CAS Number Checker
Enter a CAS Registry Number to check whether it's correctly formatted and passes the official check-digit validation.
How this works: every CAS Registry Number ends in a check digit computed from all the digits before it (read right to left, weighted 1, 2, 3, ...; the check digit is that sum mod 10). This tool only verifies that math — it confirms a number is structurally valid, not that it's the correct CAS for any particular substance. A number can pass this check and still be the wrong CAS for what you think it is, or belong to a different form (salt vs. free base, for example) of the same compound.