Important Update: Calorie Count will be shutting down on March 15th. Please click here to read the announcement. Data export is available.
Moderators: melkor

Calculating Body Fat/Lean Body Mass/Muscle Gain or Water Gain

Quote  |  Reply

Hey Everyone,

I got a Tanita scale this month and have begun tracking body fat percentage and water percentage.  SO here were my first set of numbers and here are my newest numbers..  Based on the numbers, I calculated my body fat percentage and lean body mass (which includes water, I guess?) but I'm having a hard time understanding how much muscle I actually gained and how much is just water.  The first number is body fat percentage and the second is water percentage.

thanks in advance for your explanations and help :)

 April 7, 2008

47.8%/38.7% @ 266.   [BF = 127 lbs. LBM = 139lbs of which 103 lbs = water??]

April 10, 2008

47.3%/39% @ 265.  [BF = 125 lbs. LBM = 140 lbs of which 103.35 lbs = water??]

April 22, 2008

46.3%/39.7% @ 262.6.  [BF = 121.5 lbs. LBM = 141 lbs of which 104.25 lbs = water??]


Given the increase in water percentage I think I only gained 0.75 lbs of muscle but wanted to see if others got the same calculation.

3 Replies (last)

The water in your body is not segregated to your muscles. Skin is 72% water, Blood is 83% water, your organs are approx. 75% water, your skeleton is 22% water, and fat tissue is 10% water. That being said, your skin, blood, organs, and skeleton make no change in growth when your losing weight. So the only parts of your body that are changing is your muscle and fat (which you already knew). This means that:

New weight = Old Weight - Fat lost + muscle gained 

You can see that your body fat changed from 127 to 121.5 lbs, so you lost 5.5 lbs of fat. But your weight only decreased by 3.4 lbs, so 2.1 lbs of that comes from the muscle mass that you gained. 

You don't need to separate your water mass from your muscle mass because muscle is mostly water (76%).

thanks sim!  since my percentage of water weight did go up (and i've been retaining more water lately... post-girl time ya know) does that mean that i could have potentially lost a little bit more of body fat or does the body fat percentage calculation already factor that in?  i am trying to understand why the scale includes the water percentage number at all then.

Because fat tissue is only 10% water and muscle is 76% water, you would expect your water % to go up as your losing weight.

The way your scale works is by electrical impedance. Because fat doesn't contain much water, it slows down the current which is applied through your legs. Your scale measures the resistance in the circuit and uses that to approximate your body fat. 

In simpler terms, if you are holding on to more water than usual, the scale will UNDERESTIMATE your % body fat. This is one of the reasons people use calipers to estimate their body fat. They don't depend on your state of hydration.

3 Replies