This study first clarifies the definitions of the wave runup reduction factor used in this paper, the roughness factor in the EurOtop manual used in the USA and Europe, and the coefficient of equivalent crown height used in Japan, and then shows how the mean wave overtopping discharge changes as the reduction factor changes. After referring to Goda’s diagrams of wave overtopping discharge of vertical seawalls and wave-dissipating block covered seawalls and to the runup reduction factor estimated from recently carried experiments for runup reduction of both seawalls, the runup reduction factor was modeled as a function of installed water depth of seawalls. The experimental results of EurOtop’s block-covered breakwater are well predicted by IFORM using the proper runup reduction factor, and recent experimental data of seawall covered with wave-dissipating blocks are also well predicted by IFORM with safety evaluation for small non-dimensional wave overtopping discharge less than 10^-4