7/8 inch in mm and decimal

7/8 in = 0.875 in = 22.225 mm. Closest metric drill: 20 mm.

Taps that use a 7/8 drill

Drill	Type	Inch	mm	Difference
13/16	fraction	.8125 in	20.637	-62.5 thou
53/64	fraction	.8281 in	21.034	-46.9 thou
27/32	fraction	.8438 in	21.431	-31.3 thou
55/64	fraction	.8594 in	21.828	-15.6 thou
7/8	fraction	.8750 in	22.225	0.0 thou
57/64	fraction	.8906 in	22.622	15.6 thou
29/32	fraction	.9063 in	23.019	31.3 thou
59/64	fraction	.9219 in	23.416	46.9 thou
15/16	fraction	.9375 in	23.812	62.5 thou

1 thou = 0.001 in = 0.0254 mm. Drills a few thou apart are usually interchangeable for clearance holes, but not for tap drills or reamer pilots.