For Metric sizes only.
Diameter of thread minus the pitch equals the tap drill size.
Example:
M6 x 1 thread
6 - 1 = 5mm tap drill size
Standard threads:
Example:
Tap drill size for 3/8-16 is 5/16.
The pitch is 1/16". (16 threads per inch or 1/16 of an inch per thread)
Diameter minus thread pitch = tap drill size.
3/8 - 1/16 = 5/16 tap drill size
For other sizes that don't work out so nicely, just use the closest drill size.