I don’t claim to know how Amazon determines the Bestsellers Rank of my books. Common theories include some algorithm that includes total number of books sold, number of books sold in the past X days, number of books sold in the past y hours, sales of other books, tag activity, page view activity, movement of the asteroid Ceres in relation to the constellation Orion, and so on. Apparently, Amazon also changes their algorithm from time to time. So, if you’re selling 20 books a day and getting into the #4000 range one season, you might sell the same amount next season but barely break #8000.
However, I do keep pretty meticulous sales data for my books and I can try to correlate the number of units I sell to the Amazon Kindle Bestsellers Rank.
Here’s a chart of my best-selling book, Dead Dwarves Don’t Dance. The top graph shows daily unit sales, and the bottom graph shows sales rank.
As you can see, there’s a pretty direct correlation between my spike in sales and my better sales rank. One day in April I sold 126 copies of the book and achieved my best sales rank of #333.
These days, I’m selling between 10 and 20 copies a day, and my sales rank fluctuates around #5000 to #9000.
You can see that around the beginning of March I was also selling around 15 copies a day, and my rank was around #5000.
Even at the current #8000 rank, Dead Dwarves Don’t Dance is showing up in the top #100 for a couple genre lists (Kindle SF High Tech and SF High Tech). Unfortunately, not in the top #20, which is the sweet spot to be in because your book shows up on the first page of the list.
I hope this helps you other writers get a feel for the relationship between daily sales and Amazon Bestsellers Rank.