space group number

To find the crystal class from space group symbol:

(a) Drop the initial capital letter denoting lattice type.

(b) Drop all subscripts.

(c) Change all remaining letters to m.

(d) If the symbol is not 1 or -1, discard any "1"s

system	class	number
triclinic	1	1
	-1	2
monoclinic	2	3-5
	m	6-9
	2/m	10-15
orthorhombic	222	16-24
	mm2 = m2m = 2mm	25-46
	mmm	47-74
tetragonal	4	75-80
	-4	81-82
	4/m	83-88
	422	89-98
	4mm	99-110
	-42m = -4m2	111-122
	4/mmm	123-142
trigonal	3	143-146
	-3	147-148
	32	149-155
	3m	156-161
	-3m	162-167
hexagonal	6	168-173
	-6	174
	6/m	175-176
	622	177-182
	6mm	183-186
	-6m2 = -62m	187-190
	6/mmm	191-194
cubic	23	195-199
	m-3	200-206
	432	207-214
	-43m	215-220
	m-3m	221-230