What Is CPA? (Cost Per Acquisition)
The average ad cost of getting a defined action, such as a registration, purchase or subscription.
Book a free callCPA (Cost Per Action / Cost Per Acquisition) is the average ad spend required to get a conversion action you've defined in advance. It's calculated as Total Spend ÷ Number of Conversions. The action can be a registration, a form submission, a first purchase or a subscription start. Unlike CPI, it measures not just the install but the real value that comes after it.
How to calculate CPA
The most common problem with CPA is an unclear definition. If "CPA" means registration for one team and first purchase for another, the reports stop being comparable. Best practice is to name the metric with its action: CPA (registration), CPA (first purchase) and so on.
Example
A subscription app spent $90,000 in a month and started 600 paid subscriptions.
CPA vs. CPI vs. CAC
| Metric | What it measures | Denominator |
|---|---|---|
| CPI | Cost of an app install | Number of installs |
| CPA | Cost of a defined action | Number of conversions |
| CAC | Total cost of acquiring a customer | Number of new customers |
The order is almost always CPI < CPA < CAC. Not everyone who installs registers, not everyone who registers pays, and CAC also includes sales and marketing costs beyond advertising.
Reading the three together shows where the funnel leaks: if CPI is low but CPA is very high, the problem isn't the ads; it's onboarding or the paywall.
How to set a target CPA
Target CPA isn't an arbitrary number; it's worked out backward from customer lifetime value.
When doing this math, it's critical to use net LTV: store fees, refund rate, payment processing fees and any cost of service should already be deducted. A target CPA set on gross LTV is the most common reason campaigns turn unprofitable as they scale.
tCPA (target CPA) bidding strategy
On Google and Meta, tCPA lets you tell the algorithm, "Bring me conversions at this cost per conversion." It needs two conditions to work well:
- Enough conversion volume: The algorithm generally needs at least 30-50 conversions per week to learn. Below that, tCPA behaves erratically.
- The right signal: If the conversion event fires incompletely or late, the algorithm steers toward the wrong users. Server-side measurement (CAPI, S2S) reduces that risk.
Setting the target too aggressively (well below your actual CPA) stalls campaign spend. In practice, stepping down gradually from your current CPA in 10-15% increments delivers more stable results.
How to lower CPA
- Fix the conversion funnel. Since CPA = CPC ÷ conversion rate, every improvement to the landing page or onboarding lowers CPA directly, without touching the media budget.
- Move the optimization goal down the funnel. Optimize for registrations instead of installs, and for purchases instead of registrations.
- Improve audience quality. Build lookalike audiences from converting users and exclude underperforming segments.
- Align creative and message. If the ad's promise isn't delivered on the destination page, the click is wasted.
- Account for measurement delay. Conversions are reported late, especially on iOS and SKAN; don't make panic decisions on early data.
Frequently asked questions
What is the difference between CPA and CPI?
CPI counts only the app install. CPA counts the valuable post-install action you define, such as registration, first purchase or subscription. Since not every user who installs takes that action, CPA is always higher than CPI and tells you much more about user quality.
What is a good CPA?
It depends on your own unit economics, not an industry average. If your CPA is well below your net customer lifetime value, the campaign is healthy. The same CPA can be very profitable for one app and unsustainable for another.
Why did my CPA suddenly go up?
Common causes: CTR dropping due to creative fatigue, CPM rising with seasonal competition, a change to the landing page or onboarding flow hurting conversion, or events breaking on the measurement side. Start by checking your change log.
Catch CPA spikes instantly
When costs rise above normal, Roasy AI pinpoints the problem at campaign level, so a bad day never turns into a bad week.
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