Card number checker
A number passing the checksum only means no digit was mistyped. It says nothing about whether the card exists, has funds, or belongs to whoever gave it to you — only the issuer knows that.
Runs in your browser · nothing is sent anywhere
Checks a card number against the Luhn checksum, identifies the brand from its prefix and confirms the length is right for that brand. It runs in your browser and the number is never sent anywhere.
How to use the card number checker
The Luhn algorithm exists to catch typing mistakes, not fraud. It was designed in the 1950s to detect the two errors humans actually make: a single wrong digit, and two adjacent digits swapped. It catches every single-digit error and almost every transposition — the exception being 09 swapped to 90, which it misses. That is the entire job, and it does it with arithmetic simple enough to have been done by a mechanical device.
What it emphatically does not do is tell you a card is real. Any number ending in the right check digit passes, and roughly one number in ten does by chance. Passing the checksum means "nobody mistyped this", not "this card exists" — only the issuer can answer the second question, and only through an authorisation request.
The brand comes from the leading digits, the Issuer Identification Number. Length matters too and varies by brand: American Express is 15 digits, Diners Club 14, most others 16. A 16-digit number starting 34 is not a valid Amex card regardless of its checksum, which is why the length is checked against the brand rather than just against the 12–19 range the standard allows.
One practical note: nothing you paste here leaves your browser. That is worth caring about, because a validator that posts the number to a server is a card number in someone else’s logs.
Questions
No. It means no digit was mistyped. Roughly one random number in ten passes by chance. Only the issuer knows if a card exists.