The use of this tool is simple. If the shaft of the screw doesn't easily swipe unimpeded through the gap, or alternately, if it doesn't readily fall into the circular hole, then the screw belongs to the next larger size. This tiny tool will make short work of size sorting the mixed collection of screws found in many home drawers or in the ubiquitous "screw jar."

For an improved version with an integral ruler, see my post at https://www.thingiverse.com/thing:3677953