American (unified national) threads are given as [Bolt Size]-[Thread Pitch] [Thread Series], so for a given screw (e.g. #10-32 UNC), you can look up the
dimensions and measure with calipers. Pitch is in threads per inch.
Metric threads are given as M[Bolt Size] for coarse (standard) bolts, or M[Bolt Size]×[Thread Pitch] for fine or irregular bolts. Dimensions are
here. Pitch is in mm per thread.