What is a good ACOS for books?

What is ACOS?

ACOS, advertising cost of sales, is what Amazon Ads uses to measure a campaign: ad spend divided by the sales the ads brought in, as a percentage.

ACOS = ad spend / ad sales x 100

For KDP books, "sales" means the list price customers paid for books bought within 14 days of a click. It is not your royalty. That gap is why a good ACOS for a book is lower than most people expect.

What is break-even ACOS?

Break-even ACOS is the ACOS at which the ads cost exactly what the sales earn you. It depends on one thing: how much of the list price you keep.

break-even ACOS = royalty per sale / list price
$4.39 / $14.99 = 29.3%

That is a $14.99 paperback, 300 pages at 6 x 9 in, sold on Amazon.com. At an ACOS of 29.3% each ad sale costs $4.39 in clicks and earns $4.39 in royalty. A good ACOS is any ACOS below your break-even. The further below, the more each sale makes.

What is break-even ACOS for a paperback?

Paperback royalty is a share of the list price minus the printing cost, so the break-even rises with price and falls with page count. Amazon.com, 6 x 9 in, black ink on cream paper:

"below min" means the price is under KDP's minimum list price for that book, so there is no royalty to spend. A thin, cheap paperback leaves little room for ads.

What is break-even ACOS for a Kindle ebook?

Ebooks have no printing cost, so break-even is much higher inside the $2.99 to $12.99 band where the 70% plan applies. Outside it, only 35% is available. Amazon.com, 2 MB file, no VAT:

On the 35% plan break-even sits near 35% at any price, but the money behind it does not. At $0.99 the royalty is $0.35 a sale, and that has to cover every click that led to the sale. Advertising a 99-cent book only pays if the rest of the series earns it back.

How do you check if your ACOS is good?

A worked example with the ACOS calculator's default paperback campaign:

1. Ad spend $100.00, ad sales $374.75: about 25 copies at $14.99.
2. ACOS = $100.00 / $374.75 = 26.7%.
3. Break-even ACOS = $4.39 / $14.99 = 29.3%.
4. ACOS is under break-even, so the campaign made $9.75: 25 x $4.39 royalty minus $100.00 spend.

Run your own numbers in the ACOS calculator. It works out break-even from your format and price and shows profit at each ACOS.

When can an ACOS above break-even still pay off?

ACOS only counts sales of the book you advertised. Two things it leaves out can turn a loss into a profit:

• Kindle Unlimited pages read. KDP attributes KENP read after an ad click to the campaign, but reports it apart from ACOS. At the August 2026 author-reported rate of $0.00489 a page, that money is invisible in ACOS.
• Series read-through. A reader who finishes book one may buy or borrow book two and three. The ads paid for that reader, but the later books never show in the campaign's sales.

Worked example: book one of a series in KU

A $4.99 ebook on the 70% plan earns $3.28 a sale, so it breaks even at 65.7%. One month of ads:

1. Spend $60.00, 12 sales = $59.88 ad sales. ACOS is 100.2%, well over break-even. On sales alone the ads lost $20.64.
2. KU readers read 4,000 pages after a click. At $0.00489 that is $19.58.
3. The book is 400 KENP long, so those pages equal about 10 full reads. With the 12 buyers that is 22 readers.
4. If 40% go on to each of the next 2 books, each earning $3.28, the later books earn 22 x 0.56 x $3.28 =$40.41.
5. True profit: -$20.64 + $19.58 + $40.41 = $39.35. The true break-even ACOS is 165.9%.

The 40% read-through here is an example, not a benchmark. Measure your own: divide sales (or full reads) of book two by those of book one over the same months. If you guess high, the true break-even looks better than it is and the ads lose money.

What rules of thumb do authors use?

These are common practices, not rules from Amazon, and not averages:

• Judge a campaign against your own break-even ACOS, never against someone else's ACOS.
• Give a campaign enough clicks to judge, and wait the 14-day attribution window before calling it.
• For a standalone book, keep ACOS under break-even. There is nothing downstream to pay back a loss.
• For book one of a series, accept an ACOS above break-even only up to your true break-even, worked out from your own read-through.
• Recheck break-even whenever you change the price. A price cut lowers it.