How this ROMI estimate works
Multiply incremental revenue by the contribution margin before marketing to estimate incremental gross contribution. Subtract the complete marketing investment, divide by that investment, and multiply by 100. With $15,000 in estimated incremental revenue, a 40% contribution margin, and $3,000 of marketing cost, the contribution before marketing is $6,000, the contribution after marketing is $3,000, and ROMI is 100%.
What to include
Use a contribution margin that reflects variable product, fulfillment, transaction, and return costs while excluding the marketing investment entered separately. Include media, production, agency, tools, and other campaign costs in the investment when they belong to the same activity and period. Do not count a cost twice. Keep revenue and cost in the same currency and the same analysis window.
Incrementality is the hard part
A campaign-period sales total is not incremental revenue. Customers may have bought without the campaign, through another channel, or after a longer decision cycle. A controlled test or carefully validated model can provide stronger evidence than last-click attribution, while small businesses may need to label their input as an assumption. Change the assumed revenue and margin to see how sensitive the result is; a precise-looking percentage does not remove attribution uncertainty.
ROMI and ROAS are different
ROAS divides attributed revenue by advertising spend and does not account for product costs. This ROMI scenario uses contribution after variable costs and subtracts total marketing investment. Neither metric alone captures long-term brand effects, customer lifetime value, or company-wide overhead. Google describes marketing ROI in terms of incremental gross profit and total investment, while its incrementality guidance explains why observed sales should not automatically be credited to a campaign.
Google: marketing ROI and incremental gross profit · Google: incrementality testing · AMEC: evaluation framework