Luhn Checksum Calculator
The Luhn algorithm, also known as the modulus 10 or mod 10 algorithm, is a checksum used to validate identification numbers including payment card numbers, IMEI device serials, and some national identification numbers. It is defined in the international standard ISO/IEC 7812. This calculator tells you whether a number you enter passes the Luhn check, shows the checksum total, and computes the single check digit you would append to a number that does not yet have one so that it becomes valid. Enter digits only; spaces and hyphens are stripped automatically.
Luhn algorithm steps
1. From the rightmost digit, moving left, double every second digit.
2. If a doubled value is greater than 9, subtract 9.
3. Sum every digit (doubled-and-reduced or untouched).
4. Valid if sum mod 10 == 0.
5. Check digit = (10 - (sum-without-check mod 10)) mod 10.
The check digit makes the whole number sum to a multiple of 10. When you append it, the appended digit occupies the rightmost position, so the parity of which digits get doubled is handled accordingly.
Worked example
Take 7992739871. Doubling every second digit from the right and reducing values over 9 gives a digit sum. To find the check digit for this 10-digit base, the algorithm yields 3, producing the valid 11-digit number 79927398713. Entering 79927398713 in the calculator above returns "Yes" for the Luhn check, confirming the appended digit is correct.
Luhn checksum: frequently asked questions
What is the Luhn algorithm?
The Luhn algorithm, also called the modulus 10 or mod 10 algorithm, is a simple checksum formula used to validate identification numbers such as payment card numbers and IMEI numbers. It is defined in ISO/IEC 7812. It detects any single-digit error and most transpositions of adjacent digits.
How does the Luhn check digit work?
Starting from the rightmost digit and moving left, every second digit is doubled. If doubling produces a number greater than 9, you subtract 9 (equivalent to adding the two digits). Sum all the resulting digits. If the total is a multiple of 10, the number is valid. The check digit is the value added to the end that makes the sum a multiple of 10.
Does a valid Luhn number mean a card is real?
No. The Luhn check only confirms the number is internally consistent, catching typos. It does not confirm the account exists, is funded, or is authorised. It is a first-pass format check, not a verification of validity or ownership.
What inputs does this calculator accept?
Enter digits only; spaces and hyphens are ignored. The tool reports whether the number you entered passes the Luhn check, the computed checksum total, and the single check digit you would append to a number that does not yet have one to make it valid.
Sources and method
- The Luhn algorithm is standardised in ISO/IEC 7812 (Identification cards: identification of issuers), which defines the check digit for issuer identification numbers.
- The algorithm is a fixed, deterministic procedure; this tool implements it exactly and computes every result, it does not estimate.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.