What Is CPM? Definition, Formula and How It Compares With CPC and CPA
In short: CPM (cost per mille) is the price you pay for 1,000 ad impressions. The formula is CPM = (cost ÷ impressions) × 1,000. Use it to compare how expensive it is to reach an audience. It says nothing on its own about clicks or sales.
What does CPM mean?
CPM stands for cost per mille. “Mille” is the Latin word for a thousand, so CPM is simply the cost of 1,000 impressions. An impression is counted each time an ad is shown to a person, whether or not they click on it.
Publishers and ad platforms often sell space on a CPM basis because it is simple to compare. A CPM of 3 means you pay 3 (in your currency) for every 1,000 times the ad is shown.
The CPM formula
Example: you spend 2,000 and your ad is shown 500,000 times. CPM = (2,000 ÷ 500,000) × 1,000 = 4.
Rearranged, the same relationship gives you a budget or an impression count:
You can try all three in the CPM calculator.
CPM vs CPC vs CPA
| Model | You pay for… | Best suited to |
|---|---|---|
| CPM (cost per mille) | Every 1,000 impressions | Brand awareness and reach |
| CPC (cost per click) | Each click on the ad | Driving website traffic |
| CPA (cost per action) | Each sale, sign-up or other defined action | Performance and lead generation |
With CPM the advertiser carries the risk that people may not click. With CPC or CPA, more of that risk moves to the publisher or platform, which is why the headline prices differ.
Converting between CPM and CPC
If you know your click-through rate (CTR, as a fraction such as 0.01 for 1%), you can convert:
Example: a CPC of 0.50 and a CTR of 1% (0.01) gives an equivalent CPM of 0.50 × 0.01 × 1,000 = 5.
What is eCPM?
eCPM (effective CPM) expresses earnings or cost per 1,000 impressions when the underlying pricing is not CPM. Publishers use it to compare, for example, a CPC campaign with a CPM campaign on the same ad slot:
What affects CPM?
- Audience: narrower or higher-value audiences generally cost more to reach.
- Platform and placement: the same audience can price differently across networks and ad positions.
- Ad format: video, rich media and native formats are usually priced differently from standard banners.
- Location and season: CPMs commonly rise during peak shopping periods and vary between countries.
- Competition: in auction-based systems, more advertisers bidding for the same audience pushes prices up.
Limits of CPM
- An impression does not prove that the ad was seen or noticed.
- A low CPM is not automatically good value if the audience is poorly matched.
- CPM should be read alongside outcome metrics such as CTR, conversion rate and cost per acquisition.
This guide is general educational information, not financial or professional advertising advice. Pricing and reporting rules differ between platforms, so check the definitions used by the network you buy from. See our editorial policy for how we write and review content.