The CPA formula
CPA = Ad spend ÷ Number of salesUse the amount spent and the purchases (conversions) your ad platform reports for the same period. In Meta Ads Manager, this is “Cost per purchase”.
How much can you afford per sale?
Break-even CPA = Order value − Costs per orderYour break-even CPA is the most you can spend in ads to win one sale before that sale loses money. If your CPA is below it, every sale is profitable after ads. The gap between them is your headroom.
Questions
What is a good CPA?
Any CPA below your break-even CPA. Lower is better, but a higher CPA can still be fine for products with large margins or customers who buy again.
Is CPA the same as CAC?
Close. CPA is the ad cost per sale; customer acquisition cost (CAC) is the cost of winning a new customer, often including more than ads. For a first purchase they are often the same.
Why does my CPA change so much?
Small numbers swing a lot: with 5 sales, one more or less changes CPA by 20%. Judge CPA over enough sales, usually a week or more.