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What ACoS and TACoS Actually Measure

ACoS is ad spend divided by ad-attributed sales. TACoS divides the same spend by all of a book's sales. Here is what each number can and cannot tell a KDP author, with worked math.

The BookAdsAI team · Metrics · Published October 5, 2026.

ACoS and TACoS look like cousins, and authors often quote them interchangeably. They answer different questions. ACoS tells you how expensive the sales your ads produced were. TACoS tells you how much of the book's whole business is being spent on advertising. Mixing them up is one of the most common reasons a profitable campaign gets switched off, or a losing one gets left running for months.

ACoS: the cost of the sales your ads claimed

ACoS stands for advertising cost of sales. The formula is simple:

ACoS = ad spend ÷ ad-attributed sales × 100

If you spent $30 on clicks last week and the Amazon Ads console attributed $100 of sales to those clicks, your ACoS is 30%.

Two details matter for books. First, "sales" in the Ads console is the list-price value of the units Amazon attributed to an ad click, not your royalty. A $4.99 ebook sale shows up as $4.99 of sales even though your 70% royalty is closer to $3.40 after the delivery fee. Second, Kindle Unlimited page reads are not in that sales figure. The console reports KENP pages read and an estimated KENP royalty for KDP books, but the ACoS column ignores them. A book that earns a large share of its money in Kindle Unlimited will always look worse on ACoS than it really is.

Break-even ACoS is your royalty rate

Because ACoS compares spend with list-price sales, the break-even point is roughly your royalty percentage:

• A $4.99 ebook on the 70% royalty option breaks even near 68–69% ACoS once the small delivery fee is taken out.
• A $2.99 ebook on the 70% option also breaks even near 68%, but each sale only covers about $2.03 of spend.
• A $0.99 ebook on the 35% option breaks even at 35% ACoS, which is why most authors do not advertise 99-cent books on their own unless a series is behind them.
• A $14.99 paperback earns 60% of list price minus the printing cost. If printing is $4.00, the royalty is about $5.00, so break-even ACoS is about 33%.

That last example is the one that surprises people. The same 40% ACoS is comfortably profitable on an ebook and loses money on a paperback with a high print cost. Always work out break-even ACoS per format before you judge a campaign.

TACoS: ad spend as a share of everything the book earns

TACoS means total advertising cost of sales:

TACoS = ad spend ÷ total sales (ad-attributed plus organic) × 100

If you spent $30 on ads and the book sold $300 in total that week, including sales nobody can trace to a click, TACoS is 10%. Amazon does not show TACoS in the Ads console. You build it yourself from the KDP Reports dashboard (total units and royalties) and the Ads console (spend).

For authors, a royalty-based version is usually more useful than the retail-dollar version:

Royalty TACoS = ad spend ÷ total royalties (sales plus KENP) × 100

If that number is under 100%, the book as a whole earned more than you spent on ads. If it is well under 100% and stable, the ads are part of a healthy business even when the campaign-level ACoS looks high.

What each number is good for

ACoS is a campaign and keyword tool. Use it to compare targets against each other inside the same book and format. A keyword at 25% ACoS is cheaper per attributed sale than one at 90%. That is a fair comparison because both are measured the same way.

TACoS is a business tool. Use it to see whether the ads are growing the book or just renting sales you would have had anyway. Watch the trend over weeks:

• TACoS falling while total sales rise: ads are helping organic sales grow, or organic sales are growing on their own. Either way, the spend is getting lighter relative to income.
• TACoS rising while total sales stay flat: you are spending more to stand still. Ads may be capturing buyers who would have found the book anyway.
• TACoS flat while both spend and sales grow: you are scaling at a steady cost. That is usually fine if royalty TACoS is comfortably under 100%.

A worked example

Suppose a fantasy novel sells at $4.99 in ebook and is enrolled in Kindle Unlimited. In one week:

1. Ad spend is $70.
2. The Ads console attributes 14 ebook sales, which it records as $69.86 of sales. ACoS is just over 100%.
3. The console also reports about 6,000 KENP pages read from ad clicks.
4. KDP Reports shows 40 ebook sales and 18,000 pages read in total for the week.

On ACoS alone, the campaign looks like it is losing money. Now convert to royalties. Fourteen ebook sales at about $3.40 is roughly $48. If the per-page rate that month is around $0.0045, the 6,000 ad-attributed pages add about $27. Attributed royalties are about $75 against $70 of spend, so the ads roughly break even on their own. Across the whole book, 40 sales and 18,000 pages earn around $217 in royalties, so royalty TACoS is about 32%. If the second book in the series is selling to some of those readers, the picture is better still.

The KENP rate changes every month, so treat the page math as an estimate and recheck it when KDP publishes the rate.

Mistakes that come from mixing them up

• Comparing your ACoS with someone else's TACoS. A forum post saying "my ads run at 15%" is often a TACoS figure. Your campaign ACoS is not failing because it is higher.
• Judging a KU-heavy book on ACoS. Without KENP, a borrow-heavy book can look like a loss when it is profitable.
• Judging paperbacks against the ebook threshold. Print costs change break-even, so the same ACoS can mean profit in one format and loss in another.
• Reading yesterday's ACoS. Attributed sales keep arriving for days after the click, so the most recent days always look worse than they will end up. Judge on a window that has had time to settle.

How BookAdsAI uses these numbers

BookAdsAI's dashboard reports profit as royalties minus ad spend from reports that actually landed, rather than leaning on ACoS alone. When Autopilot adjusts a bid, it uses ACoS at the keyword or target level, because that is where the comparison is fair: only after at least five clicks, and only by up to 10% a day, lowering bids on targets above a 40% ACoS ceiling and raising them on targets below a 15% floor. You can read the full rules in the Spend-Cap Autopilot Method.

The short version

ACoS answers "how expensive were the sales the ads claimed?" TACoS answers "how heavy is advertising relative to everything this book earns?" Work out your break-even ACoS per format, add KENP back in by hand, and track royalty TACoS weekly. Those three habits prevent most bad decisions about book ads.