ISBN Checker
Validate an ISBN-10 or ISBN-13 check digit
An ISBN is not a random string. Its final character is a check digit calculated from the others, which means a single mistyped number makes the whole ISBN invalid. This tool runs that checksum so you can confirm an ISBN before it goes on a cover or into a listing.
What is the ISBN check digit?
The last digit of an ISBN is a checksum derived from the preceding digits using a fixed formula. ISBN-13 weights digits alternately by 1 and 3; ISBN-10 weights them from 10 down to 2 and can end in an X, which stands for the value 10. If the calculated check digit does not match, the ISBN is wrong.
Do I need an ISBN to self-publish?
Not always. Amazon KDP gives ebooks a free ASIN and can assign a free ISBN to paperbacks. But a free KDP ISBN lists Amazon as the publisher and is tied to KDP, so authors who want to distribute widely or appear as their own publisher usually buy their own.
Do I need a separate ISBN for each format?
Yes. Each edition and format needs its own ISBN: the paperback, the hardcover and the ebook are three separate ISBNs. The same book in a new edition with significant changes also needs a new one.
Frequently asked questions
How do I know if an ISBN is valid?
Recalculate its check digit from the other digits. If it matches the final character, the ISBN is valid. This tool does that for both ISBN-10 and ISBN-13.
Do I need to buy an ISBN?
Not to publish on Amazon, which offers a free one. You need your own if you want to be the publisher of record or distribute beyond KDP on your own terms.
Can two books share one ISBN?
No. Every edition and format is a distinct product and needs its own unique ISBN.
What does the X at the end of an ISBN mean?
In ISBN-10 the check digit can be 10, which is written as the single character X.