Guide

What is a good ACoS for books?

The honest answer: it depends on what one new reader is worth to you. Here's how to work out your own target in a few minutes, and why most KU series authors can run far higher than the usual advice.

By Michael Cortez · Updated November 8, 2026

Start with break-even

A "good" ACoS is one below your break-even. For a single book, break-even ACoS equals the share of the price you keep. A $4.99 ebook at a 70% royalty breaks even at 70% ACoS: $3.49 of ad spend per $4.99 sale. Anything below that is profit on the sale itself.

Then add what ACoS can't see

For authors, two things change the answer, often by a lot:

Kindle Unlimited page reads. A borrow pays per page read and never counts as a sale, so it adds return without lowering ACoS.
Read-through. Readers who finish book 1 move on to book 2 and beyond. That revenue is real, and it's nowhere in the ad report.

Add them and break-even rises. With 45% read-through to book 2 and 80% after, a four-book series gives each new reader about 1.1 later books. That can push break-even ACoS well past 150%. The break-even ACoS calculator does this with your numbers.

Set a target below break-even

Your target is break-even minus the margin you want to keep. If your series break-even is 180% and you want to keep a fifth of what ads earn, target about 145%. Keywords above break-even get lower bids or negatives; keywords well below target can take higher bids.

Judge keywords, not just the account

An account average hides both waste and hidden winners. Some keywords will bring buyers who never read on; others will bring KU binge readers with no sales at all, which ACoS shows as infinite. The Kindle Author Ads Analyzer scores every search term on series profit.

Run your own report through the Kindle Author Ads Analyzer

Free, runs in your browser, no signup. See which terms to negate, which bids to move and which keywords pay across the series.

Analyze my report

Questions

Is 70% ACoS good for a book?

For a standalone ebook at a 70% royalty with no Kindle Unlimited reads, 70% ACoS is break-even: you earn back exactly what you spend. For a KU series it can be very profitable, because page reads and later books aren't in the ACoS figure.

Is 100% ACoS bad?

Only if the advertised book's sales are all you earn. For a series in Kindle Unlimited, 100% or even 200% ACoS can be profitable once page reads and read-through count.

What ACoS do paperbacks need?

Paperback royalties are 60% of list price minus printing cost, so the break-even ACoS is usually lower than for ebooks. Work it out from your actual royalty per copy ÷ list price.