CPA-to-CAC Scalability Matrix
Model how CPA inflates as you double and triple spend, budgets never scale linearly.
Tripling spend doesn't triple customers, at 18% decay your CAC climbs to $156 (30% higher). Check that number against LTV before you scale.
Decay compounds per doubling of spend, so returns bend as you exhaust your best audiences and placements first.
About this calculator
"Just double the budget" is the most common way a profitable channel gets wrecked, because CPA rarely scales linearly with spend. This calculator applies a decay rate you set to your current CPA to project blended acquisition cost at 2x and 3x spend, so you can see, before you commit budget, whether scaling breaks your unit economics.
How to use it
- Enter your current monthly spend and the CPA (or CAC) that spend is producing today.
- Enter your CPA inflation rate, how much CPA typically rises each time you double budget in this channel. Estimate it from your own history if you have it.
- Read CAC at 2x and 3x spend, plus the resulting customer volume and the CAC premium versus today.
- Compare 3x CAC against your LTV or max-CAC ceiling before committing to a scale-up.
Methodology
The model compounds inflation per doubling of spend: CPA at a given multiple is current CPA × (1 + decay rate) raised to the power of log₂(multiple). Tripling spend is treated as log₂(3) ≈ 1.585 doublings, so the 3x CPA sits between the 2x and a hypothetical 4x figure, not simply 1.5x the 2x number.
Customer volume at each spend level is that level's spend ÷ that level's CPA, showing that doubling spend produces less than double the customers once CPA rises.
The CAC premium at 3x is the percentage increase in CPA versus your current baseline, isolating how much more expensive each additional customer becomes as you climb.
This assumes decay compounds smoothly and uniformly, real auctions often decay in steps (a specific audience segment exhausts, then costs jump) rather than a smooth curve, so treat the projection as directional, and validate the decay rate against your own spend-history data whenever you have it.
FAQ
Auctions have a finite pool of your best-fit, lowest-cost impressions. As you spend more, the algorithm bids on progressively lower-propensity audience members to hit delivery goals, which raises the blended cost per acquisition, not the cost of your original audience.
Look at your own history: find two periods where spend roughly doubled and compare the CPA in each. The percentage CPA increase between those periods is a reasonable starting decay rate for this calculator.
Blended CAC at a spend level is the average cost across all customers acquired at that level, including the cheap ones from your original spend. Marginal CAC, the cost of the next customer specifically, rises even faster than the blended figure shown here.
Scale in smaller increments and re-measure decay at each step rather than jumping straight to 3x, expand into adjacent audiences or channels to add fresh, lower-cost inventory instead of over-saturating the current one, or accept a lower scale target that keeps CAC within your margin.