Many studies on the pressure propagation in porous media show that the relationship between the square of the radius of investigation and the producing time of a well is linear. The linear relationship is the pressure propagation equation (or pressure equation). However, these studies do not arrive at the same constant coefficient representing the linear relationship. The purpose of this study is to find the constant coefficient for the pressure propagation equation. The constant coefficient from this study is derived from analyzing pressure transient behavior and observing the boundary effect time (the starting time of wellbore pressure affected by the reservoir boundary). The results show that the constant coefficient in the pressure equation, i.e., the linear constant between the square of dimensionless radius of investigation and the dimensionless producing time is 17.82.