The CAS Registry Number is the unique numerical identifier assigned by the Chemical Abstracts Service to every chemical substance described in the open scientific literature. The format is up to ten digits split into three hyphenated groups: a base number, a two-digit sub-number, and a single check digit (e.g. 1310-73-2 for sodium hydroxide; 7664-93-9 for sulphuric acid; 67-56-1 for methanol). The CAS Registry contains over 200 million substance entries and is the largest substance database in existence. For chemical commerce, regulation, and procurement, the CAS number is the universal lingua franca, every other identifier (IUPAC name, common name, trade name, SMILES, InChI) maps back to a CAS number.
What a CAS number identifies
A CAS number identifies a single, specifically defined chemical substance. The substance can be:
- A defined molecule, methanol (67-56-1), benzene (71-43-2), aspirin (50-78-2)
- A specific salt, sodium chloride (7647-14-5) is distinct from potassium chloride (7447-40-7)
- A specific hydrate, copper(II) sulphate pentahydrate (7758-99-8) is distinct from anhydrous copper sulphate (7758-98-7)
- A specific isomer. D-glucose (50-99-7) is distinct from L-glucose (921-60-8)
- A polymer of defined repeating unit and molecular weight range, though most polymers are also identified by the CAS number of their monomer
- A complex mixture of defined origin, many essential oils, refinery products, and natural extracts have CAS numbers despite being mixtures
The specificity matters. A purchase order written for “67-56-1, methanol” is unambiguous. A PO for “methyl alcohol” without CAS is technically the same substance but creates room for the supplier to ship a different grade or a different chemistry under a similar trade name.
CAS format and validation
The CAS format has three parts separated by hyphens:
- The first part is up to seven digits, identifying the substance’s position in the CAS Registry
- The second part is exactly two digits
- The third part is a single check digit calculated from the first two parts
The check digit lets you validate a CAS number for typos. If the check digit doesn’t match the calculated value, the CAS number is incorrect (either typo’d or fabricated). A surprising number of supplier documents contain typo’d CAS numbers, always verify before relying on the number.
The check-digit calculation: take each digit of the first two parts, multiply by its position (counting from the right, starting at 1), sum the products, and the check digit is that sum modulo 10. For 1310-73-2: digits are 1,3,1,0,7,3 from positions 6,5,4,3,2,1 → sum = 1×6 + 3×5 + 1×4 + 0×3 + 7×2 + 3×1 = 6+15+4+0+14+3 = 42 → 42 mod 10 = 2 ✓.
CAS Registry vs CAS Common Chemistry
| Resource | Coverage | Access | Use |
|---|---|---|---|
| CAS Registry | 200M+ substances; full literature linkage | Paid subscription via SciFinder | Patent and literature research; full chemistry intelligence |
| CAS Common Chemistry | ~500K substances; basic identifier and structure data | Free; commonchemistry.cas.org | Routine identifier verification; SDS preparation; trade documentation |
For chemical sourcing, CAS Common Chemistry is sufficient for the vast majority of needs. The free database covers all common industrial chemicals, all REACH-registered substances, and all major commodity chemistries. The full CAS Registry adds patent literature, reaction database access, and granular sub-substance variants, needed for R&D and regulatory dossier work but not for routine procurement.
How CAS numbers map to other identifiers
| Identifier | Relationship to CAS |
|---|---|
| HS Code | Many CAS numbers; HS codes are 6-digit and group thousands of CAS numbers per code |
| EC Number / EINECS | Roughly 1:1 for substances on the EU EINECS inventory; CAS may have substances EC does not |
| IUPAC name | 1:1; IUPAC name is the systematic chemical name |
| SMILES | 1:1; SMILES is the structural notation |
| InChI / InChIKey | 1:1; InChI Key is the hashed structural identifier |
| Trade names / common names | Many-to-one (one substance, many trade names) |
For chemical regulation, the CAS-to-regulatory-inventory mapping is the operational link:
- REACH registers substances by CAS (where applicable) and EC number
- TSCA uses CAS numbers for the inventory
- IECSC uses CAS numbers
- K-REACH uses CAS numbers
- AICIS uses CAS numbers
A buyer checking regulatory status across jurisdictions queries each inventory by the same CAS number. The cross-reference is direct.
Why the CAS number matters on every PO
Three reasons CAS belongs on every chemical PO:
- Unambiguous specification. The chemical to be supplied is specified by CAS, not by trade name. A trade-name dispute (e.g. supplier ships a substance with a similar but distinct name) is closed by the CAS reference.
- Regulatory traceability. Destination customs may query the regulatory status. The CAS reference connects to the inventory entry.
- Quality alignment. The COA, SDS, and product label can be cross-checked against the PO via CAS. Discrepancy in any document is caught.
A common operational mistake is ordering “industrial-grade caustic soda” without CAS. The supplier ships UN 1823 (sodium hydroxide solid, CAS 1310-73-2). The buyer needed UN 1824 (sodium hydroxide solution, also CAS 1310-73-2, same CAS, different physical form). The CAS alone is not always enough; the PO must specify CAS, grade, form, and concentration.
Operator note: fake CAS numbers from low-tier suppliers
Smaller Chinese chemical traders occasionally write fabricated CAS numbers on documents, typo’d numbers, transposed digits, or even invented numbers that look legitimate. Run the check-digit validation on every CAS number on incoming documents. A failed check digit is a strong quality signal that the supplier’s documentation is unreliable. Expand the audit on other documents from that supplier.
Related terms
EC Number is the EU regulatory equivalent. IUPAC name is the systematic chemical name. HS Code groups CAS numbers for trade classification. SDS and MSDS carry the CAS as the primary substance identifier.