body fat % question

MileyClimb

what does the body fat percentage mean exactly? the reason I ask is because they reccomended for me since I have so much weight I need to lose to do 20% body fat. I am very happy about the results with. even though I have no idea or understand body fat percentage. does that mean like half my size or something?

ladyace2078

Body fat % literally means what percent of your body is made up of fat (vs. muscle/bone/organ/water). It can be tough for a female to drop to 18-21% body fat. Google body fat tables to understand healthy/non-healthy BF%.

CoderGal

Lean Body Mass/Body Fat (LBM/BF)

The general rule of thumb is it takes 3500 calories of energy to burn 1 pound of fat. This means that you would have to add at least 3500 calories to your actual TDEE (total daily energy expenditure) to gain a pound of fat in a day. Also, burning 3500 calories does not mean everything you burned was fat. Yes, it is true that you can gain multiple pounds in a day. This is not fat and muscle pounds. This is food and other things such as glycogen pounds, which you can read about later.

1 pound (fat) = 3500 calories

The rate you burn calories depends on your body composition/Lean Body Mass (LBM). This is the mass of your body excluding body fat (BF), which can be pretty difficult to calculate accurately. There are many tests and calculators that will give different solutions for the same variables. If you have your mass and your LBM, you can find your BF or vice versa. To sum it up the following is always the case:

LBM = mass - BF = mass - mass * (BF%)
BF = mass - LBM = mass - mass * (LBM%)
BF% = 100 * BF / mass
LBM% = 100 * LBM / mass
Example (me) based on equations below: BF% = 17.0% (via BIA) = 19% (via Covert) = 19.1% (via Military) = 23.83% (via BMI) = 25.2% (via fat2fit)
BF=20.4lb (via BIA) = 22.9lb (Military) = 34.6lb (via BMI)

Note: It's hard to compare yourself to another person based on just these numbers, as there are faults that come into play even if you are using the same medium to measure. Look at the measurements as a way to gauge the difference in yourself over months, not a way to tell you 'how fat you are'. Fat is good for you, your body needs it to live happily. There is a large deviation in my examples based on the same information due to the different ways to measure body fat (17%-25.2%) but some of those values are suppose to be more accurate then others. Also, remember that the percentages are a ratio (I know, it seems obvious now that I've said it). Some people don't seem to realize that if you lose weight, your fat percentage can stay the same (though you still might be on the right track). You're likely to lose muscle and fat while losing weight, so the ratio will only slide in your favor if you increase your muscle. If your body fat percentage is fluctuating this is normal. I'd prefer comparing values month to month rather then week to week. The body fluctuates a lot, especially if you're female.

Other BF Equations:
Brozek formula: BF = (4.57/density - 4.142) * 100
Siri formula is: BF = (4.95/density - 4.50) * 100
A few ways to measure BF/LBM:

Hydrostatic (Underwater) Weighing - This is the most accurate way of measuring since it doesn't predict body density. There is a error margin for residual volume (lung capacity). Remember, the density (mass/volume) of water changes with temperature (water becomes larger when it freezes into ice cubes for example).

density_body/density_water = density_body/(~1g/cm^3) = weight_body/(weight_body - weight_immersed)

Skin Fold Methods - A pair of calipers/callipers can be used to pinch standardized parts of the body such as your bicep and belly, determining the subcutaneous fat layer thickness. These sites are measured several times and averaged, and an average is taken of the averages to determine body fat percentage. The calculations some people use seem to vary and consist of body mass index rating scales so I will not list them here. The same person has to accurately measure you the same way and over months you can notice a trend of what's happening to your body fat. This can be done at most local gyms, demonstrations are available on youtube.

Weight Scales/Bioelectrical impedance analysis (BIA) - This functionality is built into many weight scales and other devices (such as Omron hand grippers) these days. The values can vary greatly, even if you have 2 of the same scale (according to my scale manual). Scales that can do this are actually reading your electrical impedance, so it requires skin to metal contacts (this is why you do not receive a % value if you are wearing socks). These scales can figure out your total body water (TBW), which can help calculate LBM, BF and bone mass. Since this depends on your TBW, the values can vary greatly depending on if it's that time of the month, you're hydrated, your hot (related to damp skin), you're losing weight etc etc. It can also be inaccurate depending on where your carrying your fat. For example, if you carry your weight in the top half of your body and use a scale that sends the current through the bottom half.

V=Voltage=IR=Current*Resistance=Current*|Impedance|^(Imaginary#*phase)=Complicated=you have as much equipment then me in your house.

Example (me): Thinner Glass Body Analysis Scale reading BF% = 17.0% fat
BF = 20.4lb
LBM = 120lb - 20.4lb = 99.6lb
Body Mass Index (BMI) - Your BF can be calculated from your BMI (sex=1 if male, 0 if female).
Child body fat % = (1.51*BMI) - (0.70 * Age) - (3.6 * sex) + 1.4
Adult body fat % = (1.20*BMI) + (0.23 * Age) - (10.8 * sex) - 5.4
Example (me): BF% = (1.20*18.8) + (0.23*29) - (10.8*0) - 5.4 = 23.83% fat

Anthropometric/Height and Circumference Methods - The U.S. Marine Corps and U.S. Army (Department of Defense) use this method along with Covert Bailey and fat2fit's body fat calculator. They make calculations based on mass, and the circumference of various body parts such as waist, hip, bicep, forearm and wrist.
U.S. Army (Department of Defense) Military BF Calculator: http://www.fat2fitradio.com/tools/mbf/
Example (me, from calculator): BF% = 19.1% fat
BF = 22.9lb fatLBM = 97.1lb
Covert Bailey BF Calculator: http://www.fat2fitradio.com/tools/cbbf/
Example (me, from calculator): BF% = 19% fat
BF = 23lb fat
LBM = 97lb
Fat 2 Fit BF Calculator: http://www.fat2fitradio.com/tools/bf/
Example (me, from calculator): BF% = 25.2% fat
BF = 30.2lb fat
LBM = 89.8lb