To determine the correct filter cartridge size, first calculate the volume and capacity of the pool. Use the capacity of the pool to determine the desired flow rate per minute, examine the specifications of different filters, and determine which matches the target flow rate.
To determine the flow rate of a rectangular pool, multiply the length, width and average depth. For an oval pool, multiply half the length, half the width, the average depth and 3.14 for pi. Use the volume to calculate the capacity by multiplying the number by 7.48. This gives the pool capacity in gallons.
Next, determine the flow rate, or volume or water flowing past a given point. This measurement is in gallons per minute or hour. Divide the pool's capacity by the desired turnover time in hours, which yields the flow rate per hour. Divide this number by 60 to produce the flow rate per minute. An eight-hour turnover rate is common.
Finally, look at the specifications of pool filters to determine if they can handle the capacity of the pool. Multiply the filter area by the filter rate in the specifications. This gives the filter flow rate in gallons per minute. Look for a filter that matches the desired flow rate determined by the above calculations.