What it means
ROAS (Return on Ad Spend) measures revenue per euro of ad spend: ROAS = attributed revenue ÷ ad spend. A ROAS of 4 means 4 euros of revenue per advertising euro. Mathematically, ROAS is the inverse of ACoS expressed as a decimal (ACoS 25% = ROAS 4). Margin matters here too: your break-even ROAS is the inverse of your pre-advertising margin — at a 20% margin you need at least a ROAS of 5 to advertise profitably.