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Understanding the BMI Calculator

So you’ve been working hard on your IIFYM diet and weight loss plan and are excited about the results that you should be seeing. When it comes time to assessing your progress, how do you determine whether or not you are seeing the results you were hoping for?

If you’re like some of our clients, you decide to use a BMI calculator.​

This is unfortunate, however, as the BMI calculator is not going to give you an accurate representation of how you’re doing.

Let’s explain further and go over why BMI weight loss and true weight loss are not always inline.

What the BMI Calculator Measures

The BMI calculator, which stands for ‘body mass index’ stacks your weight against your height and gives you a number that you can then use to compare against some standards derived from the general population.

Many physicians often use BMI as a guideline for establishing whether a person is at a healthy weight, overweight, or obese.

The only two numbers involved in getting to the number include your weight and your height. Age, body fat percentage, or lean body mass is not factored in.

The formula for the BMI calculators is as follows:

Body weight (in kilograms) divided by your height squared. From here, you can take your number and put it into a table that indicates body weight ranges.

These are as follows:

BMI < 18.5 = Underweight
BMI: 18.5-25 = Normal weight
BMI: 25-30 = Overweight
BMI: 30+ = Obese

If you like, take a moment to figure out your own BMI using the formula above or our BMI calculator.

Calculate Your BMI Below

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So What Are the Problems with the BMI Formula?

First, it doesn’t take into account your lean body mass.

Let’s illustrate how this comes into play.

Say you have two men, both of whom stand 5’7” tall and weigh 195 pounds. The first man is very heavy set. He doesn’t do much activity at all and works an office job. As such, he has packed on a few pounds of fat over the years and would be described as quite robust.

The second man weighs the same amount, but standing these two men side by side, you see stark contrasts. The second man takes up much less space because the weight he has on his frame is dense muscle. He’s athletic, works out regularly and keeps his body fat to a low level.

Because muscle mass tissue takes up far less space compared to body fat tissue, the 195 pound inactive man is much larger than the 195 pound active man.

But yet, when you plug their numbers into a BMI calculator, both men come back with a BMI of 30.5, putting them in the obese category.

While the first man may in fact be considered obese, the second man is far from it. But yet, by medical standards, that’s what they place him in. He may only have a few pounds of fat on his frame, but yet, he’s classified the same as someone who’s easily 30% or more total body fat weight.

A BMI calculator does not give you an accurate representation of what your body weight composition is. Is it primarily fat weight? Or is it lean muscle mass weight? That makes a big difference in terms of your overall health as well as performance.

And, from a progress-tracking standpoint, you can also run into issues with a BMI weight loss calculator.

Let's look at an example of an issue with the BMI formula for fat loss.

Say you have two people, Sarah and Susie. Sarah is interested in losing weight as quickly as possible and goes on a 1000 calorie diet plan. She doesn’t care that it isn’t necessarily healthy – she just wants that weight off in a hurry. She ends up losing 10 pounds, but 8 pounds of that is lean muscle mass and only 2 pounds of it is body fat.

Susie on the other hand, knows better. She wants to stay healthy as she focuses on fat loss. She uses a moderate calorie reduced diet of 1600 calories per day and begins a strength training program. Susie loses 4 pounds of body fat but also gains 5 pounds of lean muscle mass weight.

By BMI calculator standards, Sarah is the healthier one of the two. Her total body weight has gone done while Susie’s has actually gone up. But yet, when looking at things from a BMI standard, this isn’t the story it tells. If both women are strictly using BMI calculators as a way of assessing their results, chances are, Susie will be rather frustrated, while Sarah may think this is a healthy way to diet (which it’s not!).

Finally the last big problem with a BMI calculator is that it doesn’t help you accurately figure out your total daily energy expenditure, or TDEE. Lean muscle mass is very metabolically active tissue, so someone who has a lot of lean muscle will experience a much higher resting metabolic rate.

Going back to our example of the two men at 195 pounds, the one that maintains a higher total amount of body fat may only burn around 2400 calories per day for instance.

The one who is 195 pounds of lean muscle mass may have a total daily energy expenditure much closer to 3000 calories or even higher.

When trying to assess how many calories each of these individuals should be taking in for weight loss purposes, as you can see, this will make a big difference. If they eat the same diet, one will either underfeed while the other does fine or the other will overfeed while the other does fine.

A BMI calculator is not a useful tool for figuring out how to base your IIFYM diet on, so should not be utilized for this purpose.

The Verdict

All in all, a BMI calculator is not a great way to determine your overall health, the progress you are making on your program, or how many calories you need to eat per day.

While it can give you a general sense of your overall body weight compared to others for average individuals, for those who are active and carry lean muscle mass, it tends to be very inaccurate.

Want to lose weight without giving up the foods that you love?

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