The runoff coefficient is the percentage of rainfall that becomes runoff (and river discharge) during a storm, and it has a relevant role in engineering practice. It is, in fact, a key parameter in the so called "design-storm" method, in which hypothetical storm events of a given return period are transformed into flood discharges that are assumed to have the same return period. In order to investigate what values of runoff coefficient should be used in the design-storm method, in this study we analyse the correlation between event runoff coefficients and catchment characteristics, accounting for the relationship between the return periods of the storms and of the flood peaks.