Understanding CAS Number Check Digits
Published 2025-03-02
How the last digit of a CAS Number validates the rest of the identifier.
The final digit of a CAS Registry Number is a check digit. It is calculated by multiplying each digit of the remaining number by its position from the right, summing the products and taking the remainder modulo 10. Validating the check digit before a database lookup removes a large share of typographical errors and prevents fruitless searches.