Z-factor, or ideal gas deviation factor, describes the departure of the gas behavior from that of the Ideal Gas Law (PV = nRT), which does not account for variation in forces between molecules as a function of pressure. For molar volume (n=1), the Real Gas Law is

PV_m = zRT

where P is pressure, V_m is molar volume, R is the ideal gas constant, T is absolute temperature and z is the ideal gas deviation factor.

Note that z-factors greater than 1 mean the real gas volume is greater than the Ideal Gas Law would predict for the given pressure and temperature conditions, while z-factors less than 1 indicate that the real gas is more compressed than the ideal gas prediction. For the Bering Basin conditions described below, z-factor increases with depth.