What Is a Good ACOS for Books?
Updated 2026-07-18
ACOS — Advertising Cost of Sale — is ad spend divided by ad-attributed sales. Authors constantly ask what a good ACOS is, and the honest answer is that there's no universal number: it depends entirely on your royalty, your price, and whether readers go on to buy the rest of your series.
Start with your break-even ACOS
Break-even ACOS is simply your royalty per sale divided by your list price. If your ebook sells for $4.99 and you earn $3.25, your break-even ACOS is about 65%. Spend more than 65% of your ad-attributed revenue and you're losing money on that standalone sale.
This single number turns a vague question into a concrete target. 'Is 45% ACOS good?' becomes 'is 45% below my break-even of 65%?' — and the answer is yes, that sale was profitable.
Why the answer changes for a series
If your ad sells book 1 and a meaningful share of those readers buy books 2, 3 and 4, the true value of that first sale is much higher than one royalty. Authors with strong read-through routinely run book 1 at an ACOS well above break-even on purpose, because the profit shows up later in the series.
The catch: read-through has to be measured, not assumed. Look at your actual sales ratios between books before you justify a high ACOS with it.
Kindle Unlimited complicates the math
If you're in KU, page reads earn money that Amazon doesn't attribute to your ads. Your reported ACOS will look worse than reality because the revenue side is incomplete. Many KU authors track total royalties against total ad spend for the month as a sanity check alongside campaign ACOS.
Practical ranges — with a caveat
Treat any 'industry average ACOS' you read online as trivia, not a target. Your break-even number is the only benchmark that reflects your actual economics — everything else is someone else's book.
- Below break-even: profitable on the standalone sale
- At break-even: paying for visibility and rank at cost
- Above break-even: only justified by read-through, KU page reads, or a deliberate launch push
Work it out for your book
Our free guides cover the mechanics, and the Toolkit includes a calculator that turns your price and royalty into a break-even ACOS and a starting bid, then tracks each keyword against it automatically.