From Longman Business Dictionary
tonnageton·nage /ˈtʌnɪdʒ/ noun [countableC, uncountableU]1the total number of tons that something weighsThe aim is to obtain the maximum saleable tonnage at reasonable cost.
2TRANSPORTthe size of a ship or the amount of goods it can carry, measured in tons → gross registered tonnage → net registered tonnage