Revenue divided by spend. A ROAS of 4.0 means you made $4 for every $1 you spent. This is the standard efficiency metric for performance marketers, but it doesn't account for costs beyond ad spend like production, salaries, etc.
Formula: Total Revenue ÷ Total Ad Spend = ROAS
Example: $10,000 in revenue from $2,500 in ad spend = 4.0 ROAS