when we measure electromagneticfield intensity ,the relationship between dbm and w/m2

as i known,electromagnetic field intensity can be measure it by w/m2 or V/m,cause we can use p/4πr2,but rf receiver usually use dbm to measure it,i know rf receiver' attena get rf power from space,so how w/m2 become dbm,when rf receiver stay in the same electromagnetic field intensity ,it can get different dbm with different attena?