Loosing Fat Is Different Than Loosing Weight: BMI vs. BFP

Body Mass Index (BMI) and Body Fat Percentage (BFP) are two commonly misunderstood terms in the world of health and fitness. The primary misunderstanding is how they’re different. They are not the same, and it’s important to understand the difference, and to know what you’re measuring when you’re trying to make progress in weight loss, athletics, muscle gain, and overall health.

Body Mass Index is not actually the most useful measurement in the world. A very strong, fit person could potentially have the same BMI as someone who is unhealthy, and even slightly over weight with regards to body fat. You can calculate your BMI here.

Once you’ve got your number, you can check which category you fall into to:

  • Underweight = <18.5
  • Normal weight = 18.5–24.9
  • Overweight = 25–29.9
  • Obesity = BMI of 30 or greater

Body Mass Index is purely based on the ratio between your height and your weight. It says nothing about the composition of your body. For someone who is trying to loose weight, or lean out, trying to lower their BMI is not useful for this reason.

Since beginning to train for cycling competitively, not much has changed with regards to my BMI. My BFP has gone down, and I’ve built muscle. Muscle weighs approximately x3 more than fat, thus why two people of completely different body composition can have the same BMI. The tool on this site let’s you select a body type, then calculate BMI, and you can see why how people with different body types can have the same BMI.

Now, we tell people who are trying to “loose weight” to monitor their BFP instead of relying on their actual weight to check their progress. Why? Strength training and the building of muscle, which weighs more than fat, is a very effective way to get the body to release fat. Muscle also burns more calories than fat because of it’s mitochondria, so the food you eat gets used quicker. 

“Loosing weight” is different than “loosing fat.” Someone who is unhealthy and over weight needs to loose fat, and monitoring the scale can be discouraging because it only shows you weight, not how your body composition has changed. Someone who has a lot of muscle mass and starts to strength train less might “loose weight” as their muscle mass decreases.

The measurement for body fat is in percentages. The interest is in what percentage of your body is made up of fat. There is no mathematical equation for BFP, like there is for BMI. To determine BFP there are machines that send waves through the body that measure densities, which can usually be found somewhere in a gym. There are a couple other ways, but the only one you can try at home is based on some measurements of your body. The tool is here, but the success of this method largely depends on the person.

If you look at the chart below, you’ll see what category the different body fat percentages fall into for men and women. Women by nature have higher body fat than men. This number is, in my opinion, more important than your BMI, because it tells you about the composition of your body. If you’re loosing fat and building muscle, your BMI may not show any results, but your BFP will. Knowing the difference between the two, and what the values mean, is important. Now all you have to do is learn not get tongue tied by the many three letter acronyms in health. 

Body Fat Percentage Categories
Classification Women (% fat) Men (% fat)
Essential Fat 10-12% 2-4%
Athletes 14-20% 6-13%
Fitness 21-24% 14-17%
Acceptable 25-31% 18-25%
Obese 32%+ 25%+

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