What Size Pool Filter Do I Need?
The size of the pool filter that you need will depend on the type of pool you have, the size of your pool as well as the type of filter you are buying. Above ground pools have smaller plumbing systems for the most part than inground pool installations however the selection and sizing process for a new filter looks pretty much the same for both with only one notable exception. When people talk about a "cartridge filter" for a pool they are usually talking about a robust, expensive filter system that represents one of the best filter choices for any pool.
With above ground pools, and especially entry level above ground pools, a when someone is talking about a cartridge filter they are not meaning a large pressure vessel tank with 200, 300 or 400 square feet of filter media inside, they are usually talking about single paper filter element that is perhaps 25 or 50 square feet of media. This is a point of confusion for some people since you can end up reading that cartridge filters are the best filter option you can get for a pool, while also reading about how cartridge filters are useless and you need to upgrade to a sand filter. Let's just go ahead and clear up this confusion once and for all.
Cartridge filters for above ground pools - While cartridge filters can be the best option for pool filtration systems the ones that typically come with above ground pools are usually very small. The smaller a paper filter is the less debris it can handle before it begins to restrict flow from the pump. Also the smaller the surface area of the filter paper, the less overall flow the filter will be able to handle. Often above ground pool owners look to upgrade to a sand filter when in reality a sand filter is the lower end option of the two filters. But in this instance you are upgrading from a toy paper filter essentially, to an actual pressure vessel filter tank filled with silica sand. This is a good upgrade to make for many entry level pool owners, but even better would be to upgrade from a tiny, toy-like paper cartridge filter to a cartridge filter pressure vessel tank. In the end it all comes down to flow rate potential and the amount of surface area of the filter media.
Why are pool cartridge filters so much better than sand filters? This comes down to flow rate potential and how fine the filter media is. When sized properly for a pool system, unlike the toy filters that come with cheap pools, a cartridge filter is going to remove material up to three times finer from the water than a sand filter can. In addition to being able to filter finer particulate, the cartridge filter also has a better, more efficient hydraulic design. The multiport valve on a sand filter, which controls the function and flow of water through the head and body of the filter, is a huge restriction to flow. This makes sand filters able to handle a lot less water flow than suitably sized cartridge filter units.
1) Maximum design flow rate
2) Expected pump output flow rate
3) Volume of the pool
4) Desired turnover rate
5) Hours per day you run your pool pump
These five steps are needed for every swimming pool owner in order to determine which size of filter you should be getting for your pool. This is not everything that you need to know, just the first calculation that you need to do for every type and size of pool filter to start answering your initial question "What size pool filter do I need?" If you can answer these questions then you are almost all of the way there to making an informed decision for your next filter purchase.
What Size Sand Filter Do I Need?
If a sand filter is what you are wanting for your pool then you will see that they are sized by both physical size, like a 22" filter or a 24" filter, but you will also see them designated by weight as in a 200 lb filter, or a 300 lb filter. All of this information is useful but not the first thing you need to look at deciding what size of pool filter you need. We need to look at that list above to start narrowing down on the filter selection.
If we assume the maximum design flow rate for a sand filter you are looking at is 50 GPM, which would represent a small to medium sized sand filter value, which would correspond to perhaps 150 to 250 lbs of sand (and probably a 1.5" multiport valve). This is a published figure from the manufacturer for every filter, and you can even find this value printed right on the label on your sand filter.
Next we need to estimate the flow value from the pump (or use a flow meter of you have one. Every pump on the market has a published flow curve available for it. You might not be able to get the maximum value out of a flow curve, but you can definitely make some educated guesses for flow rate based on what you see. The flow curve will have flow rate on one axis, and the other axis will be resistance to flow measure if feet of head.
Head resistance - A pool with almost no resistance to flow would be 10 or 20 feet of head. Medium head would be 25 to 35 ft of head, and a pool with a lot going on like waterfalls, floor cleaners, solar heaters or lots of twists, turns and distance in the pipe runs might have 40, 50 or 60 foot pounds of head resistance. You can use these numbers to estimate how much flow your exact pump might put out for a low, medium, or high head pool which you should be able to estimate for yourself. Having a flow meter takes out all of this guesswork for pump flow, but without one this is the best estimation that you can do. Once you know your pump model and size in horsepower, you look up the pump curve online and compare your estimated head resistance to the flow rate that you can expect your specific pump to deliver.
We were working with the example of a sand filter that had a maximum designed flow rate of 50 GPM. If the pump we had was a Hayward Super 2 pump, 1.5 horsepower, and we assumed a medium amount of head resistance at 30, then the approximated flow rate according to the data posted by Hayward is the pump will deliver a whopping 126 gallons per minute approximately. That is two and a half times the maximum flow rate that the filter we are looking at can handle! The reason why is that the Super 2 pump from Hayward is not just "twice as good" as the Superpump. They are made for different applications. If you needed to push against 60 or even 80 ft pounds of resistance then the Super 2 would work well where the Superpump would struggle to move any water at all.
