Is BMI (Body Mass Index) Accurate & Reliable?

 There are many different methods to determine whether your current weight puts you in a healthy range or if those extra pounds are tipping you over the scales.

There are several reasons why people need to keep track of their weight, and there are also different aspects of your weight that you may actually need to keep track of. It’s not as simple as – “if you weigh a certain amount, you’re all good”.

One of the difficult things to actually figure out is whether or not the weight you are is healthy for your body type and BMI is designed to try and tackle this issue, however, there are many flaws with this method.

We’ll take a look at just exactly what BMI is, how it works and what factors can affect its reliability.

What Is BMI (Body Mass Index)?

BMI stands for ‘Body Mass Index’ and is a concept designed to quantify the amount of tissue mass (muscle, fat, and bone) in an individual, and then categorize that person as either underweight, normal weight, overweight, or obese based on that value.

It takes into account the individual’s height, and weight and compares their overall weight to that of an average, healthy person of similar height and weight.

Most BMI calculators will simply ask for your height and weight and give you a score. This score is supposed to determine whether you are currently at a healthy weight.

What Is The BMI Scoring System?

The BMI index has 11 different categories that you could fit into to, ranging from ‘Very severely underweight’ to ‘Obese Class VI (Hyper Obese)’ and a ‘Normal (healthy weight)’ that most people should find themselves in.

Below is the full BMI grading system

  • Very severely underweight: 15
  • Severely underweight: 15 – 16
  • Underweight: 16 – 18.5
  • Normal (healthy weight): 18.5 – 25
  • Overweight: 25 – 30
  • Obese Class I (Moderately obese): 30 – 35
  • Obese Class II (Severely obese): 35 – 40
  • Obese Class III (Very severely obese): 40 – 45
  • Obese Class IV (Morbidly Obese): 45 – 50
  • Obese Class V (Super Obese): 50 – 60
  • Obese Class VI (Hyper Obese): 60

As you can see, the desired grade you should be aiming for is 18.5 to 25, however, this only takes into account your height and weight which can actually make it very unreliable.

How Accurate Is BMI?

The accuracy of the body mass index is always under debate, and rightly so. If you’re looking for a general approximation of whether you’re a healthy body weight, then the body mass index will for sure give you a rough idea.

The problem with the body mass index is that it only takes into account two very simple factors of your physiology. Your height, and weight. These are not accurate factors on their own to determine if you are healthy or not in terms of your weight.

Let’s take two people of identical height where one person weighs 62 kgs, and the other person weighs in at 85 kgs (Both people are 6 feet in height/175cm).

  1. BMI grade at 62 kgs – 18.5 (Normal healthy weight)
  2. BMI grade at 85 kgs – 25.4 (Overweight)

Is This BMI Score Accurate?

The only thing that is accurate is the person’s weight. The body mass index does not take into account the difference between muscle and fat.

For sure, someone that is 6 feet tall and weighs 85 kgs could very well be overweight, however, if that person has a significantly greater composition of muscle mass compare to the average person, then they could actually be in prime health making this grade very inaccurate.

Adversely, the person scoring 18.5, and being classed as healthy, could actually have fatty deposits that account for a large amount of their overall weight, and as such, could be very unhealthy.

Muscle Vs Fat

A common misconception is that muscle weighs more than fat so you would expect someone with more muscle mass to look bigger than someone with the same amount of fat to body ratio, however, this is not the case.

Muscle is simply denser than fat which means you can fit more muscle tissue into the same space as an equal quantity of fat tissue. BMI does not account for this at all.

It’s actually common to find that a person that works out and builds muscle will weigh more than someone who does not, yet the person that works out can often look slimmer than the person who does not frequent the gym.

This is because the gym enthusiast will have a much lower percentage of body fat, and a higher percentage of dense muscle tissue.

Individual Variation

This is also another common problem with the body mass index, it does not take into account different body shapes and sizes in terms of how an individual is built.

Some people have longer legs and a shorter torso, some people have longer arms than others of the same height, some people have broader shoulders etc. All of these variations in human physiology will produce a wide range of variations in terms of a person’s weight, but it does not mean that they are healthier, or unhealthier than the next person.

Is The Body Mass Index (BMI) Reliable?

If you want to get a very, very rough idea of whether your current weight is healthy then the body mass index is certainly  a basic starting point, however, it is too generalized and does not take into account personal variations, or life style.

Conclusion

BMI, also known as Body Mass Index was a very simple concept designed in the late 70’s to gauge whether a person’s weight was ideal for their height, based on a very average and approximate set of values.

Whilst a BMI grade can give an individual a very general idea of their level of health, in terms of weight, it lacks the ability to recognize a person’s actual fitness level. BMI does not take into account the difference in muscle mass versus fat mass, and can, therefore, lead to very inaccurate measurements.

Two people could be the same height and one way more than the other, however, the person that ways more may actually be healthier due to a higher percent of muscle mass.

BMI is an OK starting point to get a very approximate grasp on whether your current weight is healthy or not. You may want to consider a more accurate measurement such as a body fat percentage measurement which can be done using calipers, or on a machine at your local gym.