Impressions are part of the CPM. CPM stands for 'Cost per thousand' (M = roman numeral for 1,000). When an advertiser purchases a web campaign, they either pay a rate for per thousand impressions or per click. Google sells campaigns normally as cost per click (CPC) and so you'll only get paid if someone clicks on a link.
So there is no direct formula between CPC and impressions, other than aggregate averages over a length of time. It also has a lot to do with the content you write about, how ads show up and the ads Google provides.
The more concise and narrow your content, the more able Google is to provide relevant ads (when available) for your content. The better matched the ads, the higher the click rate and thus the higher your revenue.
Hope that helps!