From the top of my head.
1/8 bsp requires a 8.8 drill, and has an external diameter of 9.8mm
1/4 bsp 11.8mm drill, and has an external diameter of 13.2mm
3/8 bsp 15mm drill, outer diameter 16.6mm
1/2 bsp 19mm drill, outer diameter 21.2mm
3/4 bsp 24.5mm drill, outer diameter 26.7 mm

NPT is more difficult, as it is a tapered tread, that is why I don't use it if I can avoid it.