noun
net tonnage
Maritime term for a ship's cargo-carrying capacity after deducting non-cargo spaces.
この船の純トン数は約5,000トンです。
The net tonnage of this ship is about 5,000 tons.
総トン数 is gross tonnage (total internal volume), while 純トン数 is net tonnage (cargo-carrying capacity).
Compound of 純 (net) + トン (ton) + 数 (number/measure), a direct translation of the English maritime term 'net tonnage'.