What's the difference between ISBN-10 and EAN-13 checks?

Different standards with different weighted-sum formulas — ISBN-10 is the older book-specific format, while EAN-13 is a broader retail barcode standard.

Why does Luhn double every second digit?

That pattern was designed to catch common data-entry errors, like single-digit mistakes and adjacent digit transpositions.

Who invented the Luhn algorithm?

IBM scientist Hans Peter Luhn, who filed the patent in 1954; it was granted in 1960 and has since entered the public domain, becoming the basis for validating most payment card numbers today.

Why does the test number 4111 1111 1111 1111 pass?

It's a well-known publicly documented test number that satisfies the Luhn checksum by design, intended for testing payment integrations — it isn't tied to a real account.

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Developer

Check Digit Validator

Validate card numbers, ISBNs, and barcodes (Luhn, ISBN-10, EAN-13).

🔒 Runs entirely in your browser — nothing here is ever uploaded

About the Check Digit Validator

Validates the check digit on credit/debit card numbers (Luhn algorithm), ISBN-10 book codes, and EAN-13/UPC-A barcodes — the built-in error-detection digit each format includes.

How to use it

Choose a Format: Luhn (card), ISBN-10, or EAN-13/UPC-A.
Enter the Number.
Read whether the check digit is valid in the readout.

Formula

Luhn: double every second digit from the right, sum all digits (subtracting 9 from any result over 9), and check the total is divisible by 10. ISBN-10 and EAN-13 use their own weighted-sum formulas.

The Luhn algorithm was devised by IBM scientist Hans Peter Luhn (U.S. Patent 2,950,048, filed 1954, granted 1960); it's since entered the public domain and is specified for payment cards in ISO/IEC 7812-1.

Worked example

The test card number 4111 1111 1111 1111 passes the Luhn check, confirming it's a structurally valid card number format.

Recommendations

• A passing check digit only confirms structural validity — it says nothing about whether the card, ISBN, or barcode is actually issued or real.
• The Luhn algorithm covers most major card networks, so this one check applies broadly.
• This tool treats UPC-A as a 12-digit EAN-13 with a leading zero, the standard relationship between the two formats.

Frequently asked questions

No — it only confirms the number follows the correct mathematical structure. Real numbers are also validated against the issuer's actual records, which this tool has no access to.

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Sources

Wikipedia — Luhn algorithm (patent history and ISO/IEC 7812 usage) — accessed 2026-08-20