Measuring body fat percentage

Hello people,

As far as I know, there are three ways of measuring someone’s body fat percentage: Skinfold measurements, dumping you into a water tank for a while or measuring your body’s electrical impedance.
The last one doesn’t seem too accurate at the first glance, as people’s body fat can be distributed differently around the body, depending on the individual.

How accurate is that method? I’ve so far seen these measuring devices used only by people peddling weight loss drugs and fitness instructors who have a vested interest in tagging people as fat bastards and keeping them as customers.

I myself have no weight problems at 178cm and 75kg, but I seem to have a few overweight relatives who seem to be coming to me for fitness advice… :dubious:

Eh, they’re not particularly accurate. The scales are good for measuring “relative” body fat, i.e. measure yourself everyday at the same time and see how much it varies. But not as an absolute measure.

What tremorviolet said. The only accurate method out there today is autopsy, and…well, you don’t want to know that bad, do you?

A scale that’s fairly consistent would be useful, but you’d probably need a higher-end scale to get that.

They are not accurate, but all else being equal, they are repeatable. One of the main problems is that the body’s state of hydration confounds the readings (among other things). In fact, I’ve found 'em fairly useful to informally monitor hydration in myself.

Yeah, I guess not. Although I’m not sure an autopsy would help either, unless they somehow mechanically separate the different tissues in your body. :smiley:

Perhaps I my first post was a bit unclear. What I specifically wanted to know was how accurate is the method of measuring your body’s electrical impedance compared to the other available methods.

In other words, are the electrical BFP-meters junk science?

The science agrees with water-tank testing, so it’s as good as any*, but any particular model might be a piece of crap.

*Not very good.

I’ve looked into the water tank test to see if you can do it yourself. All you need is a couple good sensitive scales and the ability to get an accurate underwater measurement of yourself. That last part is harder than I thought after some experiments - you do need a bigger deeper tank of water than most people have sitting around the backyard and a helper would make it 100% easier. The rest of the calculation is done by tables (and water density - but you can figure that out yourself).

What you’re actually looking for is a process called ‘rendering’.

What you do is take the body, chop and grind it up a bit and then boil it up until all the fat is driven from the carcass.

If you measure and weigh before you start working on the ‘recently deceased’… then it should not be a huge leap to calculate the BMI etc.

The electrical methods may not be that great, but there are some that are better than others. The bathroom-scale type that measure from your feet are pretty worthless at doing much beyond simply calculating your BMI (I went from a 35-inch waist to a 31-inch and now can see my abs again, yet according to it I’m still 27% fat). The handheld versions might be a little better but still do a pretty incomplete job. The best one’s I’ve seen use hand and foot contact points and take about a minute to do a complete reading. So far, I’ve only seen them at the gym.

I like my one at home I always use it at the same time every week (so as to reduce hydration issues) and all im watching for is the change over time rather than the actual values that show. Seems to work pretty well. When I exercize a lot it shows a downward trend, when I salck off it starts going the other way.

Actually I’ve thought further on this…

If the entire body was ‘liquidised’ in some kind of huge blender then a simple calculation of volumes could be used.

Using either a centrifuge or just letting the soup settle you could get an estimate of the percentage fat content.

The range of densities of flesh/bone/fat is narrow enough to allow a simple volume calculation but wide enough to allow gravity separation of the fat from the non fat.

In addition to the methods mentioned in the OP (skinfold, water tank, electrical impedance) there is also an estimation method devied by the US Navy which uses three measurements: height, neck circumference, abdomen circumference.

Instructions and tables (PDF, circa 150 kByte)
Calculator (download; requires Windows)
Study on the method’s accuracy

(Conversion factor for use by us metric people: 1 in = 2.54 cm)

Maybe I’m being wooshed here, but isn’t eight feet under the diving board enough for this? (Gotta close the pool tonight :()

Thanks for the handy tip. Now all I have to do is to find a human-sized centrifuge. :stuck_out_tongue:

Yeah, I spotted that fourth method too after some Googling, but thanks for the link to the comparison study. Looks like the US Navy method works pretty well.

Actually, there seem to be quite a few methods for measuring body fat:
(Disclaimer: I haven’t checked any of the other content on that site than that page. Only linking there for that list. They are probably selling something…)

I’d think that any size would do as long as you’re completely immersed in water. AFAIK they’re measuring your body weight under water so that your body density can be calculated.

Good info guys. I might have to get an electrical body fat meter then.
I’ve been weight training for a year and a half and after losing 10kg or so my weight has stayed pretty much stable. It would be nice to know how much of it is still fat. :slight_smile:

The thing with the bodyfat measuring is “does it really matter?” It’s pretty obvious when you’re over fat. If you have a burning desire to know your bodyfat exactly, find a local university that’ll do a DEXA scan for a small fee. If you’d just kinda like to just monitor as you embark on an exercise and weight loss program, get a Tanita scale or a handheld unit and take measurements at the same time everyday and you can get the relative changes. But bodyfat percentages are like weight, the specific numbers don’t matter that much and vary among individuals. Don’t stress out over the number and just eat right and exercise until you reach what you know is healthy for you.

Disclaimer: I have a Tanita bodyfat scale and use it everyday. Do I know exactly how accurate it is? Nope. But like One And Only Wanderers, when the numbers start going up, I knwo I need to do something.

heh - mine’s a Tanita too!