As a published author, I have done the same shameful thing that every other published author I know has done: followed my Amazon sales ranking obsessively (though I gave that up a few months ago). And I have had the same question as all my author friends: What the heck does that number actually mean? Does it correlate in any meaningful way with sales figures?
Now, at last, someone has cast some light upon this arcane subject:
Is it possible to correlate ranking with sales? Not officially, but there’s some information out there. For example, I found that a major publisher tracked 25 titles over a six-month period, correlating the weekly Amazon sales rank with actual reported sales from Amazon. Ranks down to 750 sold 75 to 275 books per week. From 750 to 3000 had sales of 40 to 75 per week. The sales drop the further down the list you go, and at 10,000 and above—where most of us hang out—the books sold only 1 to 5 copies per week. So you can see that at this level the number could change with the sale of as little as one unit.
The whole post, by retired physician and author Richard Mabry, is here.