Check an ISBN
The check digit, the conversion between both forms, the language group. One digit copied wrong shows up at once.
This application is served by KNPLIV and runs sandboxed: it cannot read your sign-in details, store anything on your device, or send in the background what you type into it. Everything is gone when you leave the page. Like everything published here, it was read over before going online.
An ISBN gets copied by hand more often than one would think: from an email into a form, from a form into a cover layout. One digit out of place sails through every field and only shows up once the book is printed.
The last digit of an ISBN is not a digit like the others: it is a check digit, worked out from the ones before it. It exists precisely to catch this kind of mistake. This page recalculates it and tells you whether the number stands up.
It also gives you:
the conversion between the ISBN-10, in use until 2007, and the ISBN-13, which is the EAN-13 barcode printed on the back;
the digit that was expected when the check fails, which is almost always enough to find the typo;
the language group recognised from the prefix.
What it does not do: say whether this ISBN was actually issued, or to which book. That would mean querying an outside database, and this page has no network access. A number that adds up is not a number that belongs to you.
Everything is worked out in your browser. Nothing is sent anywhere.