ACoS divides ad spend by attributed sales; ROAS divides attributed sales by ad spend. When both use exactly the same inputs, they are mathematical inverses.
Conversion formulas
ACoS % = 100 ÷ ROASROAS = 100 ÷ ACoS %
A 5× ROAS equals 20% ACoS. A 4× ROAS equals 25% ACoS.
Which presentation is better?
Marketplace teams often use ACoS because it resembles a cost percentage that can be compared with gross margin. Cross-channel teams often use ROAS because it reads as revenue per advertising unit.
The metric label matters less than consistent attribution, margin awareness and a clear definition of sales.
Sources
This guide is educational and does not provide financial, accounting, tax or legal advice.