The Marginal Efficiency Calculator finds the daily spend level where paid media stops being profitable, by modeling diminishing returns explicitly. Ads platforms report your average CPA; decisions should be made on your marginal CPA, the cost of the next conversion, which is always higher when returns diminish.
The response curve is a power function: paid conversions = a times spend to the power B, where B (between 0.2 and 0.95) expresses how hard returns diminish and the coefficient a is derived from your current spend, blended CPA, and the share of conversions that are truly incremental. You set B as a scenario assumption rather than pretending to fit it from noisy data.
From that curve: average CPA at spend S = S^(1-B) / a, and marginal CPA = average CPA divided by B, always higher than average when B is below 1. Profit at S = conversions times contribution margin minus S. The profit-maximizing spend solves in closed form at the point where marginal CPA equals contribution margin, and break-even spend is where total profit crosses zero. Your current spend lands in a growth, scale, or loss zone with a recommendation, and a sensitivity table shows which input moves the answer most.
No CSV. Five inputs: daily spend, blended CPA from your ads platform, contribution margin per conversion (your break-even CPA), the percent of reported conversions that are truly paid rather than organic, and the diminishing-returns exponent B. Presets cover typical DTC apparel, SaaS lead gen, and supplement profiles, and scenarios encode into a shareable URL.
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