Understanding eCPM and RPM in Google Adsense

Recently, we’ve heard some questions about the difference between eCPM (effective cost per thousand impressions) and RPM (revenue per thousand impressions), as the older AdSense interface references eCPM and the newer version shows RPM. We’d like to clarify and let you know that the terms are referring to the same thing: both eCPM and RPM represent your estimated earnings for every 1000 impressions.

To calculate this figure, divide your estimated earnings by the number of page views, impressions or queries that you received, then multiply by 1000.

For example:

If you earned an estimated $0.20 from 50 page views, then your page eCPM or RPM would equal

[$0.20 / 50] x 1000 =  $4.00.

Formula:
eCPM or RPM = (Estimated earnings / Number of page views) * 1000

To avoid confusion in the future, we’ll work to use only one term (RPM). RPM is a commonly used metric in advertising programs, and you may find it helpful for comparing revenue across different channels. You can learn a lot more about using RPM in our two part blog series (part 1 and part 2).

Source: http://adsense.blogspot.com/2011/11/understanding-ecpm-and-rpm.html