A comparative study was performed of three instruments used to measure the grain-size distribution of thirty sediment samples from shallow lagoonal flats: the hydrometer, the Sedigraph 5100 and the CIS-1. The hydrometer and Sedigraph are based on sedimentation whereas the CIS-1 uses the time of transition. The percentage of the samples accounted for by the <8 μm fraction was not affected by the technique used, but this was not the case with the clay fraction (<2 μm). Due to its relative independence from the analytical method applied, the <8 μm fraction can be used in ternary diagram classifications. This fraction also has an environmental significance in coastal lagoons in terms of hydrodynamics, organic enrichment and macrozoobenthos assemblages. The linear relationships obtained in this study may provide useful operational indications for similar studies.