iROAS (Incremental ROAS)
Incremental ROAS is return on ad spend calculated against incremental revenue — the revenue the ads actually caused, not the revenue the platform claims credit for.
iROAS = incremental revenue / ad spend. The key word is incremental. Platform-reported ROAS uses platform-attributed revenue, which typically overstates contribution because it includes conversions that would have happened without the ad.
iROAS is always lower than reported ROAS. The gap is the overstatement. A Meta-reported ROAS of 5× often lands at iROAS 2–3× once holdout testing strips out the organic conversions. This is not Meta being dishonest — it is a structural limitation of click-and-view-through attribution.
Measuring iROAS requires an incrementality test: geo holdout, user-level holdout (Conversion Lift), or synthetic control. Because these are slow and expensive to run, most brands only have iROAS figures for a few major campaigns per year, then use those multipliers to discount their real-time reported ROAS.