There are 7.48 gallons of water per cubic foot.
To figure gallons in your pond:(All measurements are in feet)
Rectangle: length x width x depth x 7.48 = gallons
Irregular Shape : Multiply above number by .8 to adjust for less than perfect rectangle
Circle: 3.14 x (1/2 diamter x 1/2 diameter) x depth x 7.48 = gallons
Flow rate of the pump is published by the manufacturer. Usually rated in gallons per hour, this information is found within your pump instructions.
Once you determine the gallons in the pond and flow rate on the pump, the proper UV can be selected.