The international data on tidal prism and cross-sectional area of tidal inlets and estuaries are examined as a basis for exploring the variability in the comparable UK data. The data are then used to evaluate the existing classification of estuary types; to present a number of relationships between the key bulk parameters of estuary volume, plan area, and cross-sectional area; and to test some theoretically derived relationships. The results suggest that the degree of Holocene sedimentation, and hence Holocene development, may explain much of the variability seen in the data, and this is not, as yet, fully accounted for in the theoretical relationships.