How To Size a Pool Filter

Pool Size - We suggest having a pump and filter that can cycle through your pool's volume within 8 hours. Depending on filtering and water chemistry demands, you may run the pump longer or shorter, but 8 hours is a good baseline mark.

To calculate your pool's volume in gallons, use the formula listed in this step's image. The formulas are listed according to pool shape. For example, if you have an L-shape, divide your pool into two rectangles, calculate their volume then combine the sums.

See our guide on How to Size a Pool Pump for Your In-Ground Pool for the formulas required to size your pool. For this guide, we will assume a typical in-ground pool size of 20,000 gallons.

Related Article - How To Size an Above Ground Pool Pump and Filter System

Determine Required Gallons Per Minute (GPM) - After you have the number of gallons in your pool (20,000 gallons for this example), you will have to calculate the GPM required to filter 20,000 gallons in 8 hours.

To calculate this flow rate, first, divide your calculated gallons by 8 Gallons Per Hour (GPH). Then divide that number by 60 to get Gallons Per Minute (GPM). For our example of 20,000 gallons that's: 20,000 / 8 hours or 2,500 GPH divided by 60 mins/hour or 42 GPM.

Calculate the Pool's Feet of Head (Feet of Water) - The pump's actual flow rate is determined by two factors, the horsepower rating the amount of pressure needed for the pump to prime itself. This resisting force, also called the Feet of Head, accounts for factors including pipe size, length of pipe, number of turns (elbows,) and rise in height the water has to overcome to reach the pump.

As Head increases, your pools actual GPM will decrease. Calculation of Head can be involved and is generally done during the initial installation of the pool. For a rough estimate, average the suction lines' lengths between the pool and the pump. Most basic in-ground pools have a Head of 40' to 50'.

We will assume 45' for this example.

Related Article - How To Size an Above Ground Pool Pump and Filter System

Determine Pump Size - Now that you have determined your pool's required GPM and its Feet of Head, we can select the pump.

Go to the description page for your pool pump of choice. The pump's description should include its flow chart. A pump's flow chart lets you see the pump's estimated flow rate according to the specific horsepower and your plumbing's Feet of Head.

For example, say you wanted the popular Hayward Super Pump. An abbreviated version of the Hayward Super Pump Performance Page is shown at the left. Based on the data above, we are looking for a pump to handle 42 GPM with 45 Feet of Head. According to the chart for 50 Feet of Head (closest above to 45'), we need a pump between 3/4 HP (31 GPM) and 1 HP (50 GPM). Since we always go to the higher GPM, we would select the 1 HP pump.

Determine Filter Size - The size of your pool filter is directly related to the pool pump you have selected. If your pool filter is too small for the pump, there will be additional strain on the pump motor as it tries to push water through and meets resistance at the filter. This will eventually burn out the pump motor, and your filtration will also be compromised.

We recommend over-sizing the filter to be absolutely confident it can handle the flow coming from the pump. So, in this case, instead of getting a filter rated at precisely 42 GPM, you should select one that is a little higher – around 60 GPM would be fine.

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Other Considerations - A couple of other considerations should be mentioned in your selection of a pool pump and filter. The above calculations are based on a basic pool configuration with no extra water features: fountains, spas, waterfalls, solar heating, and in-floor cleaning systems. The features generally require higher GPM rates which equates to higher HP pumps/filters.

Also if your pool requires greater than 60 GPM you may need at least 2" diameter suction pipes. Suction pipes of 1 1/2" have a physical limit of 60 GPM. 2" pipes can handle up to 100 GPM.

Related Article - PVC Pool Pipe Dimensions

Determining AG Pool Filter Size - The size of your system is mostly determined on the size of your above ground pool. The gallons found inside your pool will give you the horsepower needed for your pump. A simple rule to follow is you should be able to clear all of the gallons of your pool in an 8 hour period of time. You can use the following as a guideline.

• Above Ground Pool up to 24 Ft. Round = 1 HP pump
• Above Ground Pool over 24 Ft. Round = 1 ½ HP pump

Related Article - How To Size an Above Ground Pool Pump and Filter System