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The excavation of a shipwreck implies the quest for the ship’s tonnage. There are three methods to estimate the tonnage of a shipwreck: the graphic reconstruction, the tonnage formula and the weight of the cargo. The result is given in tons. But the confrontation with the written evidence is difficult. Quite often these sources mention tonnages in volume units, their value not being always known precisely. Beyond the issue of metrological conversion, the approach must consider the fact that the tonnages mentioned in the written sources are frequently different from the real load capacities, as they are the results of social processes.