What Is ACoS?

How is ACoS calculated?

Amazon divides what you spent on a campaign by the sales it attributes to that campaign, then expresses the result as a percentage. If a Sponsored Product campaign costs you $40 and Amazon credits it with $160 of book sales within the attribution window, your ACoS is 25%. The number lives inside your Amazon Ads console, reported per campaign, ad group, and keyword.

Attribution is the part authors trip over. Amazon credits a sale to your ad when a shopper clicks and then buys within the lookback window — even if they buy a different book of yours than the one advertised. That means ACoS reflects advertising efficiency, not total business profit. A campaign can show a tidy 20% ACoS while the royalties behind those sales barely cover the spend, which is why the next question matters more than the raw number.

One more scope note: ACoS only counts sales Amazon attributes to ads. Organic sales your book earns from also-boughts, category rankings, or your newsletter do not enter the calculation. Authors who want the full picture track TACoS — total ad spend divided by total sales — alongside ACoS, so a campaign that lifts everything does not look like a failure on the ads console alone.

What is a good ACoS for books?

There is no universal good number, because break-even depends on your royalty rate. The math is simple: if you keep 70% of each sale as royalty, any ACoS under 70% leaves money on the table in a good way — the ads cost less than you earned. If you keep 35%, your break-even sits at 35%, and a 40% ACoS quietly loses money on every attributed sale.

That simple rule has two honest complications. First, delivery costs and any print costs come out before your royalty lands, so compute break-even from what actually reaches your account, not the list price. Second, books earn beyond the clicked sale: series read-through, Kindle Unlimited page-reads that attribution misses, and reviews that lift organic rank. Many authors knowingly run launch campaigns above break-even to buy visibility, then tighten toward profitability once the book has traction.

A practical starting posture is to set a target ACoS a few points below your break-even, give a new campaign a fixed test budget and two weeks, then keep the keywords beating the target and pause the rest. Small budgets, explicit targets, and regular pruning beat large budgets with no target every time.

ACoS vs TACoS: what is the difference?

ACoS answers a narrow question: for each dollar of ad-attributed sales, how many cents did ads cost? TACoS (Total Advertising Cost of Sales) answers the wider one: for each dollar of all your sales, how many cents did ads cost? Same spend in the numerator; the denominator grows from attributed sales to everything — organic, also-bought, newsletter-driven, and all.

The two numbers tell you different things, and healthy accounts watch both. A rising ACoS with a flat or falling TACoS usually means your ads are pulling extra organic sales along — the halo effect working as intended. A rising TACoS means ads are eating a growing share of total revenue, which is the signal to cut bids, not celebrate clicks.

Neither metric knows about your costs outside Amazon. Neither subtracts delivery fees, printing, or your time. Treat them as efficiency gauges on the ads dashboard and do the profit math separately, with real royalty statements in front of you.

An example: a $4.99 ebook at 70% royalty

Take a $4.99 ebook earning the 70% royalty rate. After Amazon's delivery fee, roughly $3.40 lands in the author's account per sale — the exact figure moves with file size, which is why the delivery fee deserves its own line in your math. Break-even ACoS is royalty kept divided by attributed sales value: about $3.40 ÷ $4.99, or roughly 68%.

Now run three campaigns against that line. A campaign at 30% ACoS earns about $3.40 and spends about $1.50 per attributed sale — comfortably profitable, and a candidate for more budget. A campaign at 65% roughly breaks even: fine during a launch week when rank matters, not a forever setting. A campaign at 110% spends more than the sale pays — acceptable only if you can point to the read-through or reviews it bought.

What mistakes do authors make with ACoS?

The classic error is optimizing the percentage instead of the profit. A 12% ACoS on $30 of sales earns less than a 45% ACoS on $2,000 of sales at the same royalty rate. Percentage down, dollars up is the goal — always multiply the ratio by volume before judging a campaign.

The second error is judging too fast. New campaigns pass through a learning phase while Amazon finds the shoppers who convert; pausing after three days and $8 of spend measures noise, not performance. Set a test budget per keyword cluster in advance, spend it fully, then decide with a full fortnight of data.

The third is sending ad traffic to a listing that is not ready: a vague book blurb, a placeholder cover, or thin reviews. Ads multiply whatever the product page already does, so fix the page before funding the traffic. A sharp book description and a cover that reads at thumbnail size lower your ACoS more cheaply than any bid adjustment.

• Draft the advertised book with the How-To Guide Generator before funding traffic.
• Set a target ACoS from your real royalty margin before spending anything.
• Give each test a fixed budget and a full two weeks before judging it.
• Track TACoS alongside ACoS so halo sales count toward the verdict.
• Fix the cover and blurb first — ads amplify the page, not compensate for it.
• Prune monthly: pause keywords above target, feed winners more budget.