A CAS Registry Number is a unique numeric identifier assigned by the Chemical Abstracts Service to every chemical substance described in the open scientific literature. The format is up to 10 digits split into three groups by hyphens (e.g., 7732-18-5 for water). The last digit is a check digit. CAS numbers appear in safety data sheets, REACH registrations, and product compliance documentation.
What CAS numbers identify
The Chemical Abstracts Service, a division of the American Chemical Society, has been assigning registry numbers to chemical substances since 1965. Each CAS number uniquely identifies a single chemical substance, including elements, compounds, polymers, alloys, mixtures, and biological sequences.
As of 2026, the CAS registry contains over 230 million unique substance records. Every new substance described in a patent, journal article, or regulatory filing gets a new CAS number. The registry grows by thousands of entries per day.
What makes CAS numbers valuable for product data is their universality. Unlike trade names (which vary by manufacturer and language), a CAS number unambiguously identifies a substance worldwide. Lead is 7439-92-1 regardless of whether you call it Blei, plomb, or plomo.
The format
A CAS number has three parts separated by hyphens:
7732-18-5
|||| || |
|||| || +-- Check digit (1 digit)
|||| |+---- Part 2 (2 digits)
|||| +-----
++++-------- Part 1 (2-7 digits)
- Part 1: 2 to 7 digits (no leading zeros)
- Part 2: Always 2 digits (leading zero allowed)
- Part 3: Always 1 digit (the check digit)
The total number of digits (excluding hyphens) ranges from 5 to 10. CAS numbers are assigned sequentially, so higher numbers generally correspond to more recently registered substances. Water (7732-18-5) has a relatively low number because it was registered early. A flame retardant discovered in 2023 might have a number above 2000000-00-0.
The check digit algorithm
The CAS check digit uses a simple weighted sum:
- Take all digits except the check digit, reading right to left.
- Multiply the first (rightmost) digit by 1, the second by 2, the third by 3, and so on.
- Sum all the products.
- The check digit is the sum modulo 10.
Example for water (7732-18-5):
Digits without check digit: 7, 7, 3, 2, 1, 8
Reading right to left: 8x1 + 1x2 + 2x3 + 3x4 + 7x5 + 7x6 = 8 + 2 + 6 + 12 + 35 + 42 = 105
Check digit: 105 mod 10 = 5
This matches the check digit in 7732-18-5.
You can verify any CAS number instantly with the free CAS number validator. It checks the format and the check digit calculation.
Where CAS numbers appear in product data
Safety Data Sheets (SDS)
Section 3 of every SDS (Composition/Information on Ingredients) lists hazardous substances by their CAS numbers. If a cable contains PVC with a lead stabilizer, the SDS will list:
| Substance | CAS Number | Concentration |
|---|---|---|
| Polyvinyl chloride | 9002-86-2 | 60-70% |
| Lead stearate | 1072-35-1 | 0.5-2% |
| Dioctyl phthalate | 117-81-7 | 15-25% |
REACH registration dossiers
Under the EU's REACH regulation, every chemical substance manufactured or imported into Europe in quantities above 1 tonne per year must be registered with ECHA (European Chemicals Agency). The registration dossier identifies the substance by its CAS number. When you check whether a substance in your product is REACH-registered, you search by CAS number.
SVHC (Substances of Very High Concern) lists
ECHA's Candidate List of SVHCs uses CAS numbers as the primary identifier. When you need to declare whether your products contain SVHCs above 0.1% by weight (as required by REACH Article 33), you cross-reference your bill of materials against the Candidate List using CAS numbers.
Product compliance declarations
Manufacturer declarations for RoHS, REACH, and other regulations often include tables of restricted substances with their CAS numbers. A RoHS compliance certificate might list:
| Substance | CAS Number | Threshold | Present? |
|---|---|---|---|
| Lead | 7439-92-1 | 0.1% | Below threshold |
| Mercury | 7439-97-6 | 0.1% | Not detected |
| Cadmium | 7440-43-9 | 0.01% | Not detected |
Building product data (EPDs, LEED documentation)
Environmental Product Declarations and green building documentation reference substances by CAS number when disclosing material composition. This is increasingly relevant for electrical products used in LEED-certified buildings.
CAS numbers relevant to electrical distribution
Electrical product data teams encounter CAS numbers primarily in the context of compliance. Here are substances that frequently appear:
| Substance | CAS Number | Relevance |
|---|---|---|
| Lead | 7439-92-1 | RoHS restricted substance, solder, cable sheathing |
| Bis(2-ethylhexyl) phthalate (DEHP) | 117-81-7 | SVHC, plasticizer in PVC cables |
| Hexabromocyclododecane (HBCDD) | 25637-99-4 | SVHC, flame retardant in polystyrene insulation |
| Bisphenol A | 80-05-7 | SVHC, used in epoxy resins and polycarbonate |
| Chromium trioxide | 1333-82-0 | SVHC, used in surface treatment of metal enclosures |
Common problems with CAS numbers in product data
Missing hyphens. Some systems store 7732185 instead of 7732-18-5. Without hyphens, the check digit cannot be verified because the digit grouping is ambiguous.
Leading zeros stripped. CAS numbers with a two-digit first part (like 50-00-0 for formaldehyde) are sometimes stored as 5000-0 after integer conversion, destroying the format.
Incorrect check digits. Manual data entry errors are common. Always validate the check digit, especially for CAS numbers copied from PDFs where OCR errors can change a digit.
Outdated CAS numbers. Some substances have been assigned multiple CAS numbers over the years. Asbestos, for example, has different CAS numbers for different mineral forms (chrysotile is 12001-29-5, amosite is 12172-73-5). Product data should use the CAS number that matches the specific substance form.
Validate CAS numbers in your compliance data with the free CAS number validator before filing regulatory declarations.