How To Size a Pool Filter

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Before you can select the correct filter for your pool, you must first select the correct pump for your pool. Your pool filter must be matched to your pool pump. If the filter is too small relative to the pump. the pump will generate more water pressure in the filter than the filter is able to handle. This imbalance will quickly break down the filtering material of the filter which will greatly diminish the effectiveness of the filter. In more extreme cases, the high-pressure build-up may cause cracks in the filter tank causing it to explode. This guide covers how you should size your pool filter correctly.

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Related Article: How to Size an Above Ground Pool Filter 

Step by Step

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Step 1

SIZE POOL - A standard pool circulation rule is to be able to filter all that water in your pool at least once a day. Customers generally run their single speed pump for an 8 hour period, so they require a pump/filter system that will recycle their pool water in an 8 hr period. The first step then is to calculate the total gallons in your pool. 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.




Step 2

DETERMINE GPM REQUIRED - After you have the number of gallons in your pool (20,000 gallons for this example), you will have to calculate the Gallons Per Minute (GPM) required to filter 20,000 gallons in 8 hours. To come up with this flow rate first divide your calculated gallons by 8 for 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.

Step 3

DETERMINE Feet of Head - As water flows through the system piping between the pool and the pump, it will meet with resistance to flow according to factors like the length of the piping, number of turns (elbows) and the diameter of the piping. This resistance is called "Feet of Head or "Head". 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. However for a rough estimated use the average of the lengths of the suction lines 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.

Step 4

DETERMINE PUMP SIZE - Now that you have determined your pools required GPM and its Feet of Head, you have the information required to select your pool.'s pump size. Go to the description page of the style of pump you would like to purchase. Many pump manufacturers will provide a chart on this description page showing the HP required for your particular GPM and Foot of Head requirement. 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 that will handle 42 GPM with a 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.

Step 5

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 certain it can handle the flow coming from the pump. So in this case, instead of getting a filter rated at exactly 42 GPM, you should select one that is a little higher – around 60 GPM would be fine.

Click Here to View Available Pool Filters

Step 6

OTHER CONSIDERATIONS - There are a couple of other considerations that 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 a 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.

Step 7

DETERMINE 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 horse power 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

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(1 to 40 of 190)

 Posted: 10/18/2021  Latest

We have an 23,000 gal pool. We had all new equipment installed this summer. The company recommended a 1.65 HP pump and a 90 sq ft hayward cartridge filter. As new pool owners, we went with their advice. Recently, a different pool company told us that the cartridge filter is way undersized for our pool... what are your thoughts? what size cartridge filter should we have?
 Reply

InyoPools Product Specialist  Posted: 10/19/2021  Latest

I have to answer a question with a question, is it working for you? If it is, then it's okay. However, if you are experiencing pressure issues or need to clean the cartridge frequently to keep the filter pressure down, you may have undersized it. You have an entire pool season's worth of experience with the filter; did you have any concerns about the filter before the new pool tech mentioned it?Different pools have different variables. For example, a 15,000-gallon pool that experiences a heavy swimmer load would require a larger filter than 90 square feet. But a 25,000-gallon pool in a screened enclosure may be okay with just a 75 or 90 square foot filter.
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Anonymous  Posted: 9/18/2021 

My pool is in ground. It’s 18x30, goes from 3ft feet deep to 5.5ft. What size filter should I have. The pool contractor is trying to give me a 320
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InyoPools Product Specialist  Posted: 9/20/2021 

Your pool is about 15,000 gallons, which means you could use a cartridge filter as small as 100 square feet. But if you go with a smaller filter, you will need to clean it a lot more often than if you got a 4-cartridge model like the 320. You could find a middle ground by using a 175 sq ft Hayward Star Clear Plus or 200 sq ft Waterway ProClean Plus model. These single cartridge models are darn big, and they only have a single cartridge, so maintenance and upkeep should be cheaper than the 4-cartridge model.
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 Posted: 9/15/2021 

