دانسیته نفت در شرایط سطحی که معمولا در واحد درجه °API بیان می شود.
API = 141.5/γo-131.5
γo is the specific gravity of oil (relative to water = 1, measured at STP).
The downhole density of oil (at reservoir conditions) can be calculated from the surface density equation using:
ρorc * Bo = ρo + Rs * ρg
ρorc : is oil density at reservoir conditions (kg/m3)
Bo : is the oil formation volume factor (rm3/stm3)
ρo : is oil density at standard conditions (kg/m3)
Rs : is the solution gas : oil ratio (sm3/stm3)
ρg : is gas density at standard conditions (kg/m3)