CAS number index
- Chemicals with a CAS number
- 409.4K
- Starting digits
- 1 to 9
Browse by leading digits
Pick the first one or two digits of the CAS number. Each range lists its chemicals in CAS order.
How to read a CAS number
Every CAS registry number has three parts separated by hyphens. Water is 7732-18-5.
Take the digits before the check digit from right to left and multiply them by 1, 2, 3 and so on. Add the results. The last digit of the total must equal the check digit.
8×1 + 1×2 + 2×3 + 3×4 + 7×5 + 7×6 = 105
105 ends in 5, which matches the check digit, so 7732-18-5 is a valid CAS number.
Why buyers search by CAS number
Chemical names vary between suppliers, languages and regulations. Sodium hydroxide is also caustic soda and lye. A CAS number removes that doubt, because it points to one exact substance.
When you post a buy request on CompoundList with a CAS number, we match it to the right chemical page and send it only to suppliers who list that substance.
Older substances tend to have short numbers, such as 64-17-5 for ethanol. Newer registrations have longer first parts. Browse by name from A to Z if you do not know the number.
Read more about chemical identifiers
Guides on CAS numbers, other identifiers and classifying chemicals for trade.
- Guide · Chemical identityCAS registry numbers explained
What a CAS number identifies, how the check digit works, why one chemical can have several numbers and what a CAS number cannot tell you about a product.
- Guide · Trade and logisticsHS codes for chemicals and why they matter
How the Harmonized System classifies chemicals, the chapters that matter, the rules for chemically defined compounds and mixtures, and practical steps to find the right code.
Frequently asked questions
Short answers about CAS registry numbers and how we use them.
What is a CAS number?
A CAS registry number is a unique identifier that the Chemical Abstracts Service, a division of the American Chemical Society, assigns to a chemical substance. It has no chemical meaning of its own, but it lets buyers, suppliers and regulators refer to exactly the same substance without relying on names, which vary by language and supplier.
How do I check whether a CAS number is valid?
Take the digits before the last one, from right to left, and multiply them by 1, 2, 3 and so on. Add the results. The last digit of the total must match the final digit of the CAS number. CompoundList checks this automatically in search, product uploads and buy requests.
Can one chemical have more than one CAS number?
Yes. Salts, hydrates, stereoisomers and mixtures usually have their own CAS numbers, and some substances also carry older numbers that were later merged. On each chemical page we show the primary CAS number and list the others we know of.
How does CompoundList check CAS numbers?
We validate the check digit of every CAS number we show. Always confirm the CAS number on the supplier's label, safety data sheet or certificate of analysis before you order. CAS Registry Number is a registered trademark of the American Chemical Society.