It is not always possible to use a single technique to determine the complete grain-size distribution of a natural sediment, namely when dealing with successions of variable grain size or with sediments or soils that contain silt–clay and sand–gravel populations. In this work we assess the feasibility of using laser diffraction to measure sand-size particles by analyzing the representativeness of laser samples and quantifying the deviations between results of sieving and laser diffraction. Four groups of sediments, with different grain-size distributions and proportion of heavy minerals, were compared. It is demonstrated that the risk of laser sample underrepresentativeness is higher when measuring grains coarser than 1 mm, in particular when clay–silt particles are also frequent. Depending on the size fraction, the divergence between the results of sieving and laser diffraction due to shape and density effects is variable and not easy to predict quantitatively. Given these uncertainties it is proposed to use different computational threshold diameters that separate the fractions considered by sieving and laser diffraction and measure part of the distribution with both techniques. We developed a flexible computer application that allows users to visually compare different possibilities of combined laser-sieving grain-size distribution curves and to choose the best option of threshold diameter. The best combined results can then be used to model the grain-size distribution.