ACoS (Advertising Cost of Sale) is the percentage of ad-attributed sale revenue spent on advertising. Formula: (ad spend ÷ ad revenue) × 100. Lower ACoS means more efficient advertising. Most indie authors target ACoS under 40% for non-fiction, under 60% for fiction in Kindle Unlimited.
ACoS stands for Advertising Cost of Sale — the percentage of a product's sale revenue that was spent on advertising to drive that sale. It's the headline efficiency metric for Amazon Ads (both Sponsored Products and Sponsored Brands) and for most paid-traffic campaigns selling eBooks or digital products.
ACoS = (Ad spend ÷ Ad-attributed revenue) × 100
If you spent $100 on Amazon Ads and those ads drove $400 in book sales, your ACoS is 25%. Lower ACoS means your advertising is more efficient. Higher ACoS means you're spending more to generate each dollar of sales.
ACoS only measures revenue directly attributed to ad clicks within Amazon's attribution window (typically 14 days). It misses several important revenue streams:
Because of these gaps, Total ACoS (TACoS) — ad spend as a percentage of total sales (not just ad-attributed) — is often a better efficiency metric. TACoS of 10–15% typically indicates healthy overall economics even when ACoS on specific campaigns runs 40–60%.
Three ACoS scenarios showing when the metric helps — and when it misleads:
Other concepts you'll encounter alongside this one.
Longer-form resources that apply this concept in practice.
ACoS (Advertising Cost of Sale) and ROAS (Return on Ad Spend) are reciprocal metrics. ACoS = ad spend ÷ ad revenue (lower is better). ROAS = ad revenue ÷ ad spend (higher is better). A 25% ACoS is the same as 4x ROAS. Amazon Ads report ACoS by default; most other ad platforms (Facebook, Google) report ROAS. Use whichever your platform shows natively.
Depends on margin and goals. For indie authors: under 40% ACoS for non-fiction is typically profitable at the unit level. For fiction in Kindle Unlimited, 50–65% ACoS can still be profitable because page-read revenue adds on top. For premium specialty books ($14.99+), aim for 25–35% ACoS. Break-even ACoS equals your royalty rate (70% tier books break even at 70% ACoS ignoring KU).
New campaigns often start with high ACoS because Amazon's algorithm is still learning which keywords and targets convert. Give campaigns 14–21 days to accumulate data before optimizing. During the learning phase, ACoS over 80% is common. Optimize by pausing underperforming keywords and doubling down on winners after the learning window.
Amazon's default ACoS does NOT include KU page-read revenue — only direct ad-attributed sales. For KU-enrolled fiction where page reads are a major revenue stream, use Total ACoS (TACoS) instead: ad spend ÷ total monthly revenue (sales + KU combined). TACoS below 15% usually indicates healthy overall economics.
Four main levers: (1) pause underperforming keywords — review weekly and cut anything with >1.5x your target ACoS, (2) increase bids on top-performing keywords to win more impressions, (3) improve your book listing (cover, title, description, reviews) to boost conversion rate on clicks, (4) refine keyword targeting — negative keywords for irrelevant searches, specific-match keywords for high-intent searches.
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