A pool filter cleans out the water of your pool as it passes through the system. The size of a pool filter is determined by its surface area, as measured in square feet. The most common pool filter types are cartridge filters, diatomaceous earth (DE) filters and sand filters. The different filter types have varying rates per square foot of surface area.

Measure the dimensions of your pool. For all pools, measure the average depth. For circular pools, measure the diameter. For oval pools, measure the long diameter and the short diameter. For rectangular pools, measure the length and width.

Calculate the volume of your pool. For circular pools, multiply the diameter times the diameter times the average depth times 5.9. For oval pools, multiply the long diameter times the short diameter times the average depth times 5.9. For rectangular pools, multiply the length times the width times the average depth time 7.5.

Determine how often your pool turns over. Turnover time is how long it takes all of your pool's water to go through the filter. Most pools take about 10 hours, or 600 minutes. If your pool is heavily used, you will want a shorter time, perhaps eight or nine hours. Little used pools can have turnover times of up to 12 hours.

Determine the flow rate of your pool's filtration system by dividing the volume of the pool, found in Step 2, by the turnover time in minutes, found in Step 3. For example, if your pool's size is 13,200 gallons and your turnover time is 600 minutes, your flow rate would be 22 gallons per minute (GPM). This should be the minimum for your filter.

Divide the minimum rate found in Step 4 by the filter rate for your filter. Sand filters usually have a filter rate about 15 GPM per square foot, DE filters can filter about 2 GPM per square foot, and cartridge filters are capable of about 0.25 GPM per square foot. For example, if you needed 22 GPM and you wanted a DE filter, you would need a filter size of at least 11 square feet.
Tips & Warnings
 You will most likely want a filter sized slightly larger than necessary because as the filter functions it will lose efficiency.