Gross Tonnage (GT) is the volume of all enclosed spaces on a vessel. This includes the Engine Room and other non-cargo spaces. Most Maritime Regulations (SOLAS, MARPOL, etc.) apply to vessels based on their Gross Tonnage.
Net Tonnage (NT), on the other hand, is the volume of only the cargo-carrying spaces on the vessel. This is the tonnage that determines the earning capability of the vessel. Most port/anchorage dues apply to vessels based on their Net Tonnage.
Note:
Gross and Net Tonnage should not be confused with Suez Canal Tonnage, which uses a slightly different formula to calculate the toll for transiting the canal.
Gross Tonnage (GT) and Net Tonnage (NT) should not be confused with Gross Registered Tonnage (GRT) and Net Registered Tonnage (NRT). One still finds the occasional old seafarer referring to tonnage as GRT and NRT. GRT and NRT were used before the IMO Convention – International Convention on Tonnage Measurement of Ships (1969). Both are now obsolete terms. GT and NT replaced them in 1994 in line with the convention.