Most salterns are distributed around the south and east coasts of England between the counties of Dorset and Lincolnshire. Five main groupings can be identified: north Lincolnshire coast; south Lincolnshire, Cambridgeshire and Norfolk fens; north Essex estuaries; south Essex and north Kent Thames estuary; Dorset, Hampshire and west Sussex estuaries and coast. Inland sites are restricted to a few localities in the west Midlands, notably Droitwich, Worcestershire, and Middlewich, Nantwich, and Northwich in Cheshire (Figure 3). A large number of fenland sites which now lie several kilometres inland were much closer to the sea during the Roman period before the sea began to retreat. The present distribution is extremely unlikely to be representative of the original distribution. Most, if not all, sections of low lying coastline would probably have supported salterns, particularly the larger river estuaries such as the Tyne, Mersey, Severn and Solway Firth, but remains there have since been buried or washed away.
Salterns are unevenly distributed in the areas where they are commonest. The greatest concentrations are found at a limited number of favoured locations: on the banks of estuaries, around inlets or small bays, on parts of the coast sheltered by islands or peninsulas. Clusters of 20 examples, each separated by 250m or less from its nearest neighbour are common, and clusters of more than 20 are not unknown. Such groups continue to be discovered when intensive fieldwork is carried out around estuaries in south-east England or in the Fenlands. Some of the sites within these groupings may be chronologically separable, but in most cases they are clustered to take advantage of a suitable location. Competition for the essential resources: sea water, clay and fuel would have been limited since all were available in large quantities.
Very few salterns are known or suspected in Scotland and Wales but a great number of examples have been recorded in other parts of Europe, particularly along the north and west coasts of France, Belgium, and the Netherlands. A great deal of salt-making also took place in the Mediterranean during the Roman period, but the processes used differed from those used in northern Europe.