Editing CAS Registry Number (section)

== Format ==
A CAS Registry Number has no inherent meaning, but is assigned in sequential, increasing order when the substance is identified by CAS scientists for inclusion in  the CAS Registry database.

A CAS RN is separated by hyphens into three parts, the first consisting from two up to seven digits,<ref>2014-06-18, https://www.cas.org/content/chemical-substances/faqs</ref> the second consisting of two digits, and the third consisting of a single digit serving as a [[check digit]]. This format gives CAS a maximum capacity of 1,000,000,000 unique numbers.

The check digit is found by taking the last digit times 1, the preceding digit times 2, the preceding digit times 3 etc., adding all these up and computing the sum [[modular arithmetic|modulo]] 10. For example, the CAS number of [[water]] is 7732-18-5: the checksum 5 is calculated as (8×1 + 1×2 + 2×3 + 3×4 + 7×5 + 7×6) = 105; 105 mod 10 = 5.