Every payment card number carries a check digit calculated with the Luhn algorithm, which catches most single-digit typos and transpositions before a request ever reaches a payment processor. This tool runs that check and identifies the issuing network from the number prefix.
No. It only means the digits are internally consistent. Whether the account exists, has funds, or is active can only be answered by the issuing bank.
Starting from the right, double every second digit, subtract 9 from any result above 9, sum everything including the untouched digits, and check that the total divides by 10.
The check runs entirely in your browser with no network request. Even so, use the published test numbers rather than a live card — good habit, and it is what the tool is for.
From the leading digits: 4 for Visa, 51–55 and 2221–2720 for Mastercard, 34 and 37 for American Express, 6011 and 65 for Discover.
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