Credit card validation is a critical step in the US e-commerce checkout process. Invalid card numbers cause declined transactions, frustrated customers and lost sales. The Luhn algorithm — a simple checksum formula — provides the first line of defense by catching typos and structurally invalid card numbers before they reach the payment processor. In this guide, we cover everything US e-commerce developers need to know about credit card validation.
Why Credit Card Validation Matters
In US e-commerce, checkout friction is the enemy of conversion. Every extra step, every error message, every declined transaction reduces your conversion rate. Credit card validation catches errors early — before the customer clicks "submit" and before the payment processor returns a decline.
Without validation, a customer who mistypes their card number (e.g., transposes two digits) waits several seconds for the payment processor to decline the transaction. They then have to re-enter the number and try again. With validation, the error is caught instantly, and the customer can correct it before submission. This reduces checkout time and improves the user experience.
For US businesses, credit card validation also supports PCI DSS compliance. By validating card numbers client-side (using the Luhn algorithm), you can reduce the number of invalid submissions that reach your server, reducing your PCI scope and the risk of handling invalid card data.
The Luhn Algorithm Explained
The Luhn algorithm, created by IBM scientist Hans Peter Luhn in 1954, is a simple checksum formula that validates identification numbers. It is used for credit card numbers, IMEI numbers, National Provider Identifier numbers and many other identification systems.
The algorithm works as follows:
Step 1: Starting from the rightmost digit (the check digit) and moving left, double the value of every second digit.
Step 2: If doubling results in a number greater than 9, subtract 9 (or equivalently, add the digits of the product — e.g., 16 becomes 1 + 6 = 7).
Step 3: Sum all the digits (both the doubled and undoubled digits).
Step 4: If the total modulo 10 equals 0, the number is valid. Otherwise, it is invalid.
For example, validating the number 79927398713:
Starting from the right: 3 (check digit, not doubled), 1 (doubled to 2), 7 (not doubled), 8 (doubled to 16, 16-9=7), 3 (not doubled), 9 (doubled to 18, 18-9=9), 9 (not doubled), 9 (doubled to 18, 18-9=9), 7 (not doubled).
Sum: 3 + 2 + 7 + 7 + 3 + 9 + 9 + 9 + 7 = 56. 56 mod 10 = 6, which is not 0, so this number is invalid.
The Luhn algorithm detects all single-digit errors and most transposition errors (swapping two adjacent digits). It does not detect all errors, but it catches the most common typos.
Identifying Card Types
Credit card numbers contain a Bank Identification Number (BIN) or Issuer Identification Number (IIN) in the first 6 to 8 digits. This prefix identifies the card type:
Visa: Starts with 4. Length: 13, 16 or 19 digits.
Mastercard: Starts with 51-55 or 2221-2720. Length: 16 digits.
American Express: Starts with 34 or 37. Length: 15 digits.
Discover: Starts with 6011, 622126-622925, 644-649 or 65. Length: 16-19 digits.
Diners Club: Starts with 300-305, 36 or 38. Length: 14 digits.
JCB: Starts with 3528-3589. Length: 16-19 digits.
The Credit Card Validator from Automarkly identifies the card type based on the BIN prefix and validates the number using the Luhn algorithm. This is useful for testing checkout forms and verifying card number format.
Implementing Validation
For US e-commerce sites, implement credit card validation at three levels:
Client-side validation: Run the Luhn algorithm and card type identification in the browser as the customer types. Show the card type (Visa, Mastercard, etc.) and display an error if the number fails validation. This provides instant feedback and catches typos before submission.
Server-side validation: Re-validate on the server before sending to the payment processor. Client-side validation can be bypassed, so always validate on the server too.
Processor validation: The payment processor (Stripe, PayPal, Square) performs the final validation — checking that the card is active, has available credit and is authorized for the transaction. This is the only validation that confirms the card can actually be charged.
PCI DSS Compliance for US Sites
PCI DSS (Payment Card Industry Data Security Standard) is a set of security standards designed to ensure that all companies that process, store or transmit credit card data maintain a secure environment. The standard applies to any US business that accepts credit card payments, regardless of size.
Key PCI DSS requirements for US e-commerce sites:
Never store full credit card numbers. If you must store card data, store only the last 4 digits and a token from your payment processor.
Use a PCI-compliant payment processor. Processors like Stripe, PayPal and Square handle card data on their PCI-compliant servers, reducing your compliance scope.
Use HTTPS everywhere. All pages that handle card data must use TLS encryption.
Don't handle card data yourself. Use the payment processor's hosted checkout or embedded card fields (like Stripe Elements) so card data never touches your servers.
Validation Best Practices
Validate as the user types. Show the card type as soon as the BIN is entered (first 4-6 digits). Show a checkmark or error after the full number is entered.
Format the input. Add spaces every 4 digits as the user types (e.g., "4111 1111 1111 1111"). This makes the number easier to read and verify.
Never auto-submit on validation. Let the customer review and click submit themselves. Auto-submission can be disorienting and prevents the customer from double- checking the number.
Use clear error messages. "Invalid card number — please check and try again" is better than a generic "Error" or "Validation failed."
Never store card data in localStorage or sessionStorage. These browser storage mechanisms are accessible to JavaScript and are not secure. Use only PCI-compliant storage (tokenization through your payment processor).
Using a Free Credit Card Validator
The Credit Card Validator from Automarkly validates card numbers using the Luhn algorithm and identifies the card type based on the BIN prefix. The validation runs entirely in your browser — no card number is sent to a server, which is critical for security.
For testing, use test card numbers provided by your payment processor (e.g., Stripe's test card numbers: 4242 4242 4242 4242 for Visa, 5555 5555 5555 4444 for Mastercard). Never use real card numbers for testing.
For US e-commerce developers, credit card validation is a fundamental checkout feature. Use the Credit Card Validator to test your validation logic, implement client-side and server-side validation, and always use a PCI-compliant payment processor for handling actual card data.