About This Tool
Validate any credit card number using the Luhn algorithm and automatically detect the card network. Enter a number to see whether it passes the Luhn checksum, which network it belongs to (Visa, Mastercard, American Express, Discover, Diners Club, or JCB), and whether the digit count matches the expected length for that network. The number is auto-formatted with spaces for readability as you type. This tool is designed for developers testing payment integrations, QA teams verifying form validation logic, and students learning about the Luhn algorithm. Do not enter real card numbers. All processing happens on your device with no data transmitted or stored. Use the standard test numbers provided by payment processors (like 4111 1111 1111 1111 for Visa) to verify your checkout flows and error handling.
How the Luhn Algorithm Works
The Luhn algorithm (also called the modulus 10 algorithm) is a checksum formula used to validate identification numbers, including credit card numbers. It was created by IBM scientist Hans Peter Luhn in 1954 and is now a standard in the payment industry.
The algorithm works by processing the card number from right to left. Starting from the second-to-last digit and moving left, every other digit is doubled. If doubling produces a number greater than 9, the two resulting digits are added together (or equivalently, 9 is subtracted). All digits are then summed. If the total is divisible by 10, the number passes the Luhn check.
This algorithm catches most single-digit errors and nearly all transpositions of adjacent digits. It is not a security feature and does not prove that a card is active or funded. It simply verifies that the number follows the correct mathematical structure.
Card Network Detection
Each card network uses a distinct range of leading digits called the Issuer Identification Number (IIN) or Bank Identification Number (BIN). The first 1 to 6 digits of a card number identify the network and issuing bank.
- Visa: Starts with 4, typically 16 digits (some older cards use 13)
- Mastercard: Starts with 51-55 or 2221-2720, always 16 digits
- American Express: Starts with 34 or 37, always 15 digits
- Discover: Starts with 6011, 622126-622925, 644-649, or 65, always 16 digits
- Diners Club: Starts with 300-305, 36, or 38, typically 14 digits
- JCB: Starts with 3528-3589, 15 or 16 digits
Test Card Numbers for Development
Payment processors provide standard test card numbers for integration testing. These numbers pass the Luhn algorithm but are not connected to real accounts. Common test numbers include:
- Visa: 4111 1111 1111 1111 or 4012 8888 8888 1881
- Mastercard: 5500 0000 0000 0004 or 5105 1051 0510 5100
- American Express: 3782 822463 10005 or 3714 496353 98431
- Discover: 6011 1111 1111 1117 or 6011 0009 9013 9424
Use these numbers to test checkout forms, validation logic, error handling, and payment gateway responses without processing real transactions. Never use real card numbers in test environments.
Luhn Algorithm Limitations
The Luhn algorithm was designed for error detection, not security. It catches about 98% of single-digit substitution errors and all adjacent transposition errors (like swapping 12 to 21). However, it cannot detect all possible errors, such as swapping 09 with 90 in certain positions or more complex permutation errors.
A number that passes the Luhn check may still be invalid because the issuing bank never assigned it, the account was closed, or the number was fabricated by generating a valid checksum. Payment authorization requires contacting the card issuer through the payment network, which this tool does not do. The Luhn check is a quick first-pass filter that eliminates obviously incorrect numbers before making a network request.
Privacy and Security
This tool is built for testing and education. All validation happens on your device. No card numbers are transmitted over any network, stored in any database, or logged anywhere. There are no analytics tracking, no cookies, and no third-party scripts processing your input.
For production applications, never validate card numbers on the front end alone. Always tokenize card data using a PCI-compliant payment processor. Luhn validation on the input form is useful for improving user experience by catching typos before form submission, but all actual card verification must happen through secure, authenticated channels with the card network.