A procedure for correcting pressure measurements in a reservoir to a common datum level. This is not required for calculating kh, permeability thickness, and s, skin effect, but is required for determining average reservoir pressures or for any comparison of pressures in one area of the reservoir to those in another area. The correction is done by determining the average pressure for a given well test in which the pressure gauge is at a known level, and then adding or subtracting the calculated weight of the column of reservoir fluid in pounds per square inch from the difference in elevation between the pressure gauge and the datum level.