ok... can you give me the formula that you're using to get that? cause i'm just not understanding here...

Length x width x average depth = cubic feet if it were a rectangle

rectangular cubic feet x 0.75 free form multiplier = approximate free form cubic feet

free form cubic feet x 7.48 gallons per cubic foot = gallons

so 16ftx40ftx5ft = 3,200 cubic feet

3,200 cubic feet X 0.75 = 2,400 cubic feet

2,400 cubic feet x 7.48 gal/cubic foot = 17,952 gal

Edit: There are also a lot of steps and benches in there. The 0.75 free form multiplier is a pretty rough estimate, and should not be relied on. I'd go by the volume the manufacturer lists, as they likely know best, and it seems like a reasonable volume using rough back of the envelope calculations.