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A CAS number, formally a CAS Registry Number (CAS RN), is a unique and unambiguous identifier for a specific chemical substance, assigned by CAS (Chemical Abstracts Service), a division of the American Chemical Society. Chemical substances are described by molecular formulas, structure diagrams, systematic names, common names and trade names; a CAS RN is unique to one substance however it is described. For background on the field itself, see the guide to principles of chemistry; this guide covers the identifier as a research and compliance tool.
What Is a CAS Number?
A CAS Registry Number is a numeric identifier that can contain up to 10 digits, divided by hyphens into three parts. The right-most digit is a check digit used to verify the validity of the entire number. An example of the format is 58-08-2, the sample number shown on the CAS Common Chemistry search page.
CAS Registry Numbers do not depend on any system of chemical nomenclature, so they provide a reliable common link between the different naming systems used to describe a substance, including IUPAC systematic names. CAS describes them as a global standard that makes international communication and collaboration easier.
Who Runs It and What It Covers
CAS maintains the CAS REGISTRY, a database of disclosed chemical substances curated from scientific literature and other sources. According to CAS, it holds 300+ million substances and is updated daily; CAS states that it is the source and final authority for CAS Registry Numbers, and that other resources may associate an incorrect number with a substance. CAS is based in Columbus, Ohio.
New substances enter the registry when they are identified in literature being indexed for the CAS Content Collection, or when an entity requests a CAS Registry Number for its novel substance from CAS Registry Services. Numbers are assigned in sequential order to unique, new substances. Substance classes in the registry include:
- Organic and inorganic compounds, plus metals and alloys and minerals.
- Coordination compounds and organometallics.
- Elements, isotopes and nuclear particles.
- Proteins and nucleic acids, and polymers.
- Non-structurable materials and those of unknown or variable composition, and biologics (UVCBs).
Within academic institutions, accurate substance identifiers matter when stocking a chemistry stockroom or recording reagents in an electronic lab notebook for chemistry.
How to Search or Use It
There are two main routes to a CAS RN lookup: a free open tool for common chemicals and CAS’s licensed products for the full collection.
- Start with CAS Common Chemistry. At commonchemistry.cas.org, an open community resource, you can search by chemical compound name, SMILES, InChI or CAS Registry Number. It covers nearly 500,000 substances from CAS REGISTRY, including common and frequently regulated chemicals, and is provided under the Creative Commons CC BY-NC 4.0 license.
- Search by CAS number. Type the hyphenated number (for example 58-08-2) into the search box to reach the record for that substance; use the name, SMILES or InChI search when you have only a structure or a name.
- Move to the full collection when needed. Full access to the CAS content collection is available through CAS custom data and analytics solutions or products such as CAS SciFinder. CAS also offers a CAS Registry Lookup service through an order form.
- Verify a transcribed number with the check digit. Drop the final digit, number the remaining digits 1, 2, 3 and so on from right to left, multiply each digit by its position, add the products, and take the sum modulo 10; the result should equal the check digit. For 58-08-2: the digits 5, 8, 0, 8 read from the right give 8×1 + 0×2 + 8×3 + 5×4 = 52, and 52 modulo 10 is 2, matching the check digit. A passing check shows the number is well formed, not that it is the right substance, so confirm the record against a CAS source.
- Record the identifier with the source. Keep the CAS RN alongside the literature reference, ideally one carrying a persistent Crossref DOI, so the identification can be traced later.
Strengths and Limits
These are the practical boundaries of the CAS numbering system:
- Strengths:
- Each number designates only one substance, so it resolves ambiguity between trade, common and systematic names.
- Government and regulatory agencies rely on CAS Registry Numbers for substance identification in regulatory applications, because the numbers are unique, easily validated and internationally recognized.
- CAS scientists link each number to all available data and research about that substance.
- Limits:
- Because the number is independent of any naming system, it cannot be worked out from a structure or a name; it has to be looked up in a database.
- The open Common Chemistry tool covers nearly 500,000 substances, a small part of the registry; the rest requires CAS data or products.
- A newly disclosed substance gets a number only when it is identified in indexed literature or when its owner requests one from CAS Registry Services.
Alternatives
A CAS RN is not the only identifier for a compound. These other schemes are often used alongside it:
| Identifier | Maintained by | Typical use |
|---|---|---|
| CAS Registry Number (CAS RN) | CAS, a division of the American Chemical Society | Substance identification in catalogs, literature indexing and regulatory applications |
| InChI | IUPAC / InChI Trust | Text identifier derived from structure; searchable in CAS Common Chemistry |
| PubChem CID | NCBI / National Institutes of Health | Open chemistry repository records (see the PubChem guide) |
| IUPAC systematic name | IUPAC | Nomenclature rules that describe a structure in words |
Frequently Asked Questions
What is a CAS reference number?
It is an informal way of saying CAS Registry Number: the unique identifier CAS assigns to a specific substance.
What is the CAS number used for in chemicals?
It identifies one substance regardless of how it is named, so catalogs, publications, regulatory applications and safety documents can point to the same substance. Regulators use it because it is unique, easily validated and internationally recognized.
What does CAS stand for?
CAS stands for Chemical Abstracts Service, a division of the American Chemical Society based in Columbus, Ohio. A CAS number is short for CAS Registry Number.
What is the CAS Registry?
The CAS REGISTRY is the database of disclosed chemical substances curated from scientific literature and other sources, with 300+ million substances according to CAS. Each substance in it has its own CAS Registry Number.
Where to find CAS number on MSDS?
Safety data sheets under the OSHA Hazard Communication Standard (29 CFR 1910.1200) list ingredient information under Section 3, titled “Composition/information on ingredients”. The standard also defines “specific chemical identity” as the chemical name, CAS Registry Number, or other information that reveals the precise chemical designation of the substance. Check Section 3 of the sheet for the number.
How to find chemical CAS number?
Search CAS Common Chemistry by name, SMILES or InChI, check the safety data sheet from the supplier, or use CAS products such as SciFinder for substances outside the open collection. Confirm any number you copy with the check digit method above.
What is a CAS?
“CAS” is the organization, Chemical Abstracts Service, and is also used loosely as shorthand for a CAS Registry Number.
Sources
- CAS, a division of the American Chemical Society. CAS REGISTRY. https://www.cas.org/cas-data/cas-registry (checked 2026-11-07).
- CAS Common Chemistry. https://commonchemistry.cas.org/ (checked 2026-11-07).
- US Occupational Safety and Health Administration. 29 CFR 1910.1200, Hazard Communication, as published in the eCFR. https://www.ecfr.gov/current/title-29/part-1910/section-1910.1200 (checked 2026-11-07).
