Canadian · Validation
CAS Registry Number validator.
Check that a CAS RN is well-formed and that its check digit adds up, with every step of the sum shown.
The rule: a CAS RN is up to 10 digits in three hyphen-separated groups: 2 to 7 digits, then 2 digits, then 1 check digit. Multiply each digit before the check digit by its position counted from the right (the digit next to the check digit is 1), add them up, and the last digit of the total must equal the check digit.
Inputs
Result
Enter a CAS Registry Number and press Check.
How the CAS check digit works
A CAS Registry Number (CAS RN) is the identifier CAS assigns to a chemical substance. The last digit is a check digit, so most typing mistakes and transposed digits are caught by arithmetic alone.
- Split the number. Drop the hyphens and set the last digit aside as the check digit. For
7732-18-5that leaves773218and a check digit of 5. - Weight from the right. The digit next to the check digit gets weight 1, the next 2, and so on: 8×1, 1×2, 2×3, 3×4, 7×5, 7×6.
- Add. 8 + 2 + 6 + 12 + 35 + 42 = 105.
- Take the last digit. 105 mod 10 = 5, which matches the check digit, so the number passes.
FAQ
Does a pass mean the substance exists? No. It only means the digits are internally consistent. Many invalid numbers also pass by chance (about 1 in 10 random wrong check digits is accepted), and CAS decides which numbers are actually assigned.
How long can a CAS RN be? Up to 10 digits: 2 to 7 in the first group, 2 in the second, 1 check digit. The first group is not fixed-width, so a leading zero is not added.
Do I need the hyphens? Yes here. They mark where the groups end, which is how the format is defined.
Source and date: the format and check-digit calculation are from the CAS documentation Check digit verification of CAS Registry numbers, read 30 September 2026. The worked example is our own arithmetic on that rule.
Format and checksum only — not a CAS lookup
This tool checks the number's shape and check digit entirely in your browser. It does not contact CAS, so a pass does not confirm the number was issued or what substance it names. Nothing you type is transmitted, stored, or logged.