Guide

Amazon Ads for Kindle Unlimited books

If your series is in Kindle Unlimited, ACoS is the wrong scoreboard. Here's how to judge Amazon Ads on what a new reader is worth across every book, and how to set bids from that number.

By Michael Cortez · Updated November 8, 2026

What ACoS misses

ACoS is ad spend divided by attributed sales. For a single standalone book bought outright, that's a fair measure. For a KU series it misses two things that can be most of your income from an ad.

Page reads. A KU borrow isn't a sale. You're paid per page read (KENP), and none of it counts in ACoS.
Read-through. A reader who finishes book 1 often buys or borrows book 2, then 3. The ad found that reader; the later books never appear in the ad report.

What one new reader is worth

Work it out from your own dashboard numbers:

Book 1: royalty per sale for buyers, or book 1's KENPC × the KENP rate for a full KU read.
Later books: expected later books per reader × royalty (or KENPC × rate) per later book. With 45% going on to book 2 and 80% carrying on after that, a four-book series gives each new reader about 0.45 + 0.36 + 0.29 = 1.1 later books.

Add them up and divide by the book 1 price, and you have your series break-even ACoS. It's often well above the 70% royalty rate most advice uses. The break-even ACoS calculator does the arithmetic.

Setting bids from it

The bid that breaks even on a keyword is what a click earns: reader value divided by the clicks it takes to win a reader. If a keyword wins a reader every 10 clicks and a reader is worth $8 across the series, a click is worth $0.80. Bid below that to keep a margin. Bid much lower and you're leaving readers on the table.

Raise bids gradually (around 30% at a time) on keywords that earn more per click than they cost, and lower them on keywords that cost more than they earn. Leave keywords with only one or two readers alone until there's more data.

Timing

Page reads are logged on the day they're read, not the day of the click, and some readers take days to start. Make bid decisions on a period that ended at least two weeks ago, and expect read-through to lag by weeks.

Keep it honest

Read-through is an average. Readers from some keywords may read on more than others, and paperback buyers behave differently from KU readers. Use the series value to stop cutting profitable keywords, then check your KDP dashboard month to month to confirm the ads are moving total royalties.

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 100% ACoS bad for a Kindle Unlimited book?

Not necessarily. ACoS only counts sales of the advertised book. If most of your readers borrow through KU and go on to read later books, an ACoS well above 100% can still be profitable. Work out your series break-even before cutting anything.

What's a good read-through rate?

It varies a lot by genre and by how well book 1 ends. Use your own numbers: book 2 reads (sales plus full KU reads) divided by book 1 reads over the same period, allowing for the delay while readers get there.

Where do I find the KENP rate?

KDP publishes the per-page rate for the KDP Select Global Fund each month, and your KDP reports show your page reads and earnings. The rate moves month to month, so update it in your calculations when it changes.

Should I advertise book 1 or the whole series?

Most series authors advertise book 1, because that's where read-through starts. Advertising later books mostly reaches people already in the series. The same series-value math works for any book if you start the read-through from that book.