Luhn Algorithm
Validate credit card, IMEI, and identification numbers using the Luhn checksum algorithm. Free online Luhn checker with detailed results and check digit.
About This Calculator
The Luhn Algorithm Calculator validates credit card numbers, IMEI numbers, gift card numbers, and other identification numbers using the Luhn checksum formula (also known as mod 10 algorithm). This free online tool checks whether a given number passes Luhn validation by computing the check digit and comparing it with the actual last digit. It is commonly used by developers, e-commerce platforms, payment systems, and financial institutions for pre-validation of card numbers before processing transactions.
The algorithm works by processing each digit of the number from right to left, excluding the check digit (the last digit). Every second digit is doubled, and if the result exceeds 9, 9 is subtracted. All resulting digits are summed together, and the check digit is calculated as (10 - sum % 10) % 10. If the calculated check digit matches the actual last digit of the input number, the number is considered structurally valid. This method can detect single-digit errors and most adjacent digit swaps, making it an effective first line of validation.
Regional Notes
The Luhn algorithm is a global standard used universally across all regions. Credit card issuers worldwide including Visa, Mastercard, American Express, and Discover all use the Luhn algorithm. IMEI numbers assigned by the GSMA also use Luhn validation. Since the algorithm itself is identical across all countries, no region-specific variations exist.
Frequently Asked Questions
What is the Luhn algorithm?
The Luhn algorithm, also known as the mod 10 algorithm, is a simple checksum formula used to validate various identification numbers such as credit card numbers, IMEI numbers, and gift card numbers. It was created by German computer scientist Hans Peter Luhn.
How does the Luhn algorithm work?
Starting from the rightmost digit of the number (excluding the check digit), every second digit is doubled. If doubling results in a value greater than 9, subtract 9. Then sum all the resulting digits. The check digit is the number that makes the total sum a multiple of 10. If the calculated check digit matches the actual last digit, the number is valid.
What types of numbers use the Luhn algorithm?
The Luhn algorithm is used for credit card numbers (Visa, Mastercard, American Express, Discover), IMEI numbers for mobile devices, gift card numbers, social insurance numbers in Canada, and many other identification number systems worldwide.
Can the Luhn algorithm detect all errors?
No, the Luhn algorithm can detect single-digit errors and most adjacent digit transpositions, but it cannot detect double errors such as 22 swapped with 55, 33 with 66, or 44 with 77. It is designed as a quick pre-validation check, not a cryptographic security measure.
Does Luhn validation guarantee a real credit card?
No, passing the Luhn check only means the number is structurally valid. It does not guarantee that the number corresponds to an actual issued credit card or active account. Real card issuers perform additional validation beyond the Luhn checksum.
How do I generate a number that passes Luhn validation?
To generate a Luhn-valid number, start with your base number (without the check digit), double every second digit from the right, subtract 9 from any results over 9, sum all digits, and set the check digit to (10 - sum % 10) % 10. The resulting full number will pass Luhn validation.
Is this Luhn algorithm calculator free to use?
Yes, this Luhn algorithm calculator is completely free to use. There are no hidden charges, no account required, and no usage limits. You can validate as many numbers as you need.