Biological vs. Mechanical Filtration
Filters are used to remove suspended particles from the water (mechanical) or they are employed to process the water biologically, removing nutrients and organic pollutants using bacteria.  A mechanical filter is attached to the intake of a pump and is submerged in the pond.  The filtration media is fine-pore polyester that traps the particle until it is removed and rinsed away.  The biological filter uses the same material but is located outside of the pond on the outlet side of the pump.  The water flows through the material, which is the bacterial colony that cleans the water.  Ideally a biological filter would be pre-filtered with a mechanical filter to keep the media as clean as possible to maintain peak efficiency.

Figuring the Pond Volume
Measure the pond length, width and depth in feet.  Multiply the three numbers to obtain the cubic volume of the water.  Multiply this number by 7.5 to get the total number of gallons of water.  If the surface of the pond is irregular, try to estimate the actual square footage as close as possible before multiplying with the depth.  The depth should be an average depth, not the deepest point.  
The formula for calculating the volume of your pond is:

L = Average Length
W = Average Width
D = Average Depth
V = Volume (in Gallons of Water)
L x W x D x 7.5 = V
x = multiply

An alternative to measuring pond volume is to use a water gauge for the garden hose that can be purchased at a hardware store. Another possibility is to record the amount of time it takes to fill a five gallon bucket, then time the filling of the entire pond and divide the total number of minutes by the time it took to fill the bucket X 5.