When we switch and look at the flow curve for the Hayward Superpump 1.5 horsepower instead we see that at 30 ft lbs of head resistance the pump will likely deliver around 73 GPM. What a difference! This is why pool owners who replace their super pump with a Super 2 are almost surely overdriving their filters. The Super 2 is an application specific pump for high head applications. However, as you can see in this example, even the Superpump is too powerful at 73 GPM. The 1 horsepower Superpump model still would have 65 GPM expected output, so dropping down to the 3/4 horsepower model the expected flow rate is around 52 GPM. As you can see when pool owners shop by horsepower they are almost certainly overdriving their filters, especially if they have a sand filter.
We now have a sand filter that is rated for 50 GPM and a 3/4 horsepower superpump which will move about 52 GPM on a system with 30 feet of head. Next we need to look at the volume of the pool and the expected turnover rate. You should turn over the water in your pool three times every 24 hours as the industry standard to achieve 95% of all of the water in your pool being filtered at least one time. Assuming a 20,000 gallon pool, which is about a 16 x 32' pool with a 3.5' shallow end and 6' deep end. At 20,000 gallons in volume we need to aim for 60,000 gallons daily for filtration totals. So next, how many hours per day do you run your pool pump?
24 hours @ 60,000 gallons = 41.6 GPM
12 hours @ 60,000 gallons = 83.33 GPM
8 hours @ 60,000 gallons = 125 GPM
What the heck these numbers don't add up at all! If you only ran your pump half of the day you would need 83.33 GPM in order to filter enough water. That is more than the filter can handle and more than the 3/4 superpump we chose can put out. Even worse is 8 hours of running time a day needing 125 GPM to achieve the total filtration volume. So what is the disconnect here? The disconnect is that largely pool systems are not built efficiently, pool filtration equipment is not sized properly, and pool owners do not understand filtration volume amounts or pool turnover properly. Really a breakdown in every step of the process. No wonder pool owners have so much trouble getting an answer to such as "simple" question as what size pool filter do I need?
What we have learned in the above example is that if the minimum turnover rate of three per day is going to be met, 60,000 gallons per day, the pump is going to have to run almost 24/7 to get there. At least one good thing to say here is that at least the pump no longer doubles the maximum flow the filter can handle, but as the owner of this pool you might not want to have to run your pump almost 24 hours per day. This is noisy and expensive for electrical consumption. This is why many pool owners simply ignore the recommended filtration volumes and instead undershoot these values significantly. It is not a good idea, but it is the answer to why these numbers are so different than what you have been doing to your pool, or how other people run their filtration system.
Sizing a pool sand filter - When picking a sand filter for your pool it is important to understand that your maximum flow rate will be an issue. Unless you are also buying your pump now and can size it to the filter, then you must pick a filter than can work with the pump that you have now. It would be a mistake to assume the last person who picked your pump and filter had any understanding of this filter sizing process you are learning about now. You should try to get the largest (highest) maximum design flow rate you can find in your sand filter, and this will normally be from filters with a 2" multiport valve instead of the 1.5" multiport valve filters. As a loose guide, think along the lines of 150 pounds of sand filter media per 10,000 gallons of water. A 20,000 gallon pool would need 300 pounds of sand...but remember that maximum designed flow rate and turnover rate for your pool volume are both more important than how many pounds of sand you have.
Testing for head pressure - If you want to guess as little as possible with your system then you do not need to estimate your resistance to flow. If you put a vacuum gauge into the suction side of your pump and a pressure gauge into the pressure side of your pump then you perform the following calculation to learn see a snapshot of your total dynamic head resistance to flow. With the pump running multiply the PSI on the pressure gauge by 2.31 and then perform a similar calculation on the suction side of the pump. The vacuum gauge reads in inches of mercury. With the pump still running you would multiply your vacuum reading by 1.13. Add both the suction side and pressure side numbers together, and the result is your resistance to flow in feet of head.
What Size Cartridge Filter Do I Need?
Just as with the sand filter example above when you are sizing a cartridge filter for your pool you will need to look at the same information. You need to look first at the maximum designed flow rate for the filter you are considering. Unlike sand filters however you will find that cartridge filters have vastly higher maximum designed flow rates. This is primarily because cartridge filters do not have a multiport valve which means they have a lot less restriction to flow. Even small cartridge filter have as much or more flow potential than even large pool sand filters.
