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Amazon would never reveal details about how exactly they calculate the sales rank of different items. At the same time, and based on experience, the formula seems pretty straight forward. About every couple of hours items' rank is calculated based on the number of orders for the given time period, in a given category. This means that a book in Business & Finance category, for instance, can have a sales rank of #100 in the morning of a promotion. By noon - once the promotion has kicked in and sales have accelerated, the sales rank will be updated and can be #5, or #10, or #15 - whatever the case may be.