Each book for sale on Amazon has a “Author Best Sellers Rank” located below the publisher, which gives a general idea how well a particular book is selling.
Sales ranks are recalculated by the hour on a daily basis, and can be affected by blog features, magazine/newspaper articles, press releases, and social media, to name a few. Amazon doesn’t publish information on book sales calculations, authors speculated how its ranking system reflects on number of books sold.
The following are estimated average book sales based on rankings, courtesy of Nonfiction Authors Association:
Sales Ranking vs. Number of Books Sold Daily:
50,000 to 100k = 1 book per day
10,000 to 50,000 = 5 to 15 books per day
3,000 to 10,000 = 15 to 70 books per day
500 to 3,000 = 70 to 175 books per day
200 to 500 = 175 to 500 books per day
35 to 200 = 500 to 2,000 books per day
20 to 35 = 2,000 to 3,000 books per day
5 to 20 = 3,000 to 4,000 books per day
1 to 5 = 4,000+ books per day