LUHN Validator

What is the Luhn Algorithm?

The Luhn algorithm (also known as the "modulus 10" or "mod 10" algorithm) is a checksum formula used to validate identification numbers such as credit card numbers (PANs), IMEI numbers, Canadian Social Insurance Numbers, and US National Provider Identifiers. Invented by Hans Peter Luhn in 1954 at IBM and patented in 1960, it is in the public domain and is described by ISO/IEC 7812-1, the international standard that governs payment card numbering. The Luhn check is designed to catch the most common human entry errors: any single-digit substitution (caught 100% of the time) and any transposition of two adjacent digits (caught in all cases except 09↔90). It is not a cryptographic hash and provides zero protection against deliberate tampering — it is purely an error-detection mechanism used as the last line of defense before a card number is sent to a payment processor. Every PAN issued by Visa, Mastercard, American Express, Discover, JCB, and UnionPay carries a Luhn-valid check digit in its rightmost position, making this calculator a daily tool for anyone developing or testing payment integrations.

How to Use This Tool

1. Enter a card number or any numeric string to validate it against the Luhn algorithm.
2. The tool shows whether the number is valid and displays the full step-by-step calculation.
3. Use the generator to create valid test card numbers with a specific length and prefix (BIN).
4. Inspect the breakdown to see exactly which digits were doubled and summed.
5. Copy the validated PAN directly into your test fixtures or HSM command parameters.

Common Use Cases

• Validating credit and debit card numbers during payment application development before submitting to processors.
• Generating test card numbers (PANs) for QA, integration testing, and unit-test fixtures.
• Verifying BIN (Bank Identification Number) ranges produce mathematically valid PANs.
• Building synthetic-data generators for fraud-detection model training without using real card data.
• Quick pre-validation in client-side payment forms to catch typos before reaching the server.
• Validating IMEI numbers for mobile device verification flows.
• Auditing legacy systems that may have skipped Luhn validation and store malformed PANs.
• Educational purposes — teaching how card-number error detection works without complex cryptography.

Worked Example: Luhn Check of 4111 1111 1111 1111

Starting from the rightmost digit, every second digit moving left is doubled (subtract 9 when the double exceeds 9). Sum everything; a multiple of 10 means the number is valid:

PAN:      4  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1
Doubled:  ×2    ×2    ×2    ×2    ×2    ×2    ×2    ×2
Value:    8  1  2  1  2  1  2  1  2  1  2  1  2  1  2  1

Sum = 8+1+2+1+2+1+2+1+2+1+2+1+2+1+2+1 = 30
30 mod 10 = 0  →  VALID

Standard Test Card Numbers (Luhn-Valid)

The industry-standard test PANs published in every processor's documentation. All pass the Luhn check, none can be charged — they exist exactly so you never test with a real card:

Frequently Asked Questions