Synthetic underlayment products are frequently characterized in terms of weight or, more precisely, areal weight, so it's worthwhile to examine what exactly is meant by the “weight per unit area”.
The units for areal weight are typically given as grams per square meter (GSM). Areal weight is easy-to-measure and meaningful. It describes the amount of polymer in the underlayment, allowing synthetic underlayment products to be compared to each other and also to other types of underlayments.
Thickness values are not so meaningful because the thickness varies locally, depending on the configuration of the synthetic underlayment. Areal weight alone does not uniquely describe the product because two completely different configurations could have the same overall weight, depending on the amount of scrim.
Consider the following thought experiment:
One cube with one centimeter sides has the same volume as square sheet that is one meter by one meter by one micron thick. That’s because 10,000 micrometers add up to a centimeter, and there are 10,000 square centimeters in a square meter. Therefore textbook values of “density” or “specific gravity” are the numerically the same as the areal density for a sheet that is one micrometer thick. In other words, a one micrometer deep puddle of water that is one square meter in diameter would weigh one gram. Similarly, 1,000 grams of water could produce a puddle one millimeter deep with an area of one square meter (or a circle with a radius of 0.56 m).
A rule-of-thumb then is to multiply specific density values by 1000 to obtain the weight of a sheet that is one millimeter in thickness.
As an example, the specific density of PP ranges from 0.855 g/cc to 0.946 g/cc. A one millimeter thick layer of PP would then have an areal weight between 855 gsm and 946 gsm. For polypropylene sheets, the GSM (grams per square meter) specification can be converted into an average thickness using the volume density of polypropylene.
Density X Thickness = Areal Density
Thickness = Areal Density / Density
Polypropylene is just slightly less dense than water. It floats. Its density is about 0.946 grams per cubic centimeter. Consequently, when reading a specification, the areal density (in GSM) divided by 0.946 is approximately equal to the average thickness of a polypropylene sheet in micrometers (or microns).
Simply expressed, the average thickness (in microns) of a sheet of polypropylene underlayment is slightly greater than the areal density (in GSM).
Thickness [in microns] = Areal density [in gsm] / 0.946
PP areal densities ranging from 75 to 150 gsm correspond to thicknesses from 80 to 160 microns, or 3 to 6 mils (since 25.4 microns equals one mil).
For comparison, one square is equivalent to 9.29 square meters (since 10 feet equals 3.048 meters). Multiply 75 GSM by 9.29 m2 per square to obtain about 700 grams per square, which is about a pound and a half per square. Hence, a typical UDL 15 synthetic weighs only one tenth as much as a 15 pound felt. But it is also approximately only one tenth as thick, a fact which is often understated or ignored in advertising synthetic underlayment!