What's the deal with Specific Gravity?

Specific Gravity is a measurement of molecular weight, and is typically measured in one of two ways.  In the displacement method, Specific Gravity can defined as the ratio of the weight of a substance, to the weight of an equal volume of a substance chosen as a standard - in this case water.  Another way to measure Specific Gravity is with a piece of testing apparatus called a Helium Pycnometer. PTI has found this to be the most precise method of determining a coating's Specific Gravity. 

As it relates to the Actual Coverage Rate (ACR) for powder coatings, all else being equal, lower specific gravity is better.  This difference can be easy to visualize if you look at a Zinc Rich Primer. Depending on the loading, 50 pounds of Zinc Rich Primer often times is shipped in a box that is half the size of a Zinc Free primer; that’s because the Zinc Rich primer can have a Specific Gravity as high as 2.8, while a Zinc Free primer may have a specific gravity of 1.6 or lower.

The table on our Powder Coating Resources page illustrates how many square feet can be covered with one pound of powder with varying Specific Gravity at different film thickness, assuming one pound pound of powder covers 192.3 square feet, at 1 Mil, with a Specific Gravity of 1.0 and 100% material utilization.   Using these assumptions, the formula for ACR  =  192.3 x Material Utilization / Film Thickness x SG = Square Feet Per Pound of Powder.   Don't worry about memorizing the formula -- we've also included a comparative calculator on this page.