The CPA formula
Two terms, both slippery. The numerator should be every dollar that bought the conversions: media spend at minimum, agency and creative cost if you want the honest version (that stricter read is CAC territory). The denominator is whatever your platform counts as a conversion, and platforms do not agree.
Worked
Five examples across spend scales
| Spend | Conversions | CPA | Read |
|---|---|---|---|
| $500 | 8 leads | $62.50 | Local service tester budget. One bad week moves the number a lot; small denominators are noisy. |
| $5,000 | 100 sales | $50.00 | The clean textbook case. Same arithmetic as the homepage dial. |
| $25,000 | 410 trials | $60.98 | SaaS trial motion. The trial is not revenue; pair with trial-to-paid rate before calling it good. |
| $80,000 | 1,150 orders | $69.57 | DTC scale. Watch blended vs per-channel; one channel is usually dragging. |
| $250,000 | 2,900 conversions | $86.21 | Enterprise spend. CPA usually rises with scale as targeting broadens; budget for the curve, not the first month. |
Arithmetic-checked against the calculator engine: spend ÷ conversions, rounded to cents.
The denominator
What counts as a conversion
Google's glossary defines cost per action as total conversion cost divided by conversionsGoogle Help · checked 2026, with the advertiser choosing which actions count. Meta's cost per result divides spend by results, where the result is the campaign objectiveMeta Help · checked 2026. Same formula shape, different denominators: a Meta lead-objective result and a Google purchase conversion are not the same event. Full treatment on cost per action vs cost per acquisition.
Blended vs channel
Two CPAs, both true
Channel CPA divides one channel's spend by its attributed conversions. Blended CPA divides everything by everything. Worked: $8,000 producing 160 conversions is a $50 channel CPA; $4,000 producing 50 is $80; the blend is $12,000 ÷ 210 = $57.14. Platforms self-attribute generously, so the blend is usually the honest read under automated bidding. Blended CPA covers when to trust which.
The distortion
Attribution windows move the denominator
A 7-day click window books conversions days after the spend that bought them. Cut a budget on Monday and the window keeps crediting conversions through Sunday: CPA looks great exactly when you stopped paying. Compare CPA only across periods measured on the same window, and read week-one CPA after any change as provisional. CPA in marketing places the metric in the funnel.