Short answer
Marketing ROI is the incremental profit generated per pound of marketing spend. Measured properly it uses econometrics and experiments, not last-click attribution, because it must capture offline media, brand effects and cannibalisation.
Platform-reported ROAS counts conversions the customer would have made anyway. Incremental ROI counts only the sales that would not have happened without the ad. The gap is often 2-5x: the reason CFOs distrust marketing numbers.
Use MMM to get consistent, cross-channel incremental contribution. Calibrate the model with geo-experiments and lift tests. Convert contribution to profit (not revenue), subtract cost, divide by cost. That is your ROI.
A £1 profit-negative sale still shows revenue ROI > 1. Growth-obsessed marketing teams often burn cash chasing revenue ROAS. Profit ROI is the number that ties marketing to enterprise value.
It varies by category, but a healthy portfolio ROI (profit basis) is typically 1.5-3x for mature brands and higher for direct response.
ROAS is revenue over spend, uncontrolled for what would have happened anyway. ROI is incremental profit over spend.
Bayesian MMM, calibrated with experiments, refreshed monthly, delivered as a decision system.
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