A format check, not a fraud or balance check
Type in any card number and this checks two things, entirely in your browser: whether it passes the Luhn checksum, and whether its digit pattern matches a known card network (Visa, Mastercard, American Express, Discover, Diners Club, or JCB). That's the full scope of what it does - it cannot and does not tell you whether a card is real, open, active, or has any funds on it.
What the Luhn checksum actually catches
Every standard card number has a built-in check digit calculated from the Luhn algorithm, the same formula your browser's own autofill or a payment form runs before a card number is even sent to a payment processor. It exists purely to catch simple typos and transcription mistakes - a single mistyped or transposed digit will almost always fail the Luhn check, which makes it a fast first-line sanity check before worrying about anything else.
Why this matters for developers specifically
Building or testing a checkout form needs realistic-looking card numbers without ever touching a real one. This tool runs the exact same client-side validation logic a payment form would, so you can confirm your own form's validation behaves correctly. Major payment processors like Stripe also publish their own official test card numbers for exactly this purpose - this tool complements that by letting you quickly check any number's format and checksum on the spot.
Nothing ever leaves your browser
The checksum calculation and network pattern matching both run as plain JavaScript on the page you're looking at. No card number you type is ever sent anywhere, logged, or stored - which is also exactly why this tool can't look up whether a card is real or has funds; it genuinely never contacts any card network or bank.