To correctly size a pond filter, you will need to know the volume - how many gallons of water are in your pond. If you don't already have this calculation, you can use the formulas below (formulas are based on measurements in feet and inches).
For a rectangular or square pond: length x width x depth x 7.5 = gallons
For an oval pond with straight sides: length x width x depth x 6.7 = gallons
For a circular pond: diameter x diameter x depth x 5.9 = gallons
Now that you've calculated the number of gallons, sizing the filter is easy. The rule of thumb is that you should have a filter that is rated for about 1.5 times the number of gallons in your pond. So if you calculated your pond contains 400 gallons, 1.5 x 400 would mean a filter rated at 600 gallons per hour (gph). All pond filters will have a flow rate measured in gph.