This site uses cookies to:
  • Allow members to log in to the site;
  • Collect anonymous data for Google Analytics, so that we know which parts of the site are the most interesting;
  • To prevent this message from annoying you if you've already dismissed it;
By using the site, you are agreeing to the use of these cookies. If you have cookies disabled, some parts of the site may not work as expected.

Dismiss this message

What is Body Mass Index (BMI)?

Body Mass Index

Body Mass Index (BMI) is a figure derived from an equation used by specialists to determine whether or not a person lies within a ‘healthy’ weight range.

The BMI chart was introduced as a quick screen to determine health risk using weight and height as the two independent variables. The assumption is that a person who is suffering from excess fat storage is more likely to develop injuries and illness due to the additional stress placed upon their organs and soft tissues by this fat.

The BMI is a very common method of evaluating individual people to see if they are under or over weight. The equation compares the subjects’ weight to their height by dividing the weight measurement (expressed in kilograms) by the square of their height (expressed in metres). A BMI of below 18.5 is considered underweight, between 18.5 and 25 is an indication of healthy weight, 25 to 30 is overweight, a BMI of over 30 is referred to as obese, over 35 is known as morbid obesity, and over 40 indicates extreme obesity.

The BMI equation is:

Your Weight in Kg Divided By Your Height in Metres squared (squared means multiplied by itself)


Weight/Height x Height

If you would like to know what your BMI is and don’t want to calculate it yourself, use the NHS BMI calculator.

The BMI test does have major limitations, so is only used as a general guideline for assessing healthy weight. There are many cases where BMI would not be suitable, such as the following:

  • Pregnant women
  • Amputees
  • Athletes/people who exercise regularly
  • Babies
  • The elderly

Specialists who deal with these particular groups of people will rarely use the BMI test to assess the condition of their clients for the following reasons…

  • A heavily pregnant woman would be heavier than usual due to the weight of her unborn baby and the fluid sac that surrounds the baby for protection.
  • An amputee would often be much lighter than usual, due to the weight of their missing limb being subtracted from their body weight, which would produce figures that would suggest the patient was severely under-weight.
  • Athletes tend to be very muscular, and muscle is a heavier tissue than fat when compared in equal volume. For this reason, an athlete can be very heavy compared to their height, but in peak fitness. If this was not taken into account, many athletes would appear obese on the BMI scale.
  • Babies carry lots of fat, but this fat is naturally present in babies for warmth and nourishment and is not a cause for health concerns.
  • Elderly people often suffer from muscle wastage, which can make them very light, which if misinterpreted could lead the practitioner to believe the person was suffering from ill-health, when in reality nature has simply taken its course.

So how are these groups measured if the BMI scale is not accurate?

There are various alternative means for assessing the physical condition of these groups, such as body fat testing, waist to hip ratio, circumference measurement of the body and measurement of bone thickness.

This is a specialist subject in its own right, and is known as Kinanthropometry, which is the science of measuring the body. A Personal Trainer, health club or fitness testing facility will be able to provide you with a much better and more accurate physical assessment than a BMI reading. It is important not to get hung-up on statistics, but they can provide you with a consistent and accurate reference for progression.

If you are untrained (not a regular exerciser) and do not fall into one of the categories listed in this article then use the BMI as your starting point for assessing your current health risk.

Leave any questions or comments below if you want more information on Body Mass Index.

Published by


Owner of Personal Trainer, Father and fitness copy writer. Working hard making the world fitter and healthier!

Leave a Reply

Your email address will not be published. Required fields are marked *

More Like This