How this calculator works
Starting from the right of a complete number, leave the check digit alone and double every second payload digit. Subtract 9 from any doubled value above 9, then sum the contributions. For generation, parity is offset because the new check digit will be appended. The check is arithmetic only and makes no claim about an account being issued or active.
Formula
Valid complete number: sum(contributions) ≡ 0 mod 10. Appended digit = (10 - payload sum mod 10) mod 10.
Worked example
The payload 7992739871 has a transformed sum of 67. Appending 3 makes the total 70, so 79927398713 passes. The payload 0001 needs digit 8, giving 00018.
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Frequently asked questions
Are leading zeros preserved?
Yes. The input is a digit string and the complete result keeps every leading zero.
Does Valid mean an account exists?
No. It only means the number passes this checksum. No account service is contacted.
Which mistakes are detected?
Single-digit substitutions are detected. Some transpositions, including 09 versus 90, can pass unchanged, so this is not a guarantee against all typing errors.