In this paper the results are presented of a parametric analysis concerning the evaluation of the amplification characteristics of surficial deposits of homogeneous cohesive or granular soils with nonlinear behaviour. The analysis has led to the definition of empirical formulas, able to determine an estimate of the period for which the amplification function of the stratum exhibits the first peak. This estimated value, called 'equivalent fundamental period', can replace in the applications the fundamental period usually calculated under linear hypotheses.