What a card validity checker does
A card validity checker tests the structure of a payment card number. It does not contact an issuer. It does not confirm funds, account status, or credit line. It returns a pass or fail on format.
The Luhn check
Most checkers run the Luhn algorithm, also called the mod 10 check. ISO/IEC 7812 defines the last digit of a card number as a check digit. The tool doubles every second digit from the right, subtracts 9 from any result above 9, sums the digits, and tests whether the total divides by 10. A random 16-digit number passes about 1 time in 10. A Luhn pass says nothing about the account.
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BIN and IIN lookup
The first six to eight digits form the issuer identification number. A lookup maps that range to a brand, a card type such as credit, debit, or prepaid, and an issuing country. Brands publish range files. Issuers add and retire ranges, so any lookup file goes stale. Length rules differ by brand. Diners Club US and Canada issues 16-digit numbers. Most other Diners Club cards use 14 digits.
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Expiry and security code format
Checkers test the month field from 01 to 12, the year field, and the length of the security code. Visa, Mastercard, and Discover use 3-digit codes on the back. American Express uses a 4-digit code on the front. Format rules do not prove the code matches the account.
What authorization confirms
The issuer is the sole party that can approve a transaction. Authorization checks account status, available balance or credit line, the address verification result, and the card verification value. A format checker never touches those systems. A service that claims to verify a live card is running an authorization or an account lookup. Doing that without the cardholder's consent is a federal crime in the US under 18 U.S.C. 1029.
Common results and what they mean
- Valid: the digits and length match a published pattern. Nothing more.
- Invalid: the number fails Luhn, has the wrong length, or carries an expired date field.
- Unknown BIN: the range is absent from the file the tool uses.
Legal and compliance notes
PCI DSS bars storage of the CVV and full track data after authorization. Card data taken in a breach or sold in bulk is stolen property. Use of it carries prison terms and fines. Merchants verify cards through their payment processor, not through a third-party format tool.