How to Read a CAS Number (and Why Your SDS Depends on It)

A CAS Registry Number, usually just called a CAS number, is a unique numeric identifier assigned by the Chemical Abstracts Service to every chemical substance described in the open scientific literature. It exists because chemical names are a mess: the same compound can have a systematic name, a trade name, several common names and spellings in different languages. The CAS number cuts through this by pointing to exactly one substance.

The structure of a CAS number

A CAS number has up to ten digits in three parts separated by hyphens. The first part has two to seven digits, the second part has exactly two, and the final part is a single check digit. Take water, 7732-18-5, or ethanol, 64-17-5. The numbers carry no chemical meaning: the first two parts are simply assigned in sequence as substances are registered, so you cannot read structure from them.

How the check digit works

The final digit protects against typing errors. To verify a number, take the digits to the left of the check digit, read them from right to left, and multiply the first by one, the second by two, the third by three and so on. Add the results, and the last digit of that sum must equal the check digit. For ethanol, 64-17-5: the digits to check are 6417, read from right to left as 7, 1, 4, 6. Then 7×1 + 1×2 + 4×3 + 6×4 = 7 + 2 + 12 + 24 = 45, and the last digit is 5, which matches. A number that fails this check is certainly wrong, although passing it does not prove the number is the right one.

Why the CAS number matters on an SDS

The CAS number is how every database, customs system and regulator looks up your product. Section 1 and Section 3 of an SDS list it, hazard lookups use it, and customs declarations and restricted-substance lists reference it. Putting a wrong or missing number on an SDS can lead to the wrong classification being applied, a shipment being held, or a supplier lookup failing. Our guides on writing an SDS and the 16 sections show where the identifier belongs.

Common traps

• Hydrates have their own numbers. Copper sulphate pentahydrate is 7758-99-8, while anhydrous copper sulphate is 7758-98-7. They differ in copper content and, sometimes, in classification.
• Salts and acids differ. Sodium hydroxide, potassium hydroxide and their solutions are separate entries, and a solution such as hydrochloric acid may be listed under the CAS number of the dissolved gas (hydrogen chloride, 7647-01-0).
• Mixtures and UVCBs. Substances of unknown or variable composition, such as petroleum fractions, have CAS numbers for the group but are not a single compound. Mixed xylenes, for example, are 1330-20-7 while the single isomers have different numbers.
• Old and deleted numbers. Some numbers were replaced when records were merged, so a number from an old document can need updating.

CAS, EC and UN numbers are different things

People mix these up constantly. A CAS number identifies a substance worldwide. An EC number (or EINECS/ELINCS number) is a European Community identifier used under REACH and CLP, in the form of three digits, three digits and one check digit. An index number belongs to the CLP Annex VI harmonised classification list. A UN number is a four-digit transport code that identifies a hazardous substance or group in shipping, and it is not a substance identifier in the same sense: many different substances can share one UN number. Section 14 of the SDS holds the UN number while Section 3 holds the CAS and EC numbers.

How to check a number

Look it up in a reliable public source, such as PubChem or the ECHA classification and labelling inventory, and confirm that the name, formula and hydrate state match your product. If you hold supplier documents, compare the number against the supplier SDS and certificate of analysis. Each substance page in our chemical database lists the CAS, EC and index numbers together with a retrieval date, so you can check yours quickly.