How to find a book's sales over time?

Arianne said:

In news articles and stuff you often see "X book has sold over a million copies worldwide" and I wonder how they calculate that, and over what period of time, and what editions, and e-book or physical books etc.

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That comes from the publisher's publicity department. They're the only ones in a position to know.

Any other sales figures you see bandied about range from Complete Guesswork to Wildly Random Complete Guesswork.

BookScan is what the industry uses. Amazon provides access for authors and some writers organizations (like RWA) offer a subscription discount for its members.

Evangeline said:

BookScan is what the industry uses. Amazon provides access for authors and some writers organizations (like RWA) offer a subscription discount for its members.

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At best, BookScan only gives you part of the numbers. The industry does use BookScan, but they have to wait for numbers from several types of outlets that BookScan doesn't cover before they have anything like the complete picture.

A subscription to Bookscan is very expensive, and the figures aren't completely accurate: they're getting better but I'd guess that they still only catch about 80% of the sales.

Publishers do know how many copies they've sold, however, because they control their stock and invoices and so on.

But they're unlikely to tell you what their sales are because they're too busy to answer such queries, and it's not really any of your business.

Also, many books come out in several editions (for example, hardback and paperback, plus large print, audio etc), and in several different territories and languages, and collating the information about all of those different editions would be a mammoth task. Only an author, and his or her agent, would have that information available and they're not likely to share it with people they don't know.