1. Introduction<br> Pile foundation response during earthquakes is strongly affected by nonlinear soil-pile foundation interactions. The damages to pile foundations during the 1995 Hyogo-ken Nanbu Earthquake and the 2011 Tohoku-Chiho Taiheiyo-Oki Earthquake were obviously attributed to nonlinear interactions of soil-pile foundation-superstructure. In addition, the vibration properties of soil and superstructure made the damages complicated. Therefore, in order to sophisticated seismic evaluation of the pile foundation, the nonlinear lateral soil resistance around pile should be properly evaluated and it should be incorporated in the seismic response analysis model. However, only few studies have been reported on the evaluation method of the nonlinear lateral soil resistance around pile. Therefore, the purpose of this study is to propose the simple formula to calculate the initial stiffness and the nonlinear lateral soil resistance around pile. The parameters of the simple formula are the shear wave velocity, the internal friction angle and the diameter of the pile. An additional purpose is to propose the simple formula to calculate the group factor of the nonlinear lateral soil resistance around pile for integrated pile model.<br> 2. Method<br> In this study, the nonlinear lateral soil resistance around piles is calculated using the soil-pile system modeled by 3D-FEM. When calculating the soil spring around piles, the piles are modeled as excavated soil and uniform forced displacement is given to the whole of the piles. The soil springs that are calculated are normalized in the diameter of the pile and the shear stiffness. The normalized soil springs are then approximated using a second-order logarithmic function. From the relationship between coefficient calculated by approximation and the internal friction, the simple formulas are determined. The group factors of the soil spring for the integrated pile model are calculated. The integrated pile model is, obtained by modeled as one pile which has integrate the mass and stiffness of all piles. The simple formulas are determined from the relationships between the group factor and two of the influencing factors. Two of the influencing factors are the number of piles and the spacing of the piles.<br> 3. Conclusions<br> The major findings obtained from this study are summarized as follows:<br> (1) The simple formula for determining the initial stiffness of the soil spring is shown in Eq.(4).<br> (2) The simple formula for determining the nonlinear lateral soil resistance around pile is shown in Eq.(5) and Eq.(8). The soil spring defined in the simple formula correspond to soil spring calculated by 3D-FEM.<br> (3) The simple formulas for determining the group factor of the soil spring for the integrated pile model are shown in Eq.(10), Eq.(11) and Eq.(12). The group factors defined in the simple formula correspond these calculated by 3D-FEM.<br> (4) The calculation formulas proposed in this study are greatly useful in modeling pile foundation for the seismic response analysis.