Morphometric characteristics play an important role in the classification and modelling of fluvial systems. With increasing numbers of detailed surveys of subaquatic canyon and channel systems in lakes, reservoirs and oceans, previous research has repeatedly tried to draw parallels between poorly understood subaquatic systems and much studied subaerial fluvial systems. Often, these studies only considered a few morphometric characteristics, because an efficient tool for the determination of these characteristics was missing. We developed a Matlab script for determining the most important morphometric characteristics of subaerial and subaquatic rivers, channels and canyons. The only inputs required are the levee or bank crests that enable the definition of a centreline as the basis of a channel-centred curvilinear coordinate system and the calculation of planform characteristics (e.g., bankfull width, gradually varying curvature, sinuosity). If bathymetry is also available, it is converted into the same channel-centred curvilinear coordinate system thus enabling the determination of the longitudinal bed slope and further morphometric characteristics in cross-sectional planes (e.g., bankfull depth, cross-sectional area, and slopes of banks or levees). The script is particularly appropriate for the poorly understood subaquatic systems and will contribute to the understanding of the largest canyon and channel systems on our planet.