How to calculate eCPM
Read the article about buying methods and payment models to better understand the difference between the often interchangeable terms eCPM and CPM.
You could also get the eCPM based on the metrics CPC (Cost per click) and CTR (Click-through rate), which is quite useful when you're buying CPC and have an estimate of what your CTR will look like:
|eCPM||=||Cost Per Click||x||Click-Through Rate||x||1000|
I also recommended using the CPM calculator to make sure you get your numbers right.