Amazon ranks each book based on how often it sells relative to every other book in its catalog of some 3.5 million titles. The best-selling book is ranked 1; the slowest seller exceeds 3,500,000. Books for which Amazon hasn't recorded a sale are ranked "None."
A book's Amazon Sales Rank appears in the # Product Details# section of its detail page on Amazon. Sales ranks are recalculated hourly, and can change significantly day to day.
Since Amazon has an estimated 70 percent market share among Internet book retailers, its sales rankings are the best free, publicly available information about the relative sales performance of individual titles. The rankings include new and used books sold by third-party sellers on Amazon's Marketplace platform.
Amazon doesn't publicly discuss its sales figures for individual titles, so it's impossible to correlate the rankings with quantity of sales. However, based on anecdotal reports from various publishers, you can assume that an Amazon sales rank of 5,000 translates into about 15 to 20 sales per day, depending on seasonal factors.