How do you know if Amazon ads for your book make money?
Compare what a reader brought by the ads earns you with what that reader cost in clicks. A reader earns you a royalty if they buy, the pages they read if they borrow in Kindle Unlimited, and whatever they spend on the rest of your series.
If that is more than the clicks it took to win the reader, the ads make money, whatever ACOS Amazon shows.
Earnings per reader = buyers' share × royalty + borrowers' share × earnings per borrow + series earnings per reader
What one sale pays: the KDP royalty
KDP computes the royalty from the list price before VAT, with three cases on Amazon.com:
- Ebook at 70%: 70% × (list price − delivery cost), with delivery at $0.15 per MB. The list price must sit between $2.99 and $12.99; the ceiling was $9.99 until 7 July 2026. A $4.99 ebook of 1 MB pays 0.70 × 4.84 = $3.39.
- Ebook at 35%: 35% × list price, with no delivery cost. A $0.99 ebook pays $0.35.
- Paperback: 60% × list price − printing cost at $9.99 and above, 50% below. A 300-page black-and-white paperback costs $4.60 to print, so at $14.99 it pays 0.60 × 14.99 − 4.60 = $4.39.
What a Kindle Unlimited borrow pays
A borrow pays the pages actually read, at the KENP rate KDP sets each month: pages × share read × rate per page. A 300-page book read to 60% at $0.0047 a page pays 300 × 0.60 × 0.0047 = $0.85.
The rate moves every month: in the US it went from $0.0042 to $0.0048 a page in the first half of 2026. A borrow usually pays less than a sale, but it brings in readers who would not have bought.
Earnings per borrow = KENPC pages × share read × KENP rate per page
Why ACOS understates your book ads
Amazon's ACOS divides ad spend by sales only. The royalties of Kindle Unlimited reads are reported apart, as KENP royalties, and the series' later sales do not appear at all. So for a book in Kindle Unlimited, the ACOS Amazon shows looks worse than the truth.
The break-even point you need is therefore the ACOS, as Amazon reports it, at which everything a reader earns you covers what the reader cost:
Example: the $4.99 ebook above, half bought and half borrowed, earns $2.12 per reader. Counting sales only, it breaks even at an ACOS of 68%; counting the borrows, it breaks even at a reported ACOS of 85%. A campaign at 75% looks like a loss and makes money.
Break-even ACOS (as reported) = earnings per reader ÷ (buyers' share × list price) × 100
Series read-through: the next books count too
A reader who finishes book 1 and goes on to book 2 brings that royalty too, and the ads on book 1 are what found them. Add what the rest of the series earns, on average, per reader of the advertised book.
In the example, if half the readers go on to a book 2 that earns $3.00 per reader, add $1.50 per reader: the reported break-even ACOS climbs from 85% to 145%.
The break-even cost per click for a book
Multiply what a reader earns you by the readers you get per click. At $2.12 per reader and one reader every 10 clicks, a click must cost less than $0.21 to pay for itself.
Count readers, not sales: Amazon attributes to your ads the pages read up to 14 days after a click, as KENP read. Sales and borrows divided by clicks is the rate to use.