How it works
The same thing as ROAS, only from the other side: where ROAS answers “how much came back”, ACoS answers “how much went out”. Handier when you count in percentages of revenue.
Formula
Spend ÷ revenue × 100%
Example
The same shop: spend 40,000, revenue 180,000.
ACoS = 40,000 ÷ 180,000 × 100% = 22%. Advertising eats a little over a fifth of the revenue.