Drop Some Knowledge #7: Measuring Body Fat Percentage

Hey there! Hope you had a nice week!
Tuesday: 10 easy miles (out-and-back on Lake Merced)

Wednesday: 6 easy miles (out-and-back on Lake Merced… but super duper windy)!


Thursday: Treadmilling (since it was kind of sprinkling out and windy)- 2.5 miles w/u, 4 x 1.5 mile at 7.7mph (with 800m RI), 2.5 miles c/d. 13.0 miles done in 1:54.


Okay, now to the interesting stuff (remember, on these posts, please feel free to share your own expertise, experiences, and knowledge on the subject! It’s a space for you to drop some knowledge, too, yeah?).

Body fat percentage… we’re all kind of interested or curious about this, yeah? For the average person, it doesn’t matter a whole bunch because we don’t have to whittle down to a barely-there BF% for a competition or anything.

Yet still… we can’t help but be curious, yeah?

Here is the basic thing that we’re trying to figure out… body density. In order to obtain body density, we need body mass and body volume. Body mass is easy to obtain (“Please take off your shoes and step on the scale”). Body volume, on the other hand, is not as easy. This will be explained a bit further in a couple of the techniques that I will list.

Let’s start with the cheap inexpensive techniques (and it will get more and more expensive as we go through our list):

1.) Skinfold calipers:


We’ve all see these handy devices. You basically measure the skinfold at various sites… different ones call for 3-, 4-, or 7-skinfold sites. They get the measurements, do some calculations to obtain body density, and then put it into a population-specific equation (such as male/female) to obtain the BF%.

This technique is done with the assumption that subcutaneous fat is proportional to total body fat.


Error: +/- 3.5% assuming that an experienced person is taking your measurements. You bet your bottom dollar that if I (a noob) get your measurements, the percentage error will be far greater than 3.5%.

This is great for getting an estimate of your BF% because it is pretty darned cheap (I think a lot of gyms have them and perhaps one of the trainers can do it for you).

2.) Bioelectrical impedence

“Impedence”= resistance to flow. Basically, you may be hooked up to something on the right foot and right hand, and they will send an impulse to see how fast/slow it travels to the other side (such as from the foot to hand). It is based on the assumption that fat-free mass has more water and can be a better conductor, whereas fat mass will slow down the impulse because it does not conduct electricity as well.

Error: +/-3.5-5%. It can be highly affected by your hydration and body temperature, so it is better to do this before you work out.

It’s great because it’s easy and cheap. A lot of gyms have this device (usually just a handheld one), but the error is pretty high. If you do it regularly, you may get wildly different values.

3.) Hydrostatic weighing

Okay. Remember how I mentioned that body volume is a tricky thing to obtain? Well this technique is used to get body volume through water displacement (based off of Archimedes’ principle… which I will not bore you with). If we are able to get body volume, then we can easily get body density… which then gets put into another equation to get BF%!


Error: +/-2.5%

The person must be submerged completely underwater when weighed, with only the residual volume of air in the lungs (meaning that you go underwater and exhale as much as you can). They account for the residual air in the lungs and air in the G.I. tract with some estimates.

Hey, check out my video on my BWTT Facebook page, if you want to see how it went! (btw… I totally am not graceful… by missing a step and falling into the seat).

This is definitely one of the more accurate techniques, but you must be willing to shell out some monies to go to a lab to have this done. Or you can just be a student majoring in kinesiology and do it for freeee~

4.) Bod Pod

The Bod Pod, which looks like a mini space ship (see this blog post by afastpacedlife for more specifics on what the Bod Pod experience is really like), is used to obtain body volume by the amount of air that is displaced.

First, you go into the Bod Pod, fart, press the panic button, and come out for a few minutes to catch your breath. I JOKE, HAHA.

Okay, you pretty much go in (trying to minimize clothing or things that take up space… like hair or jewelry), breathe through a little tube thingy that will measure thoracic volume, and they will calculate everything with a secret equation to get BF%

Error: +/- 2.5-3.7%. 

Again, this is pretty accurate and correlates well with hydrostatic weighing, BUT it is going to cost you some ca-ching to get it done in a lab.

5.) DEXA scan

Kind of don’t even want to mention this because it is too expensive and unnecessary for most of us. DEXA stands for “dual energy x-ray absorptiometry”.

They take an x-ray and can distinguish between bone, muscle, organs, and fat (meaning the fat-free mass and fat mass).

Error: +/-1.8%. 

Too expensive and unnecessary (unless you can get it done for free)… I already said that, but I feel like I should say it twice.


BTW we did the skinfold caliper technique and hydrostatic weighing in class today. My measurements?

Skinfold: 19%

Hydrostatic weighing: 18.75%.

Not bad! But it doesn’t really make a difference to my running or training. I don’t like to focus on numbers like that.


Have you ever used any of these techniques to get your body fat % measured?

How much would you be willing to spend to get it measured? [Zero dollars for me, haha]

Who has a race this weekend? 

What are your Mother’s Day plans?

5 responses to “Drop Some Knowledge #7: Measuring Body Fat Percentage

  1. Pingback: Drop Some Knowledge #8: Postural and Mechanical Loading on Bones/Ligaments | blessed with thunder thighs·

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