Ways to split ad budget — even, historical, marginal efficiency
How should I split ad budget across channels?
Split on marginal efficiency, not average efficiency.
The most common budget mistake is adding spend to the channel with the lowest average CPA. Channels usually face diminishing returns, so the efficiency of the next dollar differs from the average. Moving the allocation rule from average to marginal is the point of this comparison.
Side by side
| Method | Rule | Strength | Where it breaks |
|---|---|---|---|
| Even split | Divide by number of channels | Easy to explain and unbiased | Ignores efficiency differences entirely |
| Historical share | Keep last period's spend shares | Low variance and operationally stable | Keeps growing already-saturated channels |
| Average CPA weighting | Give more to the lowest average CPA | Intuitive and simple to compute | Ignores diminishing returns and overinvests |
| Marginal efficiency | Equalize the efficiency of the next unit of spend | Maximizes total outcome | Requires estimating a response curve per channel |
Which one to use
- When spend varied enough per channel
- Allocate on marginal efficiency. The curve is estimable.
- When spend was nearly flat
- No curve emerges. Vary spend deliberately first to widen the observed range.
- When channels differ by less than 1.2x
- There is little to reallocate. Look at creative or targeting instead.
- Before shifting a large amount
- Check saturation to see whether the channel is already in the flat region.
Frequently asked questions
Why not put everything into the lowest average CPA channel?
It is not recommended. Most channels lose efficiency as spend rises, so a good average can coexist with poor efficiency on the next dollar. The decision rule is marginal, not average, efficiency.
How is marginal efficiency estimated?
Fit a curve to the relationship between spend and outcome per channel, then take the rate of change at the current spend level. If past spend barely varied, that curve estimate is unstable.
Can I execute the recommended split as-is?
Moving everything at once is not recommended. A fitted curve is only trustworthy inside the observed spend range, so shift gradually, observe the result, and re-estimate.
Tools for this
- Budget Allocation for CPI, CPA & ROASSet a total budget or target CPI, CPA, or ROAS to calculate a recommended budget and automatic channel allocation without exceeding observed maximum spend.
- Campaign Saturation AnalysisCompare marginal CPA and ROAS by channel or campaign to separate scalable spend from saturated spend.
- Campaign Performance VarianceDecompose performance change into volume, rate, and mix effects to find the channels and campaigns behind the shift.