The net registered tonnage (NRT) is derived from the gross registered tonnage by deducting a number of rooms that are included in the gross registered tonnage and which are not in the net registered tonnage: the quarters intended solely for the captain; the navigation room, the radio cabin, the boatswain warehouse etc; the spaces used by the screw shaft and the auxiliary machines with boilers and the spaces used by the auxiliary machines and boilers for the propulsion. The spaces of the machines and boilers for propulsion are included, yet the deduction is fixed.
This definition of the net registered tonnage of the ship is in accordance with the Oslo Convention and no longer applies. This was replaced by a new, more universal definition in accordance with the convention of London of 1969. According to the convention of London, the net tonnage is calculated independently from the gross tonnage.