First, the disclaimers: Since the algorithm Amazon uses to generate its sales ranking is proprietary, the details contained herein are extrapolated from research and field tests. The resulting consensus finds Amazons system to provide marginal sales data at best.

To whit, read Amazons own definition of its system, slightly paraphrased from their FAQ: The Sales Ranking system exhibits how books are selling. The lower the number, the higher the sales.

The calculation is based on sales and is updated each hour to reflect recent and historical sales of every item sold. We hope you find the Amazon.com Sales Rank interesting! This last sentence seems to indicate Amazons own perspective on the importance with which the sales rankings should be viewed.

Youre not supposed to find the sales rankings informative or helpful. Youre supposed to find them interesting.

In actuality, the process is somewhat more convoluted than they let on. Only the top 10,000 books are updated every hour and the ranking does not depend upon the actual number of books sold, but rather, on a comparison against the sales figures of the other 9,999 books within that same hour. Simultaneously, a trending calculation is applied to arrive at a computerized sales trajectory. So, hypothetically, a book that held a ranking of 2,000 at 2pm and 3,000 at 3pm, might hold a 4,000 ranking at 4pm, even if it actually sold MORE books between 3-4 than it did between 2-3.

Books with rankings between 10,000 and 100,000 are recalculated once a day, rather than once an hour. Current projections, as well as historic sales information play a key role in these calculations. In fact, the predictive nature of the Amazon ranking system is what makes it possible for a newly-released book to outrank an older established title, even though the actual sales figures for the latter far exceed the former.

Books with rankings over 100,000 are also recalculated every day and applied with historic sales information and projections, although in the case of these books, history takes a back seat. Sales projections and trending take an active role here, which is why a books ranking can leap from 900,000 to 200,000 in the span of 24 hours or less. Does this mean the book has sold 700,000 copies in 24 hours? Absolutely not! What it does mean is that recent activity (i.e. purchases) for that book is trending higher than those 700,000 books it just surpassed. But, dont get excited just yet; since the activity of those 700,000 other books range from slow to stagnant, one or two orders are sufficient to catapult a ranking.

If a books ranking breaks into the top 100,000, the sales history calculation starts to rear its head, which is why a phenomenon book has a hard time maintaining a high, legitimate ranking. A phenomenon is defined by a book that leaps from the high hundred-thousands into the lower thousands (or better) in the span of 24 hours or less, usually due to some concentrated marketing initiatives. Since Amazons sales history for that title doesnt support the leap, the spike occurs and then quickly drops again.

HOW DOES ALL THIS TRANSLATE TO ACTUAL SALES FIGURES?
Since the data is recalculated every hour and/or every day (depending upon a books current ranking), its impossible to get cumulative sales figures, although those figures are applied to the algorithm during the calculation. No, to get a very rough idea of the actual number of books being sold, the sales ranking has to be dissected dynamically, with the same immediacy as the ranking being calculated, (hourly for top 10,000 books or daily for top 100,000 books). Chart the ranking of a top 10,000 book every hour for 24 hours and divide by 24 to arrive at its average daily ranking. In the case of a top 100,000 book, take its ranking every day for 7 days and divide by 7 to arrive at its average weekly ranking.

Bear in mind that this next piece of information is extremely arbitrary, based upon sales ranking/sales figure comparisons and data received from third party sources. In other words, it's probably completely wrong. But rather than disclaiming this chart until the cows come home, I'll just say this: It is difficult to make sense of something that doesnt make sense. But it sure is interesting, and now, perhaps, even slightly helpful. If the books average ranking is:

2,000,000+ Perhaps a single inventory/consignment copy has been ordered
1,000,000+ Current trends indicate total sales will most likely be under 40
100,000+ Current trends indicate total sales will most likely be under 200
10,000+ Estimate between 1 10 copies being sold per week.
1,000+ Estimate between 10 100 copies being sold per week.
100+ Estimate between 100 200 copies being sold per week.
10+ Estimate between 200 1000 copies being sold per week.
Under 10 Estimate over 1,000 copies per week