Return on investment (ROI) is the profit an investment returns relative to its cost, expressed as (gain minus cost) divided by cost. In advertising it differs from ROAS because it is measured on profit and total cost, not on revenue and ad spend alone.
ROI vs ROAS
ROAS is revenue divided by ad spend; ROI is profit divided by total cost. A campaign can show a healthy ROAS and a negative ROI once margin, tooling, and team cost are included. ROI is the number the business actually runs on.
Why it is hard, and worth it
True ROI needs costs and revenue the ad platform never sees, so it lives in your finance and CRM data, not the dashboard. That is exactly why it is trustworthy: it does not benefit from the platform's own attribution. Pair it with blended ROAS for a channel-agnostic read.