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Body composition: what it is, how to measure it yourself, and how to see it improve

Body composition is a term often confused with body fat percentage or body mass index (BMI). But, they’re completely different things.

Body fat percentage is your total mass of fat divided by your total body mass, times 100. If a 30-year-old man had 16.4 kilograms of fat and their total body mass was 82 kilograms, they would have a body fat percentage of 20%.

16.4kg/82kg x 100 = 20 (body fat percentage)

Body mass index is your body mass divided by the square of your body height. Using a man with the same body mass as our previous example, with a height of 1.85 meters, he would have a BMI of 23.95.

82kg/(1.85)2 = 23.95 (body mass index)

BMI can be misleading from a fitness standpoint – it’s a measurement great for looking at trends in populations, but it’s not so good when your looking at an individual alone.

It’s because of how BMI values are correlated with body weight status.

Underweight = <18.5

Normal weight = 18.5-24.9

Overweight = 25-29.9

Obesity = BMI of 30 or greater

Our example man has a body fat percentage of 20% (below average for his age), but, according to his BMI, he’s overweight.

If he’s a fit guy, his muscle mass could be pushing his BMI into that higher range. He’s not an unhealthy person. On the contrary, he’s fitter than average.

Body fat percentage also doesn’t paint the whole picture. A person can starve themselves to drop their body fat: it doesn’t mean they’re healthy.

Because of the shortcomings of body fat percentage and BMI a person looking to get fitter should be paying attention to their body composition, not those other two.

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Body composition

Body composition takes into account everything that your body is made of: it’s the percentage of fat, bone, water, and muscle in your body.

Fat can be anywhere from 10% (considered very lean) to 50% (morbidly obese), muscle makes up 30 to 50%, and bone is about 15%.

The rest is minor. The brain weighs a few pounds, skin is approximately six pounds, and blood is about 7% of body weight.

Do you need some complex machine or a specialist to measure your body composition?

No. You can do it yourself with a little practice and some commitment. It can all be done with a set of calipers (to measure your body fat), a scale, a measuring tape, the mirror, and your camera.

Step one: Weigh yourself daily

Weight fluctuate 5 to 10 pounds a day depending on water retention, glycogen storage, and bowel movements. To get an accurate representation of your weight, weigh yourself daily (at the same time of day) for 7 to 10 days and average it (add up every weight from the weighing period and divide it by the number of days you weighed yourself for).

Step two: Caliper measurements

Measure one site once a week and keep track of the measurement over time. There’s plenty of calipers out there to purchase and here’s a pretty instructive video telling you how to use them.

Again, just measure one site. And you don’t need to do the whole body fat percentage extrapolation. You just need the one measurement to track a difference in you body composition over time.

Step three: Waist measurements

Use a measuring tape at the level of the belly-button. It’s crude, but it’s a decently reliable indicator of fatness.

If your waist is shrinking over time, you’re losing fat. If it’s getting bigger, you’re gaining fat.

Step four: Take pictures

It seems unscientific, but they eye can be pretty accurate in detecting changes in body composition.

Take weekly pictures from the front, back, and side. Comparing how your body looks over time will tell you what’s changing and what’s not.

Now that you know why body composition is a better indicator of fitness than body fat percentage or BMI, and you know how to measure changes in body composition over time by yourself, what kind of changes should you be looking for?

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What kind of body composition changes to look for

Bone composition is quite constant between individuals (15% of total body mass). The two variables at your disposal to manipulate are body fat percentage and muscle percentage.

Here are the average ranges of those two variables for three different levels of fitness:

Average fitness:


Body fat percentage: 17%

Skeletal muscle percentage: 47%


Body fat percentage: 27%

Skeletal muscle percentage: 41%

Athletic fitness:


Body fat percentage: 12%

Skeletal muscle percentage: 50%


Body fat percentage: 21%

Skeletal muscle percentage: 44%

Exceptional fitness:


Body fat percentage: 8%

Skeletal muscle percentage: 54%


Body fat percentage: 14%

Skeletal muscle percentage: 49%

There’s a trend to note here: as fitness level increases, skeletal muscle percentage increases while body fat percentage decreases.

If you’re looking to improve your overall fitness level, strive for a change in the ratio of skeletal muscle percentage and body fat percentage.

Here’s what an improvement in this ratio could look like using the measurement guidelines we previously discussed.

If you’re overweight and have a high body fat percentage:

You’ll notice a decrease in your caliper measurements over time, a decrease in your waist measurement, and a decrease in body weight. Also, your pictures will look as if your body is becoming “tighter”.

If you’re overweight in the body fat percentage sense, not the BMI sense, more of your body weight is represented by body fat. Therefore, if you start to lose fat you’re going to be losing it in a disproportionate rate to the muscle you’re gaining from exercising and you’ll lose body weight.

If you’re already of average fitness or better with an average body fat percentage:

You’ll notice a decrease in your caliper measurements, a decrease in your waste measurement, but no decreased, and possibly an increase, in your body weight. Everything will begin to look more toned with your picture assessment.

If you’re already in good physical condition, you’ll lose fat at a rate equal to or slower than the rate at which you gain muscle. Muscle gain will stabilize your weight where it is or increase it.


Don’t get hung up on bodyweight, or BMI, or body fat percentage if you’re trying to get fitter. There’s much more to physical fitness than these tiny components.

Focus on your body composition. And now that you know how to do it by yourself, you can.

Let me know what you think of the article by liking it or commenting below. Follow Healthy Wheys on Instagram, Facebook, and Twitter and check back weekly for new articles.

Contact me using the information you’ll find on my contact page, I’d love to hear from you.

Have a great week!


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