How does body fat % work???
 

Bare with me here...I have a scale that measures bodyfat...embarassingly it tells me I'm between 47% and 48% fat, attractive huh? Well, I got to wondering how this relates to my body composition and I found this site (http://www.healthchecksystems.com/bodyfat.htm) and decided I'd calculate my stuff out the way they do in the example. Here's what I got:
266lb woman, 48%fat, wants to lose 131 pounds
--inital body fat: 266lbsx.46= 122.6lbs of my body comp is fat
--lean mass: 266lbs-122.6lbs= 144lbs
---I therefor couldn't lose 131 lbs because I don't have 131lbs of fat to lose... so I went on with the next part...
I'd like to be at say 20% body fat...
--266x.2=53.2
--144lbs lean mass + 53.2lbs fat= 197.2lbs...WTH??
Okay, so I thought BOGUS! So I tried it a simpler way... I'm 48% fat now, I want to be 20%...I need to lose 28% of the fat...so 266x.28=74.48lbs need to be lost...which puts me at 192.
I am completely lost and confused. Does this mean I'll never see 135 or are my calcuations wrong? Or maybe I'm really like 70% fat (I found a site with a calculator where you take measurements of various body parts and it too came out about 48%) and once your over like 40% fat no fat measurments expect the super scientific come out right????
Or is fat supernatural somehow and none of this would even apply??

Some of your lean (nonfat) mass is fluid and other tissue associated with your current size. So some of that will go down. Also, it's very hard to retain *all* all muscle mass when losing that much weight. Some of that muscle exists because you're carrying around 130# or so of extra weight a day!

Also, this part of your math is wrong:
"I'd like to be at say 20% body fat...
--266x.2=53.2
--144lbs lean mass + 53.2lbs fat= 197.2lbs...WTH??"

If you want to end up at 20% body fat, you have to apply the 20% to your goal weight, not your current weight. So if your goal weight were 135, 20% body fat would be 27# of fat not 53. Also at total weight 135, lean mass of 80% would then be 108#.

If you kept *all* your lean mass (theoretically) and shot for 20% body fat, you'd weigh 180 # (144/0.8). As you have already guessed, a 5'4" woman weighing 180# with 20% body fat would be very, very muscular.

And of course, the 47-48% (144# LBW) is just an estimate. At 267#, every 1% error in body fat estimation is 2.67#.

i think Juli has that one covered!! I would like to say you are being very smart and reasonable trying to attain a "body fat" goal rather than a scale weight goal. Body fat is such a better measure of overall health and fitness, and the number on the scale, and especiall number slike BMI dont mean anything. For instance, at 5'4 i am 133 pounds. Thats is just BARELY within the acceptable BMI range. However, i am only 14% body fat, with only 19 pounds of body fat and 114# 's of FFM, or fat free mass. I cant physically really lose anymore fat, and if iw anted to drop to a lower number on the scale, i would have to sacrifice lean muscle mass in order to do so!!!

Ty for clearing that up...I was terribly confused.
I don't want to be She-Hulk, :D so I'm good with saying bye to some of the extra mass my body's added to help lift the bulk and I didn't take that into account. You always here about needing to lose fat..not muscle, so I never thought of it the way you put it. It makes sense!
Guess I have a number of pounds to go before I stress too much anyways :) but it was a momentary "bummed out" moment. I am really excited about my goal (I've never set one this low and thought it was pretty reasonable for my height/bone structure, if higher is healthier I'd be fine with that too, but 190!?!) and it depressed me to think I might be fooling myself!
TY TY TY, again.