Annotag Calculator

Make your tweetable book location annotags with this calculator. Read the introductory blog post here for an explanation of annotags. If you choose "ISBN" as your bookcode format below, this calculator will turn your unwieldy 10- or 13-digit ISBN into a shortened, more easily typable form, more suitable for squeezing into a tweet. For example, the ISBN 0743482786 will become the five-character code FB7Xw.

Bookcode Decoder

Enter the bookcode below to generate the decoded ISBN. Please note that the bookcode is the part that comes after the i, such that for an annotag like #iFB7Xw:p12, the bookcode is FB7Xw.

Technical Details

This calculator validates your ISBN by doing some modular arithmatic and looking at the checkdigit. It then removes the checkdigit, as well as any leading zeros, and encodes the remaining digits with a base-62 encoding scheme, mapping the numbers onto numbers and letters. ISBN-13s that begin in 978 are automatically converted to ISBN-10s. You can read the code for this page and all the other pages on this site here on github.


  • 2014-10-12: Version 1.1. Now supports ISBN-13s, and decodes bookcodes into both ISBN-10s and ISBN-13s.