What's the difference between BMI and Body Fat Percentage?

A few months back, BMI told me I was underweight and a victim of malnutrition. Was it true? Let's find out.

What is Body Mass Index (BMI)?

BMI gives you a value, which basically tells you whether you are underweight, normal weight, or obese. This value is determined by dividing your weight (in Kg) by the square of your height (in meters). Let's look at the below example to understand this in a better way:

Weight: 78 kg 
Height: 1.75 meters

BMI (kg/m²): 78/ (1.75 x 1.75) = 25.5

This value indicates a person overweight according to his/her height.

Below are the categories of weight and their respective BMI ranges that can easily be found on Google. 

So what's the flaw in BMI?

This whole theory is based on our weight and height. And, weight consists of 3 components: Muscle, Fat, and Water which BMI does not take into account.

Let's relook at the above example:

Weight: 78 kg (Of which, 8% is Fat, 70% is Water, and 22% is Muscle)
Height: 1.75 meters

BMI (kg/m²): 78/ (1.75 x 1.75) = 25.5

BMI says this person is overweight, but this doesn't count as overweight. This person is literally faster, stronger and agiler than an ordinary person. 
This is the flaw in BMI, it can categorize a person as overweight but in reality, that person is more fit than a normal person in every way, because he has more muscle and less fat.

BMI is therefore not useful for those who have a decent muscle mass and less fat percentage. Imagine the BMI of a bodybuilder with 120 kg of weight and 1.75 meters of height? BMI would simply declare him close to class III obese and that is really absurd. 

BMI says Arnold was Class II obese when he became Mr. Olympia (the greatest bodybuilding title) for 7 times.

Now let's have a look at Body Fat Percentage.

Body fat percentage tells you what percentage of your overall weight is fat. Below is an easy example:

Weight: 90 Kg

Fat Percentage: 10% of 90 = 9 Kg.

This means a 90-Kg person with 10% body fat has about 9 Kg of fat in his body, rest is muscle and water.

How to calculate your Body Fat percentage? 

A device called "Body Fat Caliper" helps you calculate your body fat percentage, by pinching your major fat areas (chest, triceps, belly, and thighs), then taking an average and matching with a corresponding number in the chart that comes with the device.

This device can easily be bought from Amazon.

Body fat percentage is extremely useful for body builders to track their progress and prepare for competitions. 

This does not mean a skinny 40-kg guy can say he has a cool 5% body fat. There has to be a decent muscle mass too, without both fat and muscle, you are really a victim of malnutrition and BMI is accurate in this case.

When should you use BMI and not Body fat percentage?

• When you have neither muscle nor fat, then BMI will categorize you underweight which is true
• When you have more fat and less muscle, then BMI will categorize you overweight which is also true
• When you are an average person, with normal muscle mass and fat, then BMI will categorize you Normal weighted.

When should you use Body Fat Percentage and not BMI?

• When you are a bodybuilder/athlete.
• When you have more muscle and less fat.

So was I really Underweight?

There is one more thing you should know.

Muscle is heavier than fat

This is the reason why BMI says you are overweight when you have a good amount of muscle mass because what BMI sees is your weight, it doesn't see which type of weight you have, muscle or fat.

In my case, I have a less fat percentage and I am trying to gain muscle mass, I have visible muscle definition but a good size is yet to be achieved. So I looked skinny because I lost fat at a much higher pace than I gained muscle, but I am not a victim of malnutrition as suggested by BMI because I eat 7 good meals a day (2500 calories a day). It's just that my metabolism is now high and it is very difficult to gain muscle mass without consuming adequate protein which demands higher spending. Currently, I am fulfilling about 60-70 % of my protein needs every day but it is not enough for a person who works out and wants to gain muscle mass. I will increase my protein consumption in future though. I am stronger, faster and agiler than my past version of 92 Kg obesity.