I've got a 13,000-15,000 in ground gunite pool with a Hayward 18x18 sand filter and a 1.0 HP pump. Reading that the newer 18" Hayward sand filters have problems splitting at the seams. Recommendations on other brands that offer 18" filters? Best cartridge filter to replace the sand filter?
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InyoPools Product Specialist  Posted: 9/16/2021 

We currently do not have another 18" sand filter, the closest we have is the 19" tank, Sand Dollar Top Mount Filters SD40 1.5" MPT W/Valve. A suitable cartridge filter would be a 90 square foot model like the Hayward Star Clear Plus 90 Sq Ft. Filter 2 Inch Ports.
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Anonymous  Posted: 9/7/2021 

30,000 gallon with automatic cover. Stays covered when not in use. Replacing pump and filter. The Polaris pb4-60 booster pump. Pool used 4 months a year. Thinking canister since it is easier. Have de now. Thinking Pentair products. Thinking the 420 or 520 with whatever pump makes sense, but really not sure what would be the best match.
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InyoPools Product Specialist  Posted: 9/8/2021 

The Clean & Clear Plus 420 or 520 would be good choices for that setup. I suggest going with a larger VS pump, like the Hayward TriStar VS Pump 2.7 HP - W3SP3206VSP. Your pool is covered most of the time, so you should be able to use the low RPMs schedule to keep your water clean. But, you will have the luxury of high-end performance if you need to do any chemical treatments that require turning over your water several times.
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 Posted: 8/27/2021 

I have a 2.0 HP Whisperflow Pump on a 12-15,000 gallon pool with an 20 year old Pentair Triton 2 Sand Filter. I'd like to go to a cartridge filter setup, any suggestions?
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InyoPools Product Specialist  Posted: 9/1/2021 

What is the model number of your current filter? An example of the model number formats for your filter would be the TR100, or 140264
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 Posted: 9/1/2021 

I have already removed the sand filter and replaced with a Pentair 420 Cartridge Filter. It works great.
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InyoPools Product Specialist  Posted: 9/1/2021 

Awesome!
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 Posted: 8/24/2021 

i have a 20000 gallon pool with a 1 1/2 hp motor, currently have a 60 sqft de filter and want to convert to sand. what size sand filter do i need?
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InyoPools Product Specialist  Posted: 8/25/2021 

To replace a 60 sq ft DE filter, I would suggest a 30-inch tank or larger. This will give you a close enough flowrate to the DE filter while also big enough to prevent excessive backwashing. I recommend either the Pentair Tagelus Top Mount Filter TA100 w/Valve - EC145240 or Hayward Pro Series Side Mount Sand Filter 4.91 sq. ft Less Valve
 Reply

 Posted: 8/14/2021 

Looking to replace my sand filter with a cartridge filter. 12000 gallon in ground pool. 1 1/2 Hp pump. 2 inch plumbing. What would be your recommendations
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InyoPools Product Specialist  Posted: 8/17/2021 

You could go with a 90 square foot filter or higher.
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Anonymous  Posted: 8/10/2021 

Hi ?? I have a 14x48 round pool and I’m trying to find out what filter pump I can get for this pool the one the pool brought is not that good
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InyoPools Product Specialist  Posted: 8/11/2021 

A 3/4 HP pump like the Pureline Pure Pro XL Pump - PL1500 should work.
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 Posted: 8/5/2021 

Hi, I have a 12,000 gallon In ground pool with a 3hp variable Pentair pump, but have a 320sq ft Pentair clean n clear plus cartridge filter. Is the filter to small for my pump and pool? If so which one do you recommend? I’m running for 8-10 a day at 2800 .
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InyoPools Product Specialist  Posted: 8/6/2021 

That pump and filter are a fine match with one another. If anything,  they are oversized for your pool. You should get plenty of life out of the cartridges between cleanings.
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 Posted: 8/2/2021 

I have an inground pool with a spill over spa. I'm probably close to 40k gallon for both. I have a Pentair single phase 2hp pump with 2 inch plumbing. Can I use a Hayward Swim Clear 525 sqft cartridge filter for my pool? I usually run my pool 12 hours a day.
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InyoPools Product Specialist  Posted: 8/3/2021 

