Question: how to measure water volume in pond?
Answer: Square/ Rectangular (Gallons): Multiply length x width x average water depth (feet) x 7.5. Example: 8' x 4' x 2' x 7.5' = 480 gallons.
Square/Rectangular (Acres): Multiply length x width, then divide by 43,560 (square feet per acre) ... 
Round (Gallons): Multiply diameter x itself x average water depth (feet) x 5.9.