The two most common gauges for rhinestone sizes are ’ss’ (stone size) and ‘pp’ (pearl plate). The original rhinestone sizing (’pp’) was based on the technique of pearl sizing. Back then, pearls were placed on vertically stacked plates with a series of graduated holes punched in them. As the pearls fell through the holes, they were sized (and priced) according to the last plate they landed on. This is where the designation ‘pp’ comes from. Subsequently, jewelers developed their own sizing formula called ’stone size’ or ’ss' which essentially achieved the same goal, but used a different scale.
In order for manufacturers to keep us on our toes, the ‘pp’ and ’ss’ scores are meant to indicate a size range, not an exact size.

PP size	SS size	size(ｍｍ)
14PP	SS 6	2.0 - 2.1
15PP	SS 7	2.1 - 2.2
16PP	SS 7	2.2 - 2.3
17PP	SS 8	2.3 - 2.4
18PP	SS 8	2.4 - 2.5
19PP	SS 9	2.5 - 2.6
20PP	SS 9	2.6 - 2.7
21PP	SS 10	2.7 - 2.8
22PP	SS 10	2.8 - 2.9
23PP	SS 11	2.9 - 3.0
24PP	SS 12	3.0 - 3.2
25PP	SS 13	3.2 - 3.3
26PP	SS 13	3.3 - 3.4
27PP	SS 14	3.4 - 3.5
28PP	SS 14	3.5 - 3.6
29PP	SS 15	3.6 - 3.7
30PP	SS 15	3.7 - 3.8
31PP	SS 16	3.8 - 4.0
32PP	SS 17	4.0 - 4.1
33PP	SS 17	4.1 - 4.2
	SS 18	4.2 - 4.4
	SS 19	4.4 - 4.6
	SS 20	4.6 - 4.8
	SS 22	4.9 - 5.1
	SS 23	5.1 - 5.3
	SS 24	5.3 - 5.5
	SS 29	6.1 - 6.3
	SS 30	6.3 - 6.5
	SS 32	6.7 - 6.9
	SS 34	7.1 - 7.3
	SS 40	8.4 - 8.7
	SS 42	8.9 - 9.2
	SS 47	10.5 - 10.9
	SS 48	10.9 - 11.3