Question 1

Is it safe to paste card numbers here?

Accepted Answer

Yes. This tool runs entirely in your browser using JavaScript. No card data is uploaded to any server, stored, or transmitted anywhere. For real security, never paste real card numbers into any online tool — use only test numbers.

Question 2

What is the Luhn algorithm?

Accepted Answer

The Luhn algorithm is a checksum formula that detects single-digit errors and most transpositions in identification numbers including credit cards. Valid Luhn checksum does NOT mean a card is active or has funds — only that the number is structurally valid.

Question 3

How are card brands detected?

Accepted Answer

Brand is detected from the Bank Identification Number (BIN) — the first digits: Visa starts with 4, Mastercard with 51-55 or 2221-2720, Amex with 34 or 37, Discover with 6011/65/644-649, JCB with 3528-3589, etc.

Question 4

What test card numbers can I use?

Accepted Answer

Standard test numbers include Visa 4111 1111 1111 1111, Mastercard 5555 5555 5555 4444, Amex 3782 822463 10005. These pass Luhn validation but are not real accounts — safe for testing payment integrations.