What ACOS measures
Advertising cost of sales is advertising spend divided by attributed advertising sales, multiplied by 100. If spend is 100 and attributed sales are 500 in the same currency, ACOS is 20%.
What ROAS measures
Return on advertising spend is attributed advertising sales divided by advertising spend. With the same example, ROAS is 5. The two measures describe the same relationship from different directions.
Why one number is not enough
Neither metric is profit. Product costs, marketplace fees, shipping, returns and other overheads still matter. Attribution windows and reporting delays also affect comparisons.
Use a business-specific target
Set targets with your unit economics and campaign purpose in mind. Discovery, launch and mature product campaigns can have different roles. Compare like-for-like periods and document changes.
Read the wider picture
Review total sales, stock availability and conversion alongside advertising metrics. A lower ACOS is not automatically better if it comes from restricting useful demand.
These guides explain general working principles. Check platform-specific requirements for your account and market before acting on them.
