the volume by which a liquid container falls short of being full. the quantity of liquid lost from a container due to leakage or evaporation.

What ullage means?

: the amount that a container (such as a tank or cask) lacks of being full.

What is the ullage of a tank?

Ullage is the void space in the tank measured from the top of the tank to the upper surface of the fluid. Ullage is measured when the content of a tank is highly viscous and if the tank is filled to the maximum. Sounding pipe is constructed in the tank. ... Thus, sounding of the pipe becomes sounding of the tank.