A method for calculating pore pressure of a subsurface includes the steps of obtaining a resistivity value while drilling in a region wherein there is a shallow depth (reference depth) where shale is in a hydrostatic condition, or where shale is not in hydrostatic condition but where the pore pressure at such depth can be estimated and calculating, using a processor, a pore pressure at a drilling location based on the following Formula (I): PP=OvB−(OvB−Hyd)(R/R0)øn wherein PP is pore pressure, OvB is the overburden value associated with the drilling location, Hyd is hydrostatic pore pressure, R is a measured value of resistivity, R0 is the resistivity at a reference depth or is an imposed value of resistivity, and øn is a normalized value of porosity.