British Standard Brass (BSB)tapping drill sizes.

Nominal Dia of Thread Pitch TPI Tapping drill Dia mm
1/4" 26 5.40
5/16" 26 6.90
3/8" 26 8.40
7/16" 26 10.00
1/2" 26 11.50
9/16" 26 13.00
5/8" 26 14.70
3/4" 26 17.80
7/8" 26 21.00
1" 26 24.20