Newman Tool's handy hint.
For Metric sizes only. Diameter of thread minus the pitch equals the tap drill size.
eg. M6 x 1
6 - 1 = 5mm
and you thought metric was difficult ;-)
thanks to Jay Steinbuchel of Barksdale Control Products for the following:
This works for all 75% threads, not just metric.
For example, the tap drill for 3/8-16 is 5/16.
The pitch is 1/16". (1 / threads per inch = inches per thread)
3/8 - 1/16 = 5/16.
For other sizes that don't work out so nicely, just use the closest drill size.