AO3 Chapter Wordcount Userscript
Adds the wordcount for the current chapter next to the work wordcount in the metadata box at the top of a work page. The word counting function is naive and just counts spaces, so it's not 100% reliable. Comments/bugs to firstname.lastname@example.org.
Not sure what a userscript is or how to install it in your browser? Check out the OpenUserJS Userscript Beginners How-To.