Every chemical has many names. Acetone is also propan-2-one, dimethyl ketone and 2-propanone. A CAS Registry Number cuts through that confusion: one number, one substance. This guide explains what CAS numbers are, who assigns them, how to check one is valid, and where to look up compounds.
What is Chemical Abstracts Service?
Chemical Abstracts Service (CAS) is a division of the American Chemical Society. It maintains the CAS Registry, the world’s largest database of chemical substances, which now holds more than 290 million substances. Each substance added receives a unique identifier: the CAS Registry Number, or CAS RN.
How a CAS number is built
A CAS number has up to ten digits in three parts separated by hyphens. The last digit is a check digit. Water, for example, is 7732-18-5.
To check any CAS number, ignore the final digit, multiply the remaining digits by 1, 2, 3 and so on starting from the right, add the results and take the last digit of the sum. If it matches the check digit, the number is well-formed. That does not prove it refers to the compound you think, so always confirm the name and structure too.
Why CAS numbers matter in the lab
- Ordering: searching a supplier catalogue by CAS number avoids buying the wrong isomer, salt or hydrate. Hydrates and anhydrous forms have different numbers.
- Safety: Safety Data Sheets list CAS numbers, so you can match a bottle to its hazard data.
- Regulation: REACH, TSCA and controlled-substance lists identify substances by CAS number.
- Literature: CAS numbers link papers, patents and databases reliably.
Where to look up compounds
| Database | Run by | Access | Best for |
|---|---|---|---|
| CAS SciFinder | CAS (ACS) | Subscription | Comprehensive substance, reaction and literature search |
| CAS Common Chemistry | CAS (ACS) | Free | Quick CAS number lookups for common substances |
| PubChem | US National Institutes of Health | Free | Properties, identifiers, safety and bioactivity data |
| ChemSpider | Royal Society of Chemistry | Free | Structures, names and links to sources |
| Reaxys | Elsevier | Subscription | Reactions, properties and synthesis planning |
For a quick “CAS number finder”, CAS Common Chemistry and PubChem are the easiest free starting points. For research-grade searching of organic compounds and reactions, most universities provide SciFinder or Reaxys.
Common mistakes
- Confusing a salt with its free acid or base, or a hydrate with the anhydrous form.
- Copying a CAS number from an unreliable website. Check it against the supplier’s SDS or a major database.
- Assuming a well-formed number is correct. The check digit only catches typos.
For how compound identity ties into quality, see our guide to reagent chemical grades, and for the ACS journals that report new compounds, our JACS guide.
Frequently asked questions
Who assigns CAS numbers?
Chemical Abstracts Service, a division of the American Chemical Society.
Do CAS numbers say anything about structure?
No. They are assigned in sequence and contain no chemical information.
Is there a free CAS number lookup?
Yes. CAS Common Chemistry and PubChem are free to use.
