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This is a comparison of the ways that body fat can be measured. None of the approaches is perfect, but they can be useful.
==Skinfold Calipers==
These are cheap ([http://www.amazon.com/gp/product/B000G7YW74|$6]), but tricky to use accurately. Slight changes in location of the measurement can make a big difference to the reading. With lower levels of body fat, the resolution of the calipers can also be a problem. The calipers only measure the fat under the skin, and at a few locations. There is significant person to person variation in fat distribution through the body, limiting the accuracy even with expert usage. The two big advantages of calipers are that they are cheap, and they measure the body fat directly.
http://ecx.images-amazon.com/images/I/31c6Xo-ic3L._SS500_.jpg
These Bioelectrical Impedance Analysis (BIA) scales use an imperceptible electrical signal to calculate the amount of water in the body. From this Total Body Water (TBW), it is possible to estimate body fat. This makes these scales quick and easy to use, but there accuracy is limited. The level of hydration has a significant impact on the reading, so readings need to be taken at the same time of day with the same level of hydration. I categorize these scales into three types.
===Lower Body Only===
Most body fat scales have electrodes only on the feet, so the electric current only goes from one foot to the other. This only measures the lower part of the body, not the whole body. Over the years I have found these scales to be near useless; an increase in weight showed was reported disproportionately as fat when other measures show fat levels stayed constant or dropped. I have the Tanita BF680 which retails for [http://www.amazon.com/Tanita-BF680-Scale-Monitor-Athletic/dp/B000094ZGH|$60].
http://ecx.images-amazon.com/images/I/41CNM5C5HSL._SS400_.jpg
===Full Body BIA Scales===
These scales also use BIA, but have electrodes on the hands as well as the feet, giving a measure across the whole body. In my experience, these scales show changes in body fat far better than the BIA scales that read from the feet only. I have the Omron HBF-510 which retail for [http://www.amazon.com/Omron-HBF-510W-Composition-Monitor-Scale/dp/B001IV61J4|$60].
http://ecx.images-amazon.com/images/I/41yvY8JWRdL._SS500_.jpg
===Segmental Body Fat Scales===
The latest Tanita scales use two frequencies of electrical signal and use electrodes for the hands and feet. I have not tried these scales as they are expensive (~[http://www.thecompetitiveedge.com/shop/item/123-productId.184549503_123-catId.176160808.html $300]). If these scales could transfer the measurements to a PC via bluetooth, I'd be tempted to get one. http://www.tanita.com/data/Image/products/ImageResize/Ironman---BC-558_l.jpg
==BOD POD==
This is a chamber that works out your body volume accurately. Given the volume and weight you have the density, from which you can estimate body composition. The test costs about $25 at a gym, which makes it expensive to use regularly. I believe there are two types of BOD POD, a recreational and a research model. The recreational model is used by many gyms and estimates lung function, whereas the research model actually measures lung volume. I found the BOD POD quick and easy, but requires a trip to a gym. The cost means that I do not have a sense of how the reading changes over time, and its correlation with other techniques. See http://www.bodpod.com/ for more details. http://www.lifemeasurement.com/images/ph_bpgs_2.jpg
==Underwater Weighing==
This used to be the 'gold standard' of body fat measurement. The technique involves being weighed on dry ground and then again underwater having breathed out as much as possible. This gives a value of density, so it should have similar accuracy as the BOD POD, but is more expensive and unpleasant. I have not tried this technique.
==DEXA==
Different tissues in the body absorb different frequencies of X-Rays at different levels. This technique (Deal-Energy X-Ray Absorpiometry) uses two frequencies of X-Rays and looks at the different absorption to work out body fat. This is the current 'gold standard' and I believe it will show the distribution of body fat as well as the level. I have not found anywhere locally that provides DEXA.
http://www.edb.utexas.edu/fit/images/DEXA_female.jpg
==Alternatives to Measurement==
These approaches can give some hint about your body fat, but are no substitute for a real measurement.
Looking at yourself in a mirror is a simple method that can give a sense of body fat. Obviously there is no numerical quantity to this test, but it's cheap and easy.
===Abdominal Muscle Definition===
If your body fat is low enough, the definition in your abdominal muscles (six pack) can be used to monitor body fat changes. There is no specific body fat percentage for having a six-pack, but the common thought is that you need to be 12-8% for men and 18-14 for women.