Bolt Depot lists nine standard machine screw sizes, the smallest of which is seven-sixty-fourths of an inch with a thread count of 40, while the largest is three-eighths of an inch with a thread count of 16. Some machine screws share the same diameter but feature differing thread counts.
An example of a screw size with multiple differing thread counts is the size 10 screw, which has a diameter of three-sixteenths of an inch. Machinists using this size of screw can find variations of the screw with 32 threads or 24 threads.
The five smallest U.S. machine screw sizes share a standard length of 1.5 inches, while the four largest sizes share a standard length of 2 inches. In each case, individual screw manufacturers can build longer screws as necessary, adding a multiplier at the end of the screw's name to indicate how much longer or shorter the particular variant is compared to the U.S. standard.
Screw length is not measured from where the head of the screw terminates, but from where the screw's surface is. For this reason, screw manufacturers and users measure pan-headed screws from just underneath the head and measure flat-headed screws according to their entire physical length.