Calculators
Luhn Checksum Calculator
Validate a Luhn checksum or calculate a check digit to append to a digit string, privately in your browser.
Validate the entire input, or treat that same input as a payload and append the calculated check digit. These are separate results: to generate a check digit, enter the payload without an existing check digit. Leading zeros are preserved. Use sample identifiers; calculations stay in your browser. A valid checksum does not prove an identifier exists or is authorized.
How the Luhn checksum works
The Luhn algorithm doubles every second digit from the right, starting with the second-to-last digit of a complete identifier. Subtract 9 from doubled values greater than 9, then sum all transformed digits. A complete identifier passes when the sum is divisible by 10. For 79927398713, the validation sum is 70, so it passes. To generate a check digit, enter only the payload: 7992739871 produces check digit 3 and completed payload 79927398713. The generator doubles the rightmost payload digit because the appended check digit will occupy the final position. Both outputs use the same input: validation treats it as complete, while generation treats it as a payload. Enter 1 to 1000 decimal digits; leading zeros are preserved and surrounding whitespace is accepted. Internal spaces, hyphens, decimals and signs are rejected. Validation requires at least two digits; one digit can still be used as a payload. All-zero strings can pass mathematically. Luhn detects every single-digit substitution and most adjacent transpositions, but does not detect swapping 09 with 90. It is an error-detection checksum, not encryption or proof of account existence, ownership or authorization. No issuer-specific length or prefix rules are checked. All processing stays in your browser; there is no need to enter real account details.
Frequently Asked Questions
Everything you need to know about this tool, how it works, and privacy.
How do I generate a Luhn check digit?
Enter the payload without a check digit. For 7992739871, the calculator returns 3 to append, giving 79927398713.
Does passing Luhn prove an account is valid?
No. It only verifies a mathematical checksum. It does not check account existence, issuer rules, ownership or authorization.
Are leading zeros kept?
Yes. Input is treated as a digit string, so leading zeros are preserved. Up to 1000 digits are accepted.
Which errors can Luhn miss?
Luhn detects all single-digit substitutions and most adjacent transpositions, but it misses swapping 09 and 90. Some other multiple-digit errors also pass.
Why are validation and generation different results?
Validation treats the entire input as complete, including its check digit. Generation treats the entire input as a payload and calculates one more digit to append. A one-digit input is allowed as a payload but is not validated as a complete identifier.