How AuthorLens estimates another book’s sales and royalty
Nobody can see another author’s KDP account. But Amazon publishes every book’s sales rank, and KDP’s royalty rules are the same for everyone. The estimate is built from those two things — in the free sales calculator, no sign-up.
- 1 · BSR#18,400the book’s Amazon sales rank
- 2 · sales8–14 / dayAuthorLens estimate, 10 on average (300 a month)
- 3 · royalty per copy$10.97by KDP’s rules
- 4 · per month$2,640–$4,620$3,291 on average, before ad spend
The estimate in four steps
Take the BSR — its daily history, not one hour
Best Sellers Rank shows how a book sells compared with every other book in Books (or the Kindle Store). It updates every hour, and a single sale can lift a book thousands of places. So AuthorLens reads a book’s rank day after day and works from its own history, not a single snapshot.
Each format has its own BSR: Kindle, paperback and hardcover are estimated separately, then added up.
Turn BSR into sales per day
The lower the number, the more sales. But it isn’t a straight line: between #100 and #1,000 lie hundreds of sales a day, between #100,000 and #200,000 about one. The starting point is the rank-to-sales curve the KDP author community assembled. AuthorLens corrected it against real sales: 7,457 days of AuthorLens authors’ actual Amazon.com KDP copies paired with the same books’ daily rank. The public curve overstated Kindle sales — about one and a half times between ranks 300,000 and 1,000,000 and about four times beyond, because Kindle Unlimited borrows lift a rank without being a sale — and slightly understated paperbacks.
On Amazon.co.uk and Amazon.ca the formula is checked against real sales of books there: the same BSR means roughly 5.5 and 20 times fewer sales. Other Amazon stores are scaled by their size and not yet checked against real sales, and the free calculator covers Amazon.com only.
Work out royalty per copy — the way KDP does
The price is on the book page, and so are the page count and format. The rest is ordinary KDP maths for Amazon.com:
Paperback: price × 60% − printing (50% if the price is under $9.99)Kindle: (price − file delivery) × 70% (between $2.99 and $12.99; otherwise 35%)Price $24.99 60% of the price $14.99 Printing: $1.00 + 75 pages × $0.0402 − $4.02 Royalty per copy $10.97 Multiply by sales
royalty per copy × copies a day × 30 = royalty a month$10.97 × 10 copies a day × 30 = $3,291 a month. The calculator’s range for this rank is $88–$154 a day, so $2,640–$4,620 a month. That’s before ad spend: how much the author spends on ads can’t be seen from outside.
4 · per month$2,640–$4,620
How close it gets
79% of books within 50% of their true sales, 29% within 25%. The public curve alone: 71% and 36%.
84% of books within 50% of their true sales, 67% within 25%. The public curve alone: 74% and 55%.
Checked book by book in October 2026: 14 Kindle and 58 print titles ranked between 10,000 and 3,000,000 on Amazon.com, each author’s titles judged with a curve built without them. Kindle is harder, because Kindle Unlimited borrows lift a rank without a sale, and its sample is still small: every new AuthorLens author makes the check more precise. Above that range (faster than rank 10,000) the community curve is used unchanged. One book’s sales still vary with category, season, reviews and Kindle Unlimited share, so read the figure as a range, not a forecast.
How AuthorLens shows it
- Sales estimates appear only where a book’s overall store rank is known, such as its own page and Compare. Search results and bestseller lists don’t show them: a list row carries a rank within its category, which would overstate sales roughly 25×.
- Every estimate is labelled as one and never adds into your real profit figures.
- A competitor’s real royalties and ad spend show as “not observable”, never $0: nobody but the author can see them.
What the estimate leaves out
- Kindle Unlimited pages. Borrows are not sales, so they are not in the sales figure. On a book’s page in AuthorLens, a Kindle Unlimited title on Amazon.com gets a separate estimate of pages read, at KDP’s latest announced rate per page.
- Ads. The estimate is before ad spend. If the book runs ads, the author keeps less.
- Other marketplaces. Each store is estimated from its own BSR, so a book’s Amazon.com figure leaves out its sales in the UK, Canada and elsewhere.
- Expanded distribution. Sales through other stores have a different royalty and don’t show up in Amazon’s BSR.
- Returns. A small share of sales comes back, and KDP deducts it.
- Your own books. They need no estimate: AuthorLens shows the real royalties from your KDP reports.
How to use it
Compare rather than count cents. The estimate works best for seeing whether a niche has money in it: whether the top 10 earn hundreds of dollars a month or tens.
Put your main competitors on Watch. AuthorLens checks them daily, so you see BSR and price history rather than a random hour. Watch costs 1 credit a day per format — how credits work.
Work out any book yourself. In the sales calculator enter a BSR, the price and format to see sales and royalty a day.
Your own books need no estimates: AuthorLens shows the real royalties, ad spend and net profit of every title.