The key factors that affect the ground settlement caused by shield tunnel are diameter, ground loss, engineering properties of soil and buried depth of shield tunnel. By means of the numerical analyses, this paper integrates all of these key factors into a simple formula. In addition, soil condition profoundly rules the ground settlement and thus, a modified factor in terms of soil condition multiplies by the proposed formula to be available for varied ground conditions. To verify the accuracy of the formula, the predicted results are compared with the monitored results obtained from Taipei and Kaoshiung MRT shield tunnels. It is expected that the proposed formula is able to serve as a basis for engineering practices.