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.

If I switch the Format dropdown without changing the number, what happens?

The number stays as typed and is immediately re-validated against the new format's rules — so a 16-digit card number left in place after switching to ISBN-10 will show "Invalid," since it isn't the required length.

What if I type letters or symbols with no valid digits at all?

It's evaluated as "Invalid" rather than a separate error state — only a completely empty field shows the "Enter a number" prompt.

Can the ISBN-10 check digit 'X' appear anywhere other than the last position?

This tool doesn't restrict it — it treats any 'X' character as the value 10 wherever it appears in the 10-character input, which is more permissive than the official ISBN-10 standard, where X is only valid as the final check digit.

Do I need to remove spaces or dashes from the number first?

No — non-digit separators like spaces and dashes are stripped automatically before validating in all three formats, so you can paste a number formatted the way it's normally printed.

Check Digit Validator

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.

100% Free Runs in Your Browser No Sign-Up Required

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.

Check-digit formats supported

Check-digit formats supported
Format	Length	Method
Luhn (cards)	Varies (13-19 digits)	Double every 2nd digit from the right, sum, check mod 10
ISBN-10	10 digits	Weighted sum (10 down to 1), check mod 11
EAN-13 / UPC-A	13 digits	Alternating ×1/×3 weighted sum, check mod 10

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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