Chain size

Chain size is in inches, the three digit number used to describe the chain size is based on the "rule of eights". The first digit is for chain pitch. This is the distance between the pins. The second and third digits combine for roller width, with a decimal point inserted between the second and third digits.

All measurements in inches

Size	Pitch (P) (inches)	Width Between Link Plates (W)	Roller Diam (R)
420	1/2 (4/8)	0.252 ~= (2/8)	0.306 ≠ (2/8)
425	1/2 (4/8)	0.313 ~= (2.5/8)	0.312 ~= (2.5/8)
428	1/2 (4/8)	0.313 ≠ (2.8/8)	0.335 ≠ (2.8/8)
428H	1/2 (4/8)	0.313 ≠ (2.8/8)	0.335 ≠ (2.8/8)
520	5/8	0.252 ~= (2/8)	0.400 ≠ (2/8)
525	5/8	0.313 ~= (2.5/8)	0.400 ≠ (2.5/8)
530	5/8	0.375 = (3/8)	0.400 ≠ (3/8)
532	5/8
630	3/4 (6/8)	0.375 = (3/8)	0.469 ≠ (3/8)
O-ring Chains
428	1/2 (4/8)	0.313 ≠ (2.8/8)	0.335 ≠ (2.8/8)
520	5/8	0.252 ~= (2/8)	0.400 ≠ (2/8)
525	5/8	0.313 ~= (2.5/8)	0.400 ≠ (2.5/8)
530	5/8	0.375 = (3/8)	0.400 ≠ (3/8)
532	5/8
630	3/4 (6/8)	0.375 = (3/8)	0.469 ≠ (3/8)

Exceptions

There are a number of chain sizes in use, and chain manufacturers may have slightly different specs for roller diameter, sprocket width, etc. But a 525 chain from one manufacturer will have the same pitch as a 520, 525, 530, or 532 chain from another and run smoothly on any 525 sprocket from any manufacturer.