This paper deals with the determination of the minimum number of granulometric classes that should be used to calculate sediment transport of non-uniform grain-size materials. This is very important to reduce computational efforts in hydro-morphological mathematical models based on sediment continuity by fractions. In the case of log-normal river bed grain-size distributions the increment of the geometric standard deviation itself is already sufficient to describe the corresponding augment of the number of classes. However, strongly non-uniform river bed sediments (like those encountered in gravel-bed rivers) show completely different statistical characteristics when compared with the former ones. Herein specific relationships are developed which confirm that in these cases both the geometric standard deviation and the skewness of the grain-size distribution must be taken into account to define the minimum number of grain-size classes. This paper deals with the determination of the minimum number of granulometric classes that should be used to calculate sediment transport of non-uniform grain-size materials. This is very important to reduce computational efforts in hydro-morphological mathematical models based on sediment continuity by fractions In the case of log-normal river bed grain-size distributions the increment of the geometric standard deviation itself is already sufficient to describe the corresponding augment of the number of classes. However, strongly non-uniform river bed sediments (like those encountered in gravel-bed rivers) show completely different statistical characteristics when compared with the former ones. Herein specific relationships are developed which confirm that in these cases both the geometric standard deviation and the skewness of the grain-size distribution must be taken into account to define the minimum number of grain-size classes.