Adsense calculator is a tool that will help you the estimated earning potential based on your site traffic and impressions, but for that, you must be familiar with how impressions you are getting?
Adsense calculator will take the base of the average CPC and CTR and do the calculations of what happens if you have received these many impressions and these many clicks and the earning will be estimated but closely similar but to use this tool you must be aware with the average CPC