"what in the world does my Amazon sales rank number mean?"
Very roughly, the Amazon sales rank can be taken as a measure of your book's relative success to now over 6 MILLION other books at Amazon.com. Every book that has sold at least a single copy is assigned a rank.
The Amazon sales rank is a measure of how many books YOUR book sold compared to all the other books on Amazon.com. Your rank is yours and yours alone - no two books can share the rank at any one time (books that have sold the same number have additional criteria applied). The period of time over which the sales are measured is varHowever, the ranking is updated hourly.
Amazon applies some very complex (and apparently top secret) math to maintaining rankings for their top 5,000 books. Sales are measured hourly, daily, and monthly - and rankings are determined by even the amount of time BETWEEN sales. Books in the top 5,000 keep their rankings very consistent - and Amazon enforces some "averaging" of sales to keep your book from jumping up to number one just because you got all your relatives in New Jersey to buy a copy at exactly noon on Tuesday (but, do it if you can...for about 30 minutes you'll have the most incredible ranking!)
Changes in your Amazon sales rank is a great measure of the success of your marketing efforts - hopefully a nice bump upwards in rank corresponds to a book promotion or event. These are usually temporary, as it is consistent an concerted effort to move the sales rank significantly. A general rule of thumb (first proposed by Morris Leventhal of FonerBooks) is to note your rank twice a week for four weeks, then divide by 8. This will show your "average" Amazon sales rank. Checking any more than that is really meaningless, since these ranks can change on an hourly basis. You'll find that titles that sit within the top 5,000 do not usually fluctuate by more than 20% (and Amazon is trying to contain even this level of fluctuation). Titles in the 10-20,000 range may jump or drop by as much 50 or 60%. Titles under the 50,000 mark will swing
Amazon Sales Rank - the "numbers"
So - what does all this mean? How MANY books am I selling?
Rank Weekly Sales
1,000 90 copies
10,000 60 copies
100,000 16 copies
300,000 12 copies
500,000 1 copy
1,000,000 1 copy per month
Now, this isn't going to hold true all year long on a unit basis - sale rates change per season - but it will hold in the RELATIONSHIP between sales ranks.
So, theoretically, sales ranks don't change without some action having occurred - meaning your rank won't go up without a sale, and they don't fall unless some other book has more sales in the past 24 hours (though the numbers get pretty funky in the "under 50,000" range). Your titles rank will drop if you have no sales, but the rate at which it will drop is dependent upon how consistently strong your sales were BEFORE it stopped selling - sort of... It's a bit of a bell curve that hits the middle ground most severely - books with long term, strong sales drop slowly, moderate sellers (under 50,000 to about 250,000) drop faster, and weak sellers (500,000 and down) drop positions very slowly. As we said, books ranks are calculated every hour of the day.