The short answer
Average ROAS describes all included revenue relative to all included spend. Marginal ROAS focuses on the change in revenue relative to a change in spend. It helps frame a budget expansion, but a simple before-and-after calculation does not prove that the spending change caused the revenue change.
Calculate the change explicitly
Illustrative inputs: a lower-spend scenario has $5,000 spend and $25,000 revenue. A higher-spend scenario has $6,000 spend and $27,000 revenue. Average ROAS moves from $25,000 ÷ $5,000 = 5× to $27,000 ÷ $6,000 = 4.5×. The observed marginal ratio is ($27,000 − $25,000) ÷ ($6,000 − $5,000) = 2×.
Apply the margin to the additional revenue
Assume the extra revenue has an illustrative 40% contribution margin before ads. Additional contribution is $2,000 × 0.40 − $1,000 = −$200. Despite an overall average of 4.5×, the added spending does not cover the modeled incremental costs. With a different incremental product mix, the relevant margin would also change.
Do not mistake correlation for causation
Two successive periods can differ in seasonality, inventory, prices, organic demand, or attribution maturity. The observed difference contains those influences as well as any advertising effect. Use a credible experiment or other defensible measurement design when the decision requires a causal incremental estimate.
Define the spending increment
If spend did not change, the marginal ratio is undefined. If you reduce spend, interpret the signs carefully and describe the amount of revenue lost or retained relative to savings. Do not compare changes that use incompatible currencies or reporting populations.
Use this as a budget question
Ask what the next tranche of spend is expected to return, what contribution margin applies, and how that expectation will be tested. Keep the overall campaign report alongside the marginal view. A strong historical average can coexist with an unattractive expansion opportunity, and a lower average can coexist with more total contribution. The decision depends on the added economics and their uncertainty.
Evaluate the spending increment
- Gather: Record revenue and spend in the comparison and the contribution margin of the additional sales.
- Produce: Calculate the change in contribution after the change in spend.
Decision boundary: A before-and-after revenue difference also contains influences other than the budget change.
Next reading
Examples are illustrative, not client results or benchmarks. The calculations use the inputs shown; real-world decisions depend on your records and assumptions. Report a correction.