Cost per mille is the price of a thousand impressions, cost per click the price of a visit, and cost per acquisition the price of an outcome. Each measures a different step of the same funnel, so optimising the wrong one produces cheap traffic that never converts, or expensive conversions nobody scales.
What does each one actually measure?
They measure the cost of successive steps, not competing definitions of value.
A campaign can have excellent CPM and terrible CPA, which usually means the targeting is cheap and the audience is wrong.
- CPM — what it costs to be seen a thousand times. A price of attention.
- CPC — what it costs to get someone to act on being seen.
- CPA — what it costs to get the outcome you actually wanted.
Which should you optimise for?
CPA, whenever you have enough conversions for the platform to learn from. It is the only one of the three tied to something the business cares about.
The exception is early or low-volume campaigns. With too few conversions, optimising to CPA gives the algorithm almost nothing to learn from, and CPC becomes a reasonable interim target while volume builds.
Why does a falling CPM not always help?
Because cheap impressions are usually cheap for a reason. Inventory that nobody else is bidding on is often placements that perform badly, and a campaign can show an improving CPM while every downstream metric worsens.
Read CPM as a diagnostic rather than a goal. A sudden change usually means the auction or your targeting shifted, which is worth knowing regardless of direction.
When is CPC genuinely the right target?
When the click is the product — content, awareness, or driving traffic to a page that converts through a channel the platform cannot see.
It also earns its place as a debugging metric. A good CPM with a poor CPC points at creative; a good CPC with a poor CPA points at the landing page.
Frequently asked questions
- Is a low CPA always better?
- Not if it comes with no volume. A campaign converting three people cheaply is often less valuable than one converting three hundred at a higher cost, depending on what you can spend.
- Why did my CPM rise without me changing anything?
- Auction pressure. Competitors entering, seasonal demand and audience saturation all raise the clearing price for the same inventory.
- Should these be compared across platforms?
- Carefully. An impression means something different on each platform, so CPM especially is not comparable. CPA is the most portable of the three.