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

ISBN-10 was the original 10-digit standard, used until 2007. ISBN-13 replaced it to align with the EAN/UPC barcode system used in retail — it's simply the ISBN-10's 9 identifying digits prefixed with "978" (or occasionally "979" for newer titles) and given a new EAN-style check digit. Books published after 2007 only have an ISBN-13.

Why can't every ISBN-13 convert back to ISBN-10?

Only ISBN-13s starting with the "978" Bookland prefix map to a valid ISBN-10 — that prefix is what ISBN-10 numbers were given when converted to the newer standard. ISBNs starting with "979" were issued after the ISBN-10 standard was retired and have no ISBN-10 equivalent at all.

How is the check digit actually calculated?

ISBN-10 uses a weighted sum where each of the first 9 digits is multiplied by its position from 10 down to 2, summed, and the check digit is whatever makes the total divisible by 11 (the letter X represents 10). ISBN-13 uses the simpler EAN-13 scheme: alternating weights of 1 and 3 across the first 12 digits, with the check digit making the total divisible by 10.

Why does my ISBN fail even though it looks right?

The most common cause is a single mistyped or transposed digit — the check digit exists specifically to catch that, since even one wrong digit almost always produces a checksum mismatch. Double-check the number against the book's back cover or copyright page.

ISBN VALIDATOR & CONVERTER

Validate an ISBN-10 or ISBN-13 check digit and convert between both formats instantly.

ISBN-10 or ISBN-13

Valid ISBN-13

Cleaned	9780134685991
ISBN-10	0134685997

How to validate or convert an ISBN

1
Type or paste an ISBN, with or without hyphens — either the 10-digit or 13-digit form.
2
The checker computes the correct check digit for that length and compares it against the one you entered.
3
If valid, see the equivalent ISBN in the other format, converted automatically.

Questions

What's the difference between ISBN-10 and ISBN-13?: ISBN-10 was the original 10-digit standard, used until 2007. ISBN-13 replaced it to align with the EAN/UPC barcode system used in retail — it's simply the ISBN-10's 9 identifying digits prefixed with "978" (or occasionally "979" for newer titles) and given a new EAN-style check digit. Books published after 2007 only have an ISBN-13.
Why can't every ISBN-13 convert back to ISBN-10?: Only ISBN-13s starting with the "978" Bookland prefix map to a valid ISBN-10 — that prefix is what ISBN-10 numbers were given when converted to the newer standard. ISBNs starting with "979" were issued after the ISBN-10 standard was retired and have no ISBN-10 equivalent at all.
How is the check digit actually calculated?: ISBN-10 uses a weighted sum where each of the first 9 digits is multiplied by its position from 10 down to 2, summed, and the check digit is whatever makes the total divisible by 11 (the letter X represents 10). ISBN-13 uses the simpler EAN-13 scheme: alternating weights of 1 and 3 across the first 12 digits, with the check digit making the total divisible by 10.
Why does my ISBN fail even though it looks right?: The most common cause is a single mistyped or transposed digit — the check digit exists specifically to catch that, since even one wrong digit almost always produces a checksum mismatch. Double-check the number against the book's back cover or copyright page.