Measure the dimensions of the pool--average depth and either length and width for rectangular pools or the diameter for round pools--in feet.
Calculate the volume of your pool in gallons. For a rectangular pool, multiply the length of the pool times the width times the average depth times 7.5. If you have a round pool, multiply the diameter squared times the average depth times 5.9. For example, a rectangular pool 15 feet long, 10 feet wide and 6 feet deep would have a volume of 6,750 gallons.
Calculate the gallons per hour that you want filtered by dividing the volume of your pool by the number of hours you want the filter to take to turn over. Most pools turn over every eight to 12 hours. The more you use your pool, the more often your water should turn over. For example, a 6,750-gallon pool that should turn over every 10 hours needs a filter capable of filtering 675 gallons per hour.
Divide the number of gallons per hour your filter needs to handle by 15 to calculate the size of your cartridge pool filter in square feet. According to Pool Plaza, a cartridge pool filter is capable of filtering about 15 gallons per hour. Continuing the example, you would divide 675 by 15 to find that you need a 45-square-foot filter.