CAS Number (Chemical) Validator

CAS Registry Number: the universal ID for chemical substances

A CAS Registry Number (CAS RN) is a unique numeric identifier assigned by the Chemical Abstracts Service, a division of the American Chemical Society, to every chemical substance described in the open literature. Over 200 million substances are registered. The number carries no chemical meaning — it is assigned sequentially — so it works as a stable, language-independent key across safety sheets, regulatory inventories and databases.

The format is up to 10 digits split by hyphens into three groups: NNNNNNN-NN-N — a first group of 2 to 7 digits, a second group of 2 digits, and a final single check digit. Example: water is 7732-18-5.

How the check digit works

The last digit validates the rest. Take all digits except the check digit and number them from right to left starting at 1; multiply each digit by its position, sum the products, and the check digit equals that sum modulo 10.

• Example (7732-18-5): digits 7 7 3 2 1 8 weighted from the right → 8×1 + 1×2 + 2×3 + 3×4 + 7×5 + 7×6 = 8+2+6+12+35+42 = 105; 105 mod 10 = 5 ✓.
• This catches almost every single-digit typo and most transpositions.

Where you'll see CAS numbers

• Safety Data Sheets (SDS, the Brazilian FISPQ) and GHS labels identify each component by CAS number.
• Regulatory inventories: TSCA (US), REACH/EINECS (EU), and national chemical registers index substances by CAS.
• Databases: PubChem, ChemSpider and supplier catalogs cross-reference CAS numbers.

Common pitfalls

• Not the EC/EINECS number: a CAS RN is different from the EU EC number and from UN transport numbers — don't mix them.
• Hyphens are part of the format: the three-group layout matters; a bare digit string is ambiguous.
• Deleted/replaced numbers exist: CAS occasionally retires or merges numbers, so a valid-checksum number can still be obsolete.
• Checksum ≠ existence: passing the mod-10 check proves the format is internally consistent, not that the substance is registered.

FAQ

Does the CAS number encode the molecule? No. It is a sequential registry key with no structural meaning — use SMILES or InChI for structure.

How long can a CAS number be? Up to 10 digits total (7-2-1), so the longest looks like NNNNNNN-NN-N.

Can two substances share a CAS number? No — each registered substance gets a unique CAS RN; mixtures and polymers may have their own dedicated numbers.