What is CPA?
Cost per acquisition
CPA is what you pay for one conversion — total spend divided by the number of conversions.
Formula
CPA = Spend ÷ Conversions
What counts as a conversion is your choice, and the choice matters. Counting form fills gives a low, flattering number. Counting closed sales gives a higher, more honest one. Whichever you pick, stay consistent — mixing them across reports is how campaigns come to look better than they are.
A good CPA is any CPA comfortably below what a customer is worth. A $25 lead is excellent for a roofer whose average job is $9,000 and ruinous for someone selling a $30 product. Work backwards from customer value and margin rather than looking for an industry average.
When CPA rises, check frequency first. The most common cause is that the same audience has now seen the ad too many times, which shows up as rising frequency before it shows up as anything else.