How to calculate the drill size for BSPP Thread e.g. 3/8" - 19 BSPP ?

My workshop uses drills in increments of 1/4mm - ø1, ø1.25, ø1.5, ø1.75 and so on. They do no accept exotics as hole diameter 1.33mm ;-)

This might get you pretty close. Usually a table would be referenced. That table was likely developed through a bit of trial and error. An equation will get you close, but drilling, tapping, and evaluating the results will give the correct answer:

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.

I've used this formula for some time now for cut taps. Percentage of thread as a parameter, since many threading operations don't require a 75% thread, it's nice to calculate hole size to the percentage you want.

mD - .01299(P/Pitch)

where:

mD = major diameter
P = thread percentage

For a 3/8-19 BSPP

.656 - .01299(75/19)
= .6047 or 15.4 mm