What is a good ACOS?
The honest answer: nobody can tell you without looking at your book. A 40 % ACOS can be a solid business on one title and a subsidy on the next.
Why the number alone says nothing
ACOS stands for advertising cost of sale: ad spend divided by the revenue that advertising produced. Spend $30, sell $130 worth, and your ACOS is just under 23 %.
Except you do not keep those $130. On a KDP paperback you keep the royalty — 60 % of the net price, minus printing cost. That share decides how much advertising you can afford. Two books at the same price can have completely different limits if one runs 200 pages and the other 500.
The only number that matters
Instead of hunting for a generally good ACOS, work out your break-even — the point where ad spend and royalties cancel out:
Break-even ACOS = royalty rate ÷ (1 + tax) − printing cost ÷ list priceFour typical cases at the usual KDP rates:
| Book | Royalty per sale | Break-even ACOS |
|---|---|---|
| Paperback, $12.99 | $5.29 | 40.8 % |
| Paperback, $9.99 | $3.49 | 35.0 % |
| Hardcover, $22.99 | $7.29 | 31.7 % |
| Ebook, $4.99 | $3.49 | 70.0 % |
Printing cost depends on page count and trim size; the values here are examples. Put your real numbers into the calculator.
The same 40 % ACOS is a good deal on the $4.99 ebook and a loss on the $9.99 paperback. That is the entire point.
Break-even is a ceiling, not a target
Exactly at break-even you work for nothing: every copy sold brings in precisely what it cost to advertise. A target ACOS with some room makes more sense. Keep 30 % of the royalty and your target sits at 70 % of break-even — 40.8 % becomes 28.5 %. The target ACOS calculator does that for you.
A higher ACOS can still be right, for instance on a new title that needs visibility and reviews. That is a deliberate investment rather than an accident — the difference is that you know the line you are crossing.
Careful with fresh numbers
Amazon Ads attributes a sale to the day of the click, not the day of the purchase. Someone who clicks today and buys twelve days later raises today's revenue retroactively. Recent days therefore always look worse than a week that has already matured.
Clicks, impressions and cost per click, on the other hand, are final immediately. Use them to fix bids and targeting quickly — but judge the ACOS only once enough clicks have accumulated and the most recent days have dropped out of your window.