Cost per action (CPA)
Acquisition metric. How do users find and install the app?
Cost per action (CPA) is ad spend divided by the number of users who completed a chosen in-app action, such as a sign-up or first purchase. $20k of spend and 2.5k sign-ups is a CPA of $8 per sign-up (example inputs). Use CPI to compare install prices, and CPA to judge whether those installs turn into users who do something.
What is cost per action?
Cost per action (CPA) is the average ad spend for each user who completes a chosen in-app action, such as a sign-up, a trial start or a first purchase.
Target CPA (tCPA): a bid strategy in which the ad platform tries to win as many conversions as it can at or below an average cost you set.
Conversion window: the number of days after an install in which an action still counts towards the campaign.
How to calculate CPA
CPA = Ad spend ÷ Actions completed- Ad spend
- media cost for the same campaigns and period
- Actions completed
- the number of users who completed the named action, attributed to that spend
Where definitions differ
| Variant | Formula | When to use it |
|---|---|---|
| Named CPA | Cost per sign-up, cost per trial, cost per purchase | The same formula with the action in the name. Always say which action you mean. |
| Cohort CPA | Spend on one day's installs ÷ Actions by those installs within n days | Fair for comparing campaigns, because every install gets the same time to act. |
| Period CPA | Spend in a period ÷ Actions in that period | Simple, but mixes actions from older installs into this period's spend. |
CPA worked example
Example inputs, not a benchmark. Two made-up campaigns with the same spend, to show why the cheaper install is not always the cheaper user.
- Campaign A spends $20k, gets 10k attributed installs and 2.5k sign-ups. CPI is $2 and CPA (cost per sign-up) is $20k ÷ 2.5k = $8.
- Campaign B spends $20k, gets about 13.3k installs and 1.6k sign-ups. CPI is about $1.50, but CPA is $20k ÷ 1.6k = $12.50.
- Campaign B looks cheaper on CPI and is about 56% more expensive per sign-up, because only 12% of its installs sign up against 25% for Campaign A.
So CPI answers "what does an install cost?" and CPA answers "what does a useful user cost?". Bid and budget on the CPA for the action that predicts revenue, and keep CPI as a check on media prices.
CPA calculator
Example inputs: change them to your own numbers. The examples only make the calculator show a worked answer; they are not benchmarks or targets. Runs in your browser; nothing you type is sent anywhere.
- CPA
- $8
- Install-to-action rate
- 25%
- CPI (for reference)
- $2
CPA vs CPI: when does each one matter?
CPI matters when you are comparing media prices across networks or creatives, because every campaign is judged on the same install event. CPA matters when you are deciding where to put budget, because it prices the users who actually sign up, subscribe or buy.
Is CPA the same as CAC?
No. CPA prices any action you choose and is usually worked out from ad spend alone, while CAC prices a new customer and normally includes costs beyond media. They only match when the action is the first payment and you count ad spend only.
How CPA differs by vertical
| EdTech | The action is often a free trial or first lesson; cost per paid enrolment is the stricter view. |
|---|---|
| Fintech | Track a CPA for each step: sign-up, KYC complete, first deposit or first trade. |
| OTT and streaming | Trial start or subscription. A cheap trial CPA flatters when trial-to-paid is low. |
| Gaming | Actions such as tutorial complete or first purchase, often used as the event the ad platform optimises towards. |
Common mistakes
- Not naming the action, so two teams quote different CPAs for the same campaign.
- Comparing a 1-day CPA with a 7-day CPA.
- Optimising towards an easy action that does not predict revenue.
Questions
What is cost per action in mobile marketing?
It is ad spend divided by the number of users who completed a chosen in-app action, such as a sign-up or first purchase. It shows what you pay for users who do something, not just for installs.
How is CPA calculated?
Divide ad spend by the actions attributed to it within your chosen window. $20k of spend and 2.5k sign-ups gives a cost per sign-up of $8.
What is target CPA?
Target CPA is a bid strategy, not a measurement: the ad platform tries to get as many conversions as possible at or below the average cost you set. Your measured CPA is the result, and it can differ from the target.
Does the calculator send my numbers anywhere?
No. It runs entirely in your browser, and nothing you type is stored or sent anywhere.
Why do tools report different values for the same metric?
Because definitions differ: what counts as active, which attribution window applies, whether organic users are included and how days are cut by timezone. Align the definition before comparing numbers.
Which currency do the calculators use?
The calculators show US dollars, but every formula works in any currency: enter your own figures in the currency you report in and read the results in that currency.