptible to infection.

Sometimes, a person may also be underweight due to an underlying, undiagnosed medical condition.

What you can do

Persons who are overweight or obese may be able to get their BMI into a healthy range through a combination of diet and exercise. A GP can help with the implementation of a routine which can help someone lose weight at a healthy rate.

However, in those cases where weight is posing a serious risk to health, and diet and exercise alone has proved insufficient in helping to achieve results, a diet and exercise programme also including weight loss medication may be advised.

Weight loss medication is usually not needed by or prescribed to those who have a BMI of 27 or less.


The Body Mass Index is a popular formula and straightforward to apply, but it is not always an accurate indicator of a person’s fat to body mass ratio.

For instance, it measures weight, but cannot differentiate between fat and muscle. As muscle weighs more than fat, persons with a muscular build who may have very low percentage body fat, such as professional athletes, may still be identified by the index as overweight or even obese.

Another drawback of the Body Mass Index formula is that it does not take age into account, and this is particularly relevant for older persons. Muscle mass decreases as we get older, and the formula does not detect where this may be compensated for in weight by increased fat. Consequently, an older person may have a BMI in the healthy range, when their body fat percentage is actually unhealthy.

Page last reviewed:  01/10/2020