Ullage

 

The empty volume in a tank that remains after any existing product is subtracted from the maximum capacity of the tank. Or, the volume in a storage tank that does not contain product.

 

Example: 12,000 Gal (maximum capacity of tank), 7,500 Gals (product in the tank) - the remaining available space (ullage) is equal to 4,500 Gals. (12,000 - 7,500 = 4,500).