A CAS number is a unique numerical identifier assigned to every known chemical substance. Managed by the Chemical Abstracts Service, a division of the American Chemical Society, these numbers function like a universal ID system for chemicals. Each number points to exactly one substance, eliminating the confusion that comes from chemicals having multiple names across languages, industries, and scientific fields.
Why Chemical Names Alone Aren’t Enough
A single chemical compound can go by dozens of different names. The antibiotic ampicillin, for instance, is also known as ampicilina, aminobenzylpenicillin, penbritin, totacillin, ABPC, AMP, and more than a dozen other identifiers depending on who’s referencing it and in what context. Multiply that kind of ambiguity across millions of substances and the potential for miscommunication becomes serious, especially in safety-critical settings like pharmaceutical manufacturing, hazardous materials handling, and regulatory compliance.
Chemical compounds can be described by molecular formulas, systematic names, generic names, trade names, and common names. Two researchers in different countries might use completely different terminology for the same substance. A CAS number cuts through all of that. It’s language-independent, globally recognized, and maps to one specific substance with no room for confusion.
How the Number Is Structured
CAS numbers follow a consistent three-part format: up to six digits, a hyphen, two digits, another hyphen, and a single final digit. Written out, the pattern looks like NNNNNN-NN-N. The final digit is a check digit, calculated from the preceding numbers, which helps catch typos or data entry errors. If someone transposes a digit, the check digit won’t match, flagging the number as invalid.
The numbers themselves carry no chemical meaning. They don’t encode the molecular weight, structure, or properties of a substance. They’re assigned sequentially as new substances are identified and added to the registry. A lower CAS number generally means the substance was catalogued earlier, but the number itself tells you nothing about the chemistry involved.
Some familiar examples: water is 7732-18-5, caffeine is 58-08-2, ethanol is 64-17-5, and sodium chloride (table salt) is 7647-14-5.
What Gets a CAS Number
The CAS Registry covers far more than simple organic molecules. It includes inorganic compounds, metals, alloys, polymers, biochemical substances, isotopes, and even complex biological sequences. If a substance has been disclosed in scientific literature or other vetted sources, it’s eligible for inclusion. CAS scientists curate the database, reviewing new substances as they appear in published research, patent filings, and regulatory documents before assigning a number.
The registry has grown enormously since it was established in the 1960s and now contains hundreds of millions of unique substance records, making it the largest curated collection of chemical information in the world. Nearly 500,000 of those substances are freely searchable through CAS Common Chemistry, a public tool covering commonly referenced and frequently regulated chemicals.
Where You’ll Encounter CAS Numbers
The most common place non-chemists run into CAS numbers is on Safety Data Sheets. U.S. federal regulations administered by OSHA require that Section 3 of every Safety Data Sheet include the CAS number for each hazardous ingredient. For chemical mixtures, any component classified as a health hazard must be listed with its CAS number and its concentration or concentration range. When a CAS number isn’t available or is claimed as a trade secret, the preparer must provide an alternative unique identifier and note the source.
Beyond safety documentation, CAS numbers are used across scientific publishing, chemical databases, procurement catalogs, environmental regulations, and international trade. If you’re ordering a reagent from a chemical supplier, the CAS number is the most reliable way to ensure you’re getting exactly the substance you need, regardless of the supplier’s naming conventions. Regulatory agencies worldwide use them to track controlled or restricted substances, and researchers use them to search literature databases without getting tripped up by naming inconsistencies.
How to Look Up a CAS Number
The easiest free tool is CAS Common Chemistry (commonchemistry.cas.org), which lets you search by chemical name, CAS number, or structural notation. It covers about 500,000 substances relevant to education, regulation, and general public interest. For the full registry, access typically requires a subscription through research databases like SciFinder.
If you’re reading a Safety Data Sheet and want to verify a CAS number, you can enter it directly into Common Chemistry to confirm it matches the substance listed. This is a practical way to double-check labels, especially when dealing with unfamiliar chemicals or products from international suppliers where naming conventions may differ from what you’re used to.