NOTE: This has actually turned into a much broader post - the Body Fat Scale stuff is near the bottom.
Most of us need some means of measuring our progress - either to make adjustments to our program if needed or just to reassure us that things are going well. But what should that measurement be?
It's the easiest measurement to take, but it has lots of pitfalls just waiting to snag us.
First - what's your goal weight? This is not too serious an issue - at least at the beginning - because we really are only looking for progress at this point. We can pick a goal weight off a chart somewhere or remember a weight at which we felt good or shoot for the weight that a friend is at. The point here is that it is pretty easy to come up with a specific target weight and work towards it knowing we can always change the goal later.
Then, the measurements themselves generally have enough error to cause some people a hopeless amount of frustration - especially if we don't have a good scale. You can find out a bit about what makes a scale good by looking at: Accuracy and Precision of Weight Scales
But even with a good scale it is difficult to interpret the measurements in an accurate and meaningful way. The scale may not budge or might even go up for a few weeks even though you might be losing fat during that time. This can be caused by a combination of muscle building and water retention in the now empty fat cells. So unless you are comfortable with this and can truly accept it, the misleading behavior of the scale readings might lead to dispair and abandonment of your efforts.
This is a much better indicator of progress than a scale is - but it has drawbacks as well.
First - what's your goal? As with scale weight, we are really only looking for progress initially but at some point we need to start moving toward our final, long-term goal. This is much harder. I have an idea of about where I would like my waist size to end up, but I have much less of an idea where I should aim for in terms of chest or hip size and absolutely no idea about what my final thigh or calf size should be.
Then, there are some of the same issues that can frustrate scale weight users - but not as many. If you build replace fat with the same volume of muscle, your scale weight goes up considerably but your inches would remain the same. More typically, you might build enough muscle to show a scale weight gain - or no change or only a slight loss - but your fat loss would be enough to show a discernable inch loss. That's good. But if your fat cells are holding on to water to keep their volume up, that fat loss would still not show up in your inch measurements. So these can still be misleading and frustrating if you aren't willing to take them with a grain of salt and exercise patience and persistence.
Body Fat Percentage?
This is a pretty good measure of progress - IF you can get good numbers. That's where the problem is.
First - what's your goal? This is not too tough as there are quite a few good tables of ideal body fat percentages and they are in pretty good agreement with each other. And, as with any of these, you are looking for progress at first and can change the target goals as you get closer.
But now we need to have a way to actually measure body fat percentage. Good luck. The only really good way to measure body fat percentage is to strip the fat off and weigh it. While that works fine for cadavers (and this is how all other methods are calibrated when all is said and done) it would have long-term health implications for you or me. We need a non-destructive means of estimating body fat percentage fo us.
The Gold Standard for these measurements are the hydrostatic tank measurements where your overall body density is measured directly. But these are not without error primarily owing to imperfect knowledge about such things as bone density and bone volume. So your frame size is usually estimated by taking wrist or elbow measurements and your bone density is estimated by looking up your racial make-up on a chart. After doing that, you get a pretty decent number. More importantly, you get a number from which future changes can be compared and that's what is really important for a long time.
But hydrostatic body fat measurements are difficult to make, require special equipment that is hard to find, costs quite a bit of money (more than you would want to spend to get weekly or monthly measurements) and are inconvenient and time consuming. So we need something else - preferably something we can do ourselves at home.
There are LOTS of formulas out there for estimating body fat percentage using a combination of your weight and key body circumference measurements. Some are better than others but all are pretty shakey as far as accuracy goes. But, as we have found with every other measure - accuracy is not terribly important as long as we have good enough precision (repeatability) to let us see changes over time. As long as we use the exact same method and forumulas every time, this type of measurement can serve us well provided we do not try to put to much faith in the actual number that results.
In a similar fashion, skin fold measurements can be used to estimate body fat percentage and the have all of the same issues. In addition, getting repeatable measurements on yourself or on someone that is considerably overweight can be very difficult whereas girth measurements are pretty easy to make. On the flip side, once you get reasonably close to goal, skin fold measurements taken by an experience technician yield reasonably accurate results.
Finally, we come to the Bioelectric Impedance Analyzers (BIA). These come in two varieties (as far as I'm concerned): Laboratory Grade and Consumer Grade. To understand the difference, let's first discuss what BIA is and how it relates to body composition.
The basic idea behind BIA is that the electrical impediance (resistance) of fat is different from that of muscle and bone and blood and other body structures. So, if we measure the body's electrical impedance we can gain insight into the relative quantities of these various substances. So far so good. But there are real problems both with taking a meaningful measurement and with making a reasonable interpretation of the results.
First we have the same issues that the hydrostatic tests have and we need to make adjustments based on bone size and bone volume using charts and tables.
In actually taking the measurment, we have to overcome the skin resistance - which dominates the overall resistance. Consider tha most people, with dry skin, can get somewhere around 100V across their body (say wrist to wrist) and won't even feel any sensation - the body's resistance is sufficiently high to limit the resulting current to a level below our sensation threashold. Now consider that if you get past the skin resistance by piercing the skin with a needle in each wrist then a 9V battery is more than sufficient to deliver a lethal current.
In addition to the dominating nature of the skin resistance - skin resistance is highly variable. The quality of the contact, the surface area of the contact, and the degree of moisture on and in the skin at that moment all have a major impact on the effective contact resistance.
To get around this problem, Laboratory Grade BIA instruments use four wires. Two generate the actual current and the other two, located on the body between the other two, measure the voltage drop that is produced by the current flowing through the body. This is called a four-wire measurment and is the only reliable means of getting around both skin resistance and lead-wire resistance.
Furthermore, once we get current into the body, we have to ensure that this current flows through a representative cross-section of the body. This normally entails injecting current into one ankle and extracting it from the opposite wrist. That way it travels through one leg, the abdomen, the chest, and one arm. Any measurement that goes foot-to-foot or hand-to-hand is going to be seriously flawed right from the beginning.
Finally, the distance between the electrodes and the cross sectional area of the body structures through which the current is flowing are important and are measured during Laboratory BIA tests.
If all of these factors are taking into account and good measurements are made using several thousand dollars worth of equipment then BIA measurements can produce results almost on par with hydrostatic measurements.
So, what do Consumer Grade BIA instruments measure? To be frank, I haven't the faintest idea! They are all two-wire measurments, they all use a non-representative cross-section of the body, all of them neglect electrode distance and limb cross-sction. About the only thing most of them can possibly be measuring is skin resistance. Some of them try to play some games to subtract this out, but the methods used are generally pretty suspect. This is why an obese person can sometimes obtain a lower body fat percentage reading than a skinny person and why a given person's measurements generally vary all over the place from day to day and week to week.
So, my recommendation is to not waste your money on any consumer grade BIA devices.
So what should you use? Personally, go by how you feel first off. Then, use your clothes as a guide - the infamous pant-o-meter. Third, track your key measurements and pick one type of body fat measurement technique (usually, the more complicated the better) and run those numbers every few months. Finally, the scale is quite useful IF you can use it only to track the long-term trends and can ignore the short-term (month or less) fluctuations.