CAS Registry Numbers are unique numerical identifiers assigned by the "Chemical Abstracts Service" to every chemical described in the open scientific literature and including elements, isotopes, organic and inorganic compounds, organometallics, metals, alloys, coordination compounds, minerals, and salts; as well as standard mixtures, compounds, polymers; biological sequences including proteins & nucleic acids; nuclear particles, and nonstructurable materials. They are also referred to as CAS RNs, CAS Numbers, etc.

The Registry maintained by CAS is an authoritative collection of disclosed chemical substance information. Currently the CAS Registry identifies more than 56 million organic and inorganic substances and 62 million sequences, plus additional information about each substance; and the Registry is updated with an approximate 12,000 additional new substances daily.

A CAS 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 Registry Number is separated by hyphens into three parts, the first consisting of up to 7 digits, the second consisting of two digits, and the third consisting of a single digit serving as a check digit. The check digit is found by taking the last digit times 1, the previous digit times 2, the previous digit times 3 etc., adding all these up and computing the sum modulo 10. For example, the CAS number of water is : the checksum 5 is calculated as (8×1 + 1×2 + 2×3 + 3×4 + 7×5 + 7×6) = 105; 105 mod 10 = 5.

CAS number search engine

CHEMINDEX Search via Canadian Centre for Occupational Health and Safety
ChemIDplus Advanced via United States National Library of Medicine
Common Chemistry TM via Australian Inventory of Chemical Substances
European chemical Substances Information System via the website of Royal Society of Chemistry
HSNO Chemical Classification Information Database via Environmental Risk Management Authority
Search Tool of Australian Inventory of Chemical Substances