In this paper, we examine several methods for calculating the reflection of irregular waves from a perforated-wall caisson breakwater using a regular wave model. The first method is the traditional regular wave approximation, which approximates the irregular waves as a single regular wave whose height and period are the root-mean-squared wave height and significant wave period, respectively. The second is to use the regular wave model, repeatedly, for each frequency component of the irregular wave spectrum. The wave period is determined as the reciprocal of the frequency, and the root-mean-squared wave height is used for all the frequencies. The third method is the same as the second one, but the wave height corresponding to the energy of each component wave is used. Comparison with experimental data of previous authors shows the second method is the most adequate, giving reasonable agreement in both overall reflection coefficients and reflected wave spectra. The first method, which uses the most conventional approximation of irregular waves and has been widely used because of its simplicity, gives large errors even in the overall reflection coefficients, leaving aside the fact that it cannot resolve the frequency-dependent nature of wave reflection.