Verhoeff Checksum Validator

How it is calculated

The Verhoeff scheme appends one check digit to a number so that typing errors can be caught. The tool first removes every character that is not a digit (spaces and hyphens are ignored) and needs at least two digits. It then reads the digits from right to left. For each position i it permutes the digit using row i mod 8 of the permutation table, and combines the result with the running value c through the dihedral-group multiplication table: c = d[c][p[i mod 8][digit]]. If the final c is 0, the number is Valid. To show the correct check digit, the tool does the same with the last digit replaced by 0 and takes the inverse table entry of the result. The scheme catches every single wrong digit and every swap of two neighbouring digits. It only tells you if a number is consistent, not whether it belongs to a real person or account.

Common mistakes

• Confusing Verhoeff with the Luhn algorithm used on payment cards. The two use different rules, so a number valid in one is usually not valid in the other.
• Including the check digit in the part you want to compute it for. The displayed check digit is based on all digits except the last one, so enter the number without a check digit followed by any placeholder digit, or read the shown digit as the correct last digit.
• Reading Valid as proof that an ID is genuine. It means only that the digits are internally consistent; a made-up number can pass.
• Entering a single digit. At least two digits are needed, otherwise the tool gives no result.
• Expecting letters or other symbols to count. Everything that is not a digit is silently dropped, so a typo with a letter can shift the digits.