Proper evaluation of forces exerted on a solid boundary by liquid sloshing is difficult. If the free board in a liquid storage tank is insufficient, the sloshing waves caused by seismic excitation will collide with the tank roof and may cause major damage. The current study investigated the sloshing wave impact force (SWIF) in full-scale liquid storage tanks using numerical simulation based on the lattice Boltzmann method (LBM). Several shaking table tests have been conducted on a small-scale rectangular tank to validate the numerical model. The results of a standard dam break test have been used to express the validity of the proposed numerical model. This comparison confirms the validity of the numerical strategy for simulating the effect of sloshing. After validating the numerical model, it has been applied to a practical parametric study of SWIF in full-scale liquid tanks. The results of numerical simulation indicate that the simplified method recommended by related codes and standards for calculating SWIF in liquid tanks significantly underestimates the sloshing force. This confirms that the dynamic nature of sloshing should be considered in the design process of liquid storage tanks.