How is marginal ROAS different from average ROAS?
Average ROAS divides all the revenue a channel produced by everything you spent on it - a single, blended, backward-looking ratio. It is genuinely useful for reporting and reconciliation. Marginal ROAS asks a sharper, forward-looking question: of the next pound you are about to spend, how much new revenue will it create? The two routinely disagree, and the gap between them is where most budget mistakes are made.
Because paid media response curves flatten as spend grows, marginal ROAS is almost always lower than average ROAS at scale. A channel showing a comfortable blended 5.0 might be returning a marginal 1.5 on its last tranche of spend - the average is propped up by all the efficient early spend, even after the next pound has stopped paying its way. For the full treatment of why the two diverge and how to act on it, see marginal ROAS versus average ROAS.
Why marginal ROAS matters for planning
Budget decisions are made at the margin. When a team decides whether to add spend, cut it, or move it between channels, the only number that answers the question is the return on the next pound - not the average of every pound that came before. Planning on the average systematically over-invests in channels that merely harvest existing demand and under-invests in the channels that create it, because the harvesting channels post the prettiest headline ROAS while contributing the least incremental growth.
This is the same logic that underpins incremental return on ad spend - both metrics exist to stop you optimising for an attractive average that does not survive the next budget decision. The practical rule is simple: when the marginal return of one channel falls below that of another, budget should move, regardless of which channel has the better-looking average.
A paid media example
Suppose Google Search currently reports a 5.0 ROAS at £100,000 of spend. The headline looks strong, so the instinct is to scale it. But branded demand is largely exhausted, auctions are more competitive at the margin, and impression-share expansion is increasingly inefficient - so the next £20,000 might only return at 2.5. Average ROAS still reads 5.0 and falling slowly; marginal ROAS has already halved. Acting on the average pours money into the flat part of the curve; acting on the margin redirects it to wherever the next pound still works hardest.
How marginal ROAS relates to saturation
Marginal ROAS is the slope of the spend-to-revenue response curve at your current budget. Near the origin that slope is steep, so marginal return is high; as spend approaches saturation the slope flattens towards horizontal, and marginal return collapses towards zero even though total revenue is still rising. That curve is exactly what saturation curves and budget ceilings describe, modelled mathematically with the Hill function and shaped over time by adstock. Marginal ROAS is simply the way that curvature shows up in the one number planners care about.
How ElenIQ uses marginal ROAS
ElenIQ is built around the principle that budget should be allocated at the margin, before the money is committed. Rather than reporting a single blended ROAS, it fits a response curve to your historic channel data and forecasts the marginal return at every budget level - so you can see exactly where the next pound stops being worth it on each channel, and where it equalises across them to give the efficient split for your budget.
You can pressure-test that directly with the marginal ROAS calculator, model a full reallocation in the ad spend forecasting tool, or build the whole plan around forecasted response when you create a media plan.
Related terms
- Marginal ROAS vs average ROAS - the full comparison of next-pound efficiency against the blended average.
- iROAS - the incremental revenue advertising actually creates, net of baseline demand.
- Hill function - the curve used to model how response flattens near saturation.
- Adstock - how advertising impact carries over after exposure.
Frequently asked questions
What is marginal ROAS?
Marginal ROAS is the return on the next pound of advertising spend - the additional revenue created by adding a little more budget to a channel. Unlike average ROAS, which blends every pound spent into a single backward-looking ratio, marginal ROAS describes the efficiency of the next increment, which is what actually determines whether scaling a channel is worthwhile.
How is marginal ROAS different from average ROAS?
Average ROAS is total revenue divided by total spend - a blended figure that looks backward across everything spent. Marginal ROAS is forward-looking: it is the return on the next pound. Because paid media response curves flatten as spend grows, marginal ROAS is almost always lower than average ROAS at scale, so a channel can show a healthy average while its next increment of spend is barely breaking even.
Why does marginal ROAS matter for budget planning?
Budget decisions are made at the margin. The question is never what a channel returned on average last quarter, but how much new revenue the next pound will create. Planning on average ROAS systematically over-invests in channels that are already near saturation, because their blended average stays attractive even after marginal return has collapsed. Marginal ROAS reframes allocation around the efficiency of the next pound.
How do you calculate marginal ROAS?
Marginal ROAS is the slope of the spend-to-revenue response curve at your current budget - the extra revenue produced by a small increase in spend, divided by that increase. It cannot be read off a single blended ratio; it requires a model of how response changes with spend, which is why it is estimated from a fitted saturation curve rather than from a fixed historic multiplier.