CPM measures cost per 1,000 impressions, CPC measures cost per click, and CPA measures cost per defined action. They describe different stages of advertising performance, so a campaign can have a low CPM and still have an expensive CPA.
Use them together to understand where costs change between exposure, response, and results.
The formulas
| Metric | Formula | What it helps you assess |
|---|---|---|
| CPM | Spend ÷ impressions × 1,000 | Cost of delivered exposure |
| CPC | Spend ÷ clicks | Cost of attracting a click |
| CPA | Spend ÷ actions | Cost of the chosen outcome |
Specify the click type and action. A link click and a profile click are different. A newsletter sign-up and a purchase are also different.
Google’s average CPC definition distinguishes the observed average from a maximum CPC bid. More generally, a reporting metric is not necessarily the rule used to charge for or optimize the campaign.
One campaign, three cost measures
Imagine a fictional campaign with €600 spend, 100,000 impressions, 1,200 link clicks, and 30 registrations.
- CPM: €600 ÷ 100,000 × 1,000 = €6.
- Link CPC: €600 ÷ 1,200 = €0.50.
- Registration CPA: €600 ÷ 30 = €20.
These figures describe the same campaign. None is a complete verdict by itself.
If the registrations are low quality, a €20 CPA might be poor value. If they become profitable customers, the same figure may be acceptable.
How CTR connects CPM and CPC
For matched spend, impressions, and click definitions:
CPC = CPM ÷ (1,000 × CTR as a decimal).
At a €6 CPM and 1.2% link CTR, the calculation is €6 ÷ 12 = €0.50 per link click.
This explains why cheaper impressions do not always produce cheaper traffic. If a lower-cost placement attracts much less response, the CPC can still rise.
How conversion rate connects CPC and CPA
When the action count is based on the same clicks:
CPA = CPC ÷ click-to-action conversion rate as a decimal.
A €0.50 CPC and a 2.5% click-to-registration rate produce a €20 CPA. If the conversion rate falls to 1%, CPA becomes €50 even though the click cost stays the same.
Do not apply this shortcut to mismatched reports that include different attribution windows or view-through conversions.
Which metric should you focus on?
For exposure planning, CPM helps estimate delivery costs. For traffic, CPC helps assess the cost of response. For a lead or purchase objective, CPA brings the chosen action into the comparison.
Still check quality. A campaign optimized for inexpensive clicks may attract people who rarely complete the intended action.
Calculate and explain the result
Use the CPM calculator, CPC calculator, and CPA calculator with numbers from the same reporting scope.
When a result changes, describe which part changed: impression cost, click response, or post-click conversion. That is more useful than saying only that advertising has become expensive.