(Image borrowed from All America Threaded Products)
It will probably come as no surprise that the Metric system of measuring screws is straight forward and the imperial system is…less so.
A typical metric screw would be designated like this.
M8-1.0 x 20
M is the Metric designation
8 is the Major Diameter in mm (Nominal)
1.0 is the pitch in mm
20 is the length of the screw in mm
Imperial size screws follow a similar format but their features are calculated differently. (For screws under 1/4” the Major Diameter is express by a number 0-12)
10-32 x 1”
10 is the Major Diameter (Nominal) and is calculated as follows:
10 x .013” + .060”
So a number 10 screw has a Major Diameter of .190”
A number 8 screw would have a diameter of .164” (8 x .013” + .060”)
32 is the Threads Per Inch (tpi). (The pitch of the screw is calculated by divinding 1” by the tpi. The pitch of a 32tpi screw is .03125” )
1” is the length
(Flat head screws are typically measured as the overall length while screws designed to have the heads sit above the surface are measured from below the head to the end of the threads.)
Once the Major Diameter of a screw is known the pitch options are narrowed to a few possibilities usually referred to as Coarse, Fine, and UltraFine. Here are a couple of charts I use daily showing the Major Diameters and standard pitches for common screws along with other useful information.