How to use your ACOS and budget numbers
What ACOS means for a book
ACOS (advertising cost of sales) is your ad spend divided by the sales those ads brought in, as a percentage. For an author, the number that matters is not the sale price but your royalty, because the royalty is what you actually keep. Your break-even ACOS is your royalty per sale divided by your book price: at that ACOS the ads cost exactly what they earn.
- A $9.99 book that pays you $4.00 per sale has a break-even ACOS of 40%.
- A $4.99 eBook at a 70% royalty pays about $3.49 before delivery costs, so its break-even ACOS is roughly 70%.
- Above your break-even ACOS you pay more than you earn on each ad sale; below it you make a profit on the sale itself.
A worked example
A paperback priced at $12.99 that pays a $3.00 royalty, with a 10% conversion rate and a $0.50 cost per click:
| Number | Calculation | Result |
|---|---|---|
| Break-even ACOS | $3.00 / $12.99 | 23.1% |
| Max CPC at break-even | $3.00 x 10% | $0.30 |
| Max CPC at a 70% target | $0.30 x 0.7 | $0.21 |
| Cost per ad sale at $0.50 CPC | $0.50 / 10% | $5.00 |
| Your ACOS at $0.50 CPC | $5.00 / $12.99 | 38.5% |
At a $0.50 click this book loses about $2.00 on every ad sale. A $10 daily budget buys about 20 clicks and 2 sales, which earns $6.00 in royalties against $10.00 of spend, a loss of about $4.00 a day. To make the ads work you would need cheaper clicks, a listing that converts better, or a reason to accept the loss, such as readers going on to buy other books in your series.
What to do with the results
- Bid no higher than the maximum CPC for your target ACOS on new keywords.
- If your estimated ACOS is above break-even, improve conversion first: cover, title, description, reviews and price.
- Run a test for two to four weeks and replace the example conversion and CPC with the real numbers from your campaign.
- Our guide on how much to spend on Amazon Ads explains how to set the daily budget and when to scale or stop.
Want us to run the campaigns?
We set up, audit and manage Amazon Ads for authors, with measurable campaign structure and clear reporting. We do not guarantee sales, and ad spend is paid directly to Amazon. See our Amazon Ads management service.
Frequently asked questions
What is a good ACOS for book ads?
It depends on your royalty. Guides often call under 30% good for paperbacks and a higher figure acceptable for eBooks, but the real limit is your break-even ACOS: your royalty divided by your book price.
How do I calculate break-even ACOS?
Divide your royalty per sale by your book price. A $12.99 paperback that earns $3.00 has a break-even ACOS of about 23%.
What conversion rate should I assume?
Guides for KDP authors commonly assume 5 to 15 percent. Use 10 percent as a placeholder, then replace it with your real conversion rate once your campaign has data.
What is a normal cost per click for a book?
Third-party guides commonly put it at roughly $0.30 to $0.75 for books, but it depends on your genre and keywords. Your own campaign data is the only figure that matters.
Does this include KENP page reads or other books in my series?
No. The calculator looks only at the royalty from the ad sale. If readers go on to buy or read your other books, the real return is higher than shown.