Locate the National Fenestration Rating Council (NFRC) label on your window. Most new residential windows are rated by the NFRC, and should have a label.
Note the solar heat gain coefficient (SHGC) on the label. It has replaced the shading coefficient (SC) as the standard indicator of a window's shading ability. SHGC is expressed as a number between 0 and 0.87. SC is expressed as a number between 0 and 1. You can use the following formula: SHGC = SC --- 0.87.
Divide SHGC value by 0.87 to compute SC.