Is BMI accurate?

Whenever you talk about monitoring your weight, you will invariably hear the letters BMI referred to. BMI, or Body Mass Index, has been around for quite a while. The basic concept of BMI was actually introduced sometime around the 1830s! BMI is widely used as a simple basis for determining whether an individual is under or overweight, or within a “normal” weight range.

Your BMI is calculated with a simple formula, of your weight (in kilograms) divided by your height (in metres) squared. It is accepted that a BMI of less than 18.5 means an individual is underweight, between 18.5 and 25 is healthy, 25-30 is overweight and anything above 30 is obese.

For athletes, or for those of us who like to lead a healthy, active lifestyle, BMI is often used as a reference point to maintain a healthy weight. But is your BMI actually a useful tool for monitoring your weight, or is it an out-dated system?

As stated already, the basic formula for BMI was calculated in the 1830s. The actual name of Body Mass Index was introduced in 1972. But obviously, over the years science has progressed, and we now have a far greater understanding of the human body than we did 50 years ago. As a result, BMI as a measurement has quite a few flaws.


Not every individual is the same. Different ethnicities, and even people from the same ethnic backgrounds, have different body types. As we are all individuals, people are rarely anatomically identical. Body Mass Index fails to take this into consideration.

People of the same height can have vastly different bone structure. This obviously results in different frame sizes, as well as different skeletal composition. Some people have longer limbs and a short torso, while others are the opposite. In addition, some people have naturally larger frames, while others are naturally smaller in frame size. A good indicator of this is wrist circumference……those with smaller wrist sizes for their height have a smaller frame, while a larger wrist circumference indicates a bigger frame size.

Healthy levels of body fat also vary between individuals. Differences in age, sex and ethnic background can all show different levels of body fat which are considered healthy.

Having a look at the formula for BMI, it is clearly obvious that frame size and skeletal composition are not considered at all. Those with a smaller frame may be considered underweight; while individuals with a larger frame size may have a comparatively high BMI, incorrectly indicating an unhealthy weight…..and by not considering body fat levels, BMI doesn’t give a full picture of an individual’s health.


Just because an individual has a BMI within the healthy weight range, it does not mean they are actually healthy. High levels of visceral fat, sometimes known as “skinny fat”, can occur where an individual has a high amount of non-visible fat.

At some point in time, most of us have probably met that person who does absolutely no exercise and eats whatever junk food they want, but seems to have a slim figure…..but their external appearance does not mean they are healthy on the inside. A lack of muscle, combined with a higher level of visceral fat, is not a good thing.

Within your body, fat is stored around the major organs of the abdominal cavity…..this is visceral fat. When this builds up too much, it can result in a condition known as metabolic obesity, which is associated with the same risk factors to your health as overweight and obese people.

Again, BMI does not refer to this at all. Just because somebody has a healthy BMI, it does not necessarily mean they are healthy.


Active individuals, especially those who are involved in strength based exercise or competition, often have more muscle. It comes with the territory. Usually, competitive athletes have a fairly low amount of body fat too…..but if you look at their BMI, some would be considered overweight and unhealthy!

Have a look at sports life Australian Rules Football and American Football, or those involved in body building. These athletes have a huge amount of muscle and are at the peak of their fitness, and are usually quite healthy individuals. But as muscle is quite dense and heavy (it is heavier than fat), their BMI would put them within the range of being overweight, but obviously this is a bit misleading.

Once again, BMI can be inaccurate. The simplistic formula fails to accommodate for body composition, meaning some individuals with more muscle and a low amount of body fat may fall into the unhealthy, overweight category.


Clearly, Body Mass Index as a measure for determining a healthy weight is flawed. There are alternatives to help monitor your weight though.

Waist circumference is considered an alternative to whether an individual maintains a healthy weight or not. It is considered that a waist circumference less than a 94cm for men and 80cm for women is generally healthy. Although this measurement also has it’s downsides……

  • Metabolic obesity can still be an issue for “healthy” measurements
  • Body composition changes are hard to track
  • Smaller and larger frame sizes are not considered at all

Skinfold measurements can be used as an indicator of body fat percentage. This system requires the use of a set of callipers to measure skin folds across various body sites, then a formula is used to calculate body fat. Of course, this also has it’s limitations….

  • Accurate, repeatable measurements can be difficult to attain without experience
  • It is not considered to be highly accurate
  • It doesn’t determine full body composition, just subcutaneous body fat

Biometric Impedance Analysis is where a small electrical impulse is sent through the body to determine body composition. Total body water, muscle mass, fat mass and bone mass can all be calculated from the body’s impedance to this current. But again, this system does have a few flaws….

  • Machines are varying in accuracy
  • Results can change as a result of hydration status, time of day, food intake and exercise


Clearly, BMI is a rather out-dated system, with many flaws. But it is still a good starting point if you are choosing to monitor your weight as part of your fitness goals. Remember to take into consideration the many different variables which can make it so inaccurate, and don’t use it as your only reference point.

Ideally, you should consider your overall body composition……muscle, fat and water. Also remember that the type of exercise you do can change the balance of your body composition measurements……if you spend a lot of time lifting weights in the gym, your body type will be vastly different to that of an endurance runner!

Remember, our bodies, and our weight, change over the course of every single day as a result of your food and fluid intake, exercise and daily stresses……..your body composition at 8am will be different compared to when you go to bed!

It is important to not get too obsessed with your weight. Focussing too much on your weight can unfortunately lead to disordered eating or obsessive training, which is not healthy at all. Your weight is just a part of your overall health and fitness goals!!

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