RPM in
Google Adsense means Page Revenue Per Thousand Impressions. RPM does not represent how much you have actually earned, it is estimate of your earnings for per thousand impressions. Suppose your average earnings on 100 Impressions is $0.20, then your RPM will be $2, and this is your estimate earnings for 1000 impressions according to your result of earnings for 100 page impressions. RPM always vary, not fixed. RPM just give us an Idea of our earnings per 1000 page views or what will our earnings for 1000 page impressions.
Formula to find out RPM
(Estimated earnings/Number of page views)*1000
For example you have earned $0.20 for 100 page views, then your RPM would equal;
$0.20/100*1000=$2.