How much fat does a sinker have?

January 22, 2007 3:02 AM

Given that I sink in a pool with my lungs full, how much is my percent body fat/bmi?

Possibly relevant info:
- male
- ~84kg, 177cm
- unknown lung capacity but as I bike and swim and don't smoke, whatever is average for healthy 22 year old (6L?)
- happens in waters of varying temperatures (20-30C) and chlorine levels, but not so well in seawater
- naked or with regular swimsuit
- fwiw, I don't sink the second I get in necessarily; rather, if I take a big breath and dive down and lie down (anywhere from .5m-2m), I stay down
Thanks!

posted by shokod to Health & Fitness (13 answers total)

Not enough information, though I couldn't say what other info is needed. I don't sink to the bottom, but to about six feet below the surface. This happens when I'm a 6% body-fat 170 lb, and it happens about the same at my current 195 (and blobby)
posted by notsnot at 3:19 AM on January 22, 2007

I think you'll need an actual measurement to find body fat percentage. Your BMI can be calculated with the formula here.
posted by equalpants at 3:23 AM on January 22, 2007

You can use the formulas contained here to estimate your proportion of lean tissue to fat tissue. Pure H20 is 1.000 g/cm^3. Honestly, one of those scales that calculates composition would probably be more accurate for you. The most accurate way would be underwater weighing done by a pro.
FYI, your BMI is 26.8. normal is 18.5—24.9; Overweight = 25—29.9; Obese = 30+.
posted by grieserm at 3:51 AM on January 22, 2007

Response by poster: Alright, thanks so far for the answers. I was hoping more could be ascertained.
I think that BMI formula is too general because it is unable to differentiate between overweight and overfat. Maybe I will get a professional test done; I was hoping something could be gleaned this way.
posted by shokod at 4:13 AM on January 22, 2007

bodyfat scales are cheap ($40 on ebay). Take their calibration with a grain of salt, but they're cheap enough you might as well have one.
posted by -harlequin- at 4:26 AM on January 22, 2007

Also, 6 L is an impressively large estimate for lung capacity. When I was a tuba player, I got my lung capacity measured at 6.2 L, and the person measuring me made me repeat the test several times. But then again, I am 196 cm tall and built like a linebacker.
posted by rachelpapers at 5:29 AM on January 22, 2007

The BMI is not entirely accurate - it's good for an easily calculated general guideline but there are always people it won't apply to particularly well.

Incidentally, I'm a bit bemused by notsnot's experience. Given that water is going to be the same density everywhere (assuming there's no change in its temperature?) then once you're entirely under the surface it shouldn't matter a great deal how far under you are. I could see that as you go down the greater pressure would compress your lungs, but that would make you less bouyant and he should then sink further...

What would be the mechanism for him staying at a particular depth?
posted by edd at 6:17 AM on January 22, 2007

I think it's the difference in pressure from the underside of me to the topside of me. If I try to lie on the surface of the water, my feet sink to ~6 ft depth, and my head to ~2ft depth. Every time.
posted by notsnot at 6:36 AM on January 22, 2007

Your position in the water greatly affects how well you float. After years of believing that I didn't float, I was proven wrong in a swim clinic. I don't fully understand it, but I can reasonably say that how well you float won't be able to lead you to a body fat percentage.
posted by advicepig at 9:34 AM on January 22, 2007

Seconding harlequin's advice - BIA scales are everywhere now, you can probably get one at Target. Regarding calibration, just use it the same every time and you'll at least get a good idea of how your body fat is changing.
posted by rkent at 10:02 AM on January 22, 2007

Floating has more to do with what you're doing with your body while you're in the water than with how much you weigh. So there's no way you could really get the data you want from the inputs you're providing.
posted by Kololo at 12:12 PM on January 22, 2007

You can't know how much fat you have, but you can know that if you're neutrally buoyant then you have the same density as liquid water does. There is more in your body that is less dense than water -- yes, fat, but also several tissues and all that air in your lungs. A lot of it is more dense than water -- bones, muscles, et c.

So, there are models that give reasonable estimates of density of the various parts that an Average Joe has in his body. You could use those numbers as fixed values in you, and pretend that the only variable to solve is fat amount. Your calculation is only as accurate as the model is.
posted by cmiller at 3:16 PM on January 22, 2007

FWIW, I was a very chubby kid, and I sank. I am now a much healthier, leaner adult, and I float. So I'm skeptical of your floating/body fat theory.
posted by obliquicity at 11:10 AM on January 23, 2007