Yes, the Hayward SwimClear 525 sq ft would be plenty of filter for a pool that size.
 Reply

 Posted: 7/21/2021 

I have an 18,000 gallon fiberglass pool that is 40x16 shallow end is 4' 1.5 and deep end is 6'5.5" what size cartridge filter do I need
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InyoPools Product Specialist  Posted: 8/3/2021 

For a pool that size a 120 square foot filter like the Star Clear Plus 120 Sq Ft. Filter 1.5" Ports or the Waterway Pro-Clean 125 Sq Ft Filter
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Anonymous  Posted: 6/12/2021 

I have Haywood 1100 filter need new filter should I get Haywood 1200 or newvswimclear Haywood sizec100s orc1505s
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InyoPools Product Specialist  Posted: 6/17/2021 

Ether would work. I always suggest going with the same size or bigger when replacing a filter to prevent any pressure issues. Plus, if you go bigger, you may go longer between cleanings.
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Anonymous  Posted: 6/2/2021 

I have a 21 ft round above ground pool, 52inches deep. I bought a 1.5 horsepower pump. What size sand filter should I have?
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InyoPools Product Specialist  Posted: 6/4/2021 

At least an 18" tank sand filter would be needed for that 1.5 HP pump. For example, the Sand Filter with Top Mount Valve 18" Tank - W3S180T would work pretty well with a 1.5 HP pump.
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 Posted: 5/17/2021 

Would a 24 inch, 300 lb sand filter work for a 24,000 gallon pool?
 Reply

InyoPools Product Specialist  Posted: 5/21/2021 

As mentioned in the guide, the size of the pump also plays a part in the size of the filter. What is the model number of your pump?
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 Posted: 5/15/2021 

The tank on my 20 year old Purex Triton Nautilus FNS DE filter burst. I need to replace the filter system. The existing filter has pipes at 9 1/2 and 17 inches above the concrete pad. New systems that I have seen have pipes at 8 and 15 1/2 inches. Can I put pressure treated boards under the new filter to raise it 1 1./2 inches? Or what would be my solution?
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InyoPools Product Specialist  Posted: 6/4/2021 

I'd prefer a cement block over wood paneling just for the longevity of the material.
 Reply

 Posted: 5/3/2021 

Have in ground pool approximately 7200gal have older model pentair de filter with 1hp motor. Would like to get a cartridge filter. What size filter should I get.
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InyoPools Product Specialist  Posted: 5/13/2021 

A pool that size could use the Clean & Clear 75 sq/ft Cartridge Filter - 160315.
 Reply

 Posted: 4/29/2021 

I have a 24,000 gallon in ground pool with 2" plumbing. There's a water feature w/1.5" plumbing that we only use occasionally. All of my equipment is old and starting to have issues. Would like to replace the DE filter with a cartridge type. Need help sizing the pump, motor, and filter.
 Reply

InyoPools Product Specialist  Posted: 5/14/2021 

Please provide model numbers of the equipment you are replacing. This is the easiest way for me to figure out what type of equipment should replace it.
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 Posted: 4/13/2021 

Looking for the correct size filter for an 18000 gal pool, with a 1.5 hp pump.
 Reply

InyoPools Product Specialist  Posted: 4/16/2021 

First, is this an inground or above ground pool? Second, what type of filter are you looking for? Sand, Cartridge, or DE
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 Posted: 3/3/2021 

I have an 18ft round pool 48 deep. What size pump and sand filter do I need?
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InyoPools Product Specialist  Posted: 3/4/2021 

This article is mainly for inground pool systems. Is your 18' round pool an above ground model? If so, this guide would be more helpful to you: How To Size an Above Ground Pool Pump and Filter System.For an above ground pool of your size, you would need a 1 HP pump, and at least a 16-inch tank sand filter. For example, this would be a suitable system for your pool: Carefree 16" Sand Filter w/ Hi-Flo II 1.0 HP Pump - 520-5307-6S
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