How to validate a card number with the Luhn check

1. Enter the card number

   Paste the digits into the "Card number" field. Spaces and dashes are stripped, so `4111 1111 1111 1111` and `4111-1111-1111-1111` work the same as the bare digit string.

2. Press Run

   Result returns `digitsLen` (digit count after stripping) and `luhnValid` (boolean). The Luhn pass means the checksum is internally consistent — NOT that the card exists at any bank.

3. What it doesn't tell you

   Luhn doesn't verify the issuer, the cardholder, expiry, CVV, or available balance. Many test numbers (4242…, 5555…) pass Luhn but are sandbox-only. Don't use this for fraud screening.

4. Quick examples that should pass

   Visa test: `4111111111111111` (16 digits). Mastercard test: `5555555555554444`. Amex test: `378282246310005` (15 digits). All three are well-known sandbox numbers.