Ullage

Ullage is the empty space above a liquid in a tank, measured as the distance from the cargo surface to a fixed reference point at the tank top, the opposite of sounding, which measures liquid depth from the bottom. On tankers ullage readings convert through calibration tables to volume, then to weight after temperature and density correction, giving the cargo quantity for the bill of lading. Crews leave deliberate ullage for thermal expansion so a warming cargo does not overflow. Ullage is taken by tape, float gauge, or radar-level system, and the figures underpin custody transfer and loss control.