ACoS is a reporting ratio. A break-even check needs a separate cost record for the product and marketplace being reviewed.
Calculate the ratio with its scope
Divide ad spend by ad-attributed revenue and express the result as a percentage. ROAS reverses those two amounts. Keep a zero denominator marked as undefined. Amazon’s ACoS and ROAS guide.
Retain the market, currency, dates and attribution definition. Do not substitute total retail sales for the denominator and still label the result ACoS.
Build the cost record
Start with the actual product price and the applicable treatment of taxes. List product cost, selling fees, fulfillment, inbound costs and other costs included in the decision. Mark which values are estimates and which are charges.
Amazon’s pricing page provides fee information and a calculator for estimates. Check the local marketplace and actual product inputs. Amazon’s selling costs and Revenue Calculator.
State the break-even assumptions
Compare the amount available before advertising with the advertising cost allocated to the same scope. Specify whether the sheet concerns an order, product or reporting period, and how returns and other costs are handled.
For a product with positive revenue, calculate its before-ad contribution margin ratio as (revenue − variable non-ad costs) ÷ revenue × 100. The contribution remaining after variable costs covers fixed costs and profit. Contribution-margin definitions and calculations.
Match the included units and revenue basis to the ad report before comparing this ratio with ACoS. At equal ratios, the allocated ad spend consumes that contribution, leaving nothing for fixed overhead in the sheet. This calculation does not establish the incremental value of ads.
Keep missing costs visible. A favorable ACoS alone does not establish profit, and a product-specific calculation should not become a universal target for unrelated products.