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CPM Definition
CPM, or cost per thousand impressions, is the marketing world's metric for judging the merits of different media buys. Offline, CPM is calculated by taking the total cost of a given ad buy, dividing it by the total estimated viewership of a given advertisement, and multiplying the total by 1000. Here's an example: You buy a magazine ad for US $5,000. The magazine's subscriber base is 50,000. Therefore, the CPM will be ($5,000/50,000) x 1,000, or $100.
On the Web, CPM is a little different. Since it's so difficult to accurately determine the total number of visitors to a Web site, the CPM is calculated using the number of actual ads served. The distinction is subtle, but critically important: in the offline world, marketers simply guess how many times an ad is seen, whereas on the Web, we know.
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