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Luhn Algorithm Test: How to Validate a Number

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Luhn Algorithm Test: How to Validate a Number

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Luhn Algorithm Test: How to Validate a Number

What is the Luhn algorithm test?

The Luhn algorithm test is a checksum check that verifies whether a string of digits, such as a payment card number, has the correct structure. It catches any single-digit error and most swaps of adjacent digits. A number that passes is not proven valid; it is only proven consistent with the Luhn formula.

How do you run a Luhn test by hand?

Work from the rightmost digit, which is the check digit. Double every second digit moving left, subtract 9 from any result above 9, then add all digits together. If the total ends in 0, the number passes.

  1. Write the digits in order.
  2. Starting with the second digit from the right, double it. Skip the next digit, double the next, and continue.
  3. If a doubled value is 10 or more, subtract 9.
  4. Add the check digit and all modified digits.
  5. Divide the sum by 10. A remainder of 0 means the number passes.

What is a worked Luhn test example?

Use 79927398713, a standard published example. The digits from the right are 3, 1, 7, 8, 9, 3, 7, 2, 9, 9, 7. Double the second, fourth, sixth, eighth, and tenth digits to get 2, 16, 6, 4, 18, then reduce 16 to 7 and 18 to 9. The sum is 3 + 2 + 7 + 7 + 9 + 6 + 7 + 4 + 9 + 9 + 7 = 70. Because 70 ends in 0, the number passes.

Change the leading 7 to 8 and the sum becomes 71. The number fails because 71 does not end in 0.

What does the Luhn test not tell you?

The Luhn test does not confirm that an account exists, that funds are available, or that a card is active. It only confirms that the digits satisfy a modulus 10 checksum. A random number can pass the test, so a passing result is never proof of validity.

Where is the Luhn test used?

Card networks, banks, and payment processors use the Luhn check as a first filter before authorization. It appears in ISO/IEC 7812 for identification cards and in many validation libraries. The test is also used for IMEI numbers, Canadian Social Insurance Numbers, and some account identifiers.

How can you test a number without errors?

Use a script or a trusted validation tool rather than mental math for long numbers. The algorithm is small enough to implement in a few lines in any language. Always test with known valid and known invalid samples to confirm the implementation.

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