The CPM formula
CPM is the cost of one thousand ad impressions. The three values are tied together, so knowing any two gives you the third:
- CPM = cost / impressions x 1,000
- Cost = impressions / 1,000 x CPM
- Impressions = cost / CPM x 1,000
Use the switch at the top of the calculator to choose which value you want to find.
Worked examples
Finding CPM. A campaign costs $500 and serves 200,000 impressions. 500 / 200,000 x 1,000 gives a CPM of $2.50.
Planning a budget. A publisher quotes a $12 CPM and you want 50,000 impressions in a newsletter. The cost is 50 x 12, which is $600.
Checking reach. You have $1,000 to spend on a network charging $4 CPM. That buys 250,000 impressions.
For publishers: CPM and RPM
If you sell ad space on your own site, CPM tells you what to quote advertisers. RPM, revenue per thousand page views, is what you end up earning after the ad network takes its cut and after pages that show no ads at all. RPM is almost always lower than the CPM an advertiser pays.
Why CPMs vary so much
- Audience. Business buyers, high-income readers and narrow interests cost more to reach.
- Country. Impressions in the US, UK, Canada and Australia usually cost several times more than in other markets.
- Season. CPMs climb in November and December as retailers bid for holiday shoppers.
- Format. Video and native placements cost more than standard banners.
A low CPM is not a win if nobody clicks or buys. Pair this with the CTR and ROAS calculators to see whether cheap impressions turn into sales.
