Guides and calculators
Everything here comes from actually reconciling real Amazon exports rather than repeating what other articles say. Short, specific, and honest about what can't be known.
Guides
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Why your Amazon Ads ACOS is lying to you Guide
The console measures your spend against the price the customer paid Amazon, not the royalty you received. A keyword showing a 14× return can lose money on every sale. Here is the gap, the arithmetic that corrects it, and why Kindle Unlimited pushes the error in the opposite direction.
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Why your KDP and Amazon Ads reports never agree Guide
Four purchases in one report, three units in another, and both are correct. Two clocks, a settling window, separate identifiers for each edition, and a dashboard that lists the same sale on three different sheets. A field guide to reconciling them.
Calculators
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Break-even ACOS calculator Free tool
Enter your list price and format. Get the ACOS your book actually needs, the ROAS that means you broke even, and the most you can pay for a sale before you lose money. No signup, nothing to upload.
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KENP royalty calculator Free tool
Work out what your Kindle Unlimited page reads earned, what a full read-through of your book is worth, and how a borrow compares to a sale.
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Campaign break-even check Free tool
Drop in an Amazon Ads report and see which search terms actually made money once your royalty is the yardstick. Runs entirely in your browser — your file is never uploaded.
What's coming
Guides get added as I hit things worth writing down. On the list: reading the search-term report without fooling yourself, what to do when a book can't be advertised profitably at any bid, and how the attribution model change affects historical comparisons.
If there's something you keep getting stuck on, tell me and I'll write about it: hello@authortally.com.