In the sand filter example the pump had to be downsized to avoid sending far too much water through the filter. Since cartridge filters have such a high designed flow rate, our example filter will be able to handle 120 GPM even though it is only a medium sized filter at best. Next we have the same 20,000 gallon pool, which needs the same 60,000 gallons of flow through the filtration system every day. In this example we do not need to downsize the pump for the sake of protecting the filter, so the original 1.5 HP superpump at 73 GPM would work fine. Even the Super 2 high head pump from the first example at 126 GPM could probably even match up with this medium sized cartridge filter. This would give you a lot more filter options than with the sand filter which is going to need to run almost 24 hours per day. With the 1.5 HP superpump here are the 8, 12 and 24 hour turnover values:
24 hours @ 73 GPM = 105,120 Gallons
12 hours @ 73 GPM = 52,560 Gallons
8 hours @ 73 GPM = 35,040 Gallons
Wow, do you see how much of a difference the filter makes when choosing between sand and cartridge? With the sand filter we had to run a much smaller pump 24 hours per day almost to reach out 3x turnover goal of 60,000 gallons each day. With the 1.5 HP superpump instead of the 3/4 HP, and with a cartridge filter that can handle higher flow, you can now meet your total daily turnover a little over 12 hours. That is an improvement however this is only scratching the surface of how to make a filtration system more functional, and especially more economical, which you can learn more about in this article about how much electricity a pool pump uses. What if you had a huge pool volume and you needed more filtering than the Superpump can provide? What about the 1.5 HP Super 2 that we were looking at that was rated for 126 GPM? If you were to run that with the 120 GPM cartridge filter:
24 hours @ 126 GPM = 181,440 Gallons
12 hours @ 126 GPM = 90,720 Gallons
8 hours @ 126 GPM = 60,480 Gallons
Holy bananas look at how much water volume that is! On a 24 hour schedule that is enough water to turnover a 60,000 gallon pool three times. That is a 30 x 50' pool with an average depthjust over 5' deep. It is incredible to think that you could actually turn over a pool that large with something as small as a 150 st cartridge filter like this new model from Hayward, the C150S which has a 120 GPM maximum designed flow rate. However you would not want to filter a 30 x 50' pool with a single 150 sf cartridge element even if it can handle an impressively high maximum flow rate. You need to consider the effective filration area when sizing a pool cartridge filter. This will determine how long you will be able to go in between needing to open and clean your filter elements.
Sizing a pool cartridge filter - Since both a small cartridge filter and a large cartridge filter both have impressively high maximum flow rates then technically you could use either one on a given pool size. The only factor that would change between a smaller filter versus a larger filter is simply the amount of time you go between cleanings. When your filter pressure rises 7 to 10 PSI then it is time to clean your filter elements. If you have a 300 sf cartridge filter instead of a 150 sf cartridge filter then you will be able to go twice as long in between filter cleanings. When sized appropriately for the pool a cartridge filter should be able to go months in between needing to be cleaned, assuming the water was clean when you started the clock.
With all of these calculations and variables I am sure you are starting to see how dynamic the question really is, and asking what size filter you need is going to get you a lot of information but not necessarily a lot of answers. For the established commercial standards for swimming pool filtration and filter sizing we would use multipliers called FMR - Filter Media Rate. These established standards are applied to the filter media to determine how much (in square footage) you need for your pool filtration system. In order to calculate the actual surface area of filter media you would use the equation FA (filter area) = FR (flow rate) / FMR (filter media rate).
You can calculate the surface area for your specific filter, but this information is already provided on the manufacturers label, probably right next to where the maximum design flow rate was printed. If your filter provides a maximum design commercial filtration rate, that is the FA = FR / FMR calculation mentioned above. For residential pools instead of ultra-conservative FMR calculations it is accepted indusrty practice to use 150 square feet of surface area for cartridge filters per 10,000 gallons of water. A 20,000 gallon pool would use a 300 sf cartridge filter. Remember that cartridge filters are very capable, both small and large, and the only appreciable number you are changing is how long you will be able to go in between filter cleanings.
Too long, didn't read (TLDR) - In order to know what size filter you need for your pool you need to know your pool volume and then decide if you will follow the industry standard of three turnover of your pool volume daily. Decide on the total volume you want to filter in 24 hours (or the number of hours you want to run your pump daily) and take your filtration total in gallons and then divide by the hours your pump runs, and divide that number ny 60 to give you your GPM (gallons per minute) needed to be handled by your filter. Check the maximum designed flow rate for your filter and make sure your flow rate will not exceed this value. Check your pump flow curve and estimate (or calculate) your total dynamic head resistance (measured in feet of head) to see how much flow your pump will output on your system and make sure this matches up with your maximum design flow rate on the filter. For sand filters sizing is usually around 150 pounds of sand per 10,000 gallons of water where for cartridge filters it is normally around 150 square feet per 10,000 gallons of water.
There is another way that is much easier, much more simple, removes a lot of these hard limitations and actually ends up filtering your pool better for way less money and that is to get a variable speed pool pump. It will be law July 2021 that all pools must use a variable speed pump (or similar efficiency threshold) and this will allow you to set flow rates to whatever maximum or minimum value that you need for your pool or your specific pool filter...assuming you have a flow meter and know how much water your system is moving.
Match a variable speed pump with a suitably sized cartridge filter (150sf / 10,000 gallons) and you have a monster combination of energy efficiency as well as huge potential maximum flow volumes. If you want to filter your water efficiently, effectively and for as little money (and confusing calculations) as possible, that is the best you are going to do. A variable speed pump, a large cartridge filter and a flow meter so you can dial in your 3x turnover based on a 24 hour running schedule. It does not get any better than that when it comes to pool filtration.
A note on DE filters - I do not endorse DE filters (diatomaceous earth) as pool grade DE is not safe like food grade DE is for humans. Pool grade DE is toxic, is an airborne irritant, can cause silicosis since DE is not purged from the body and repeated doses can accumulate, which has been shown to cause nonmalignant respiratory diseases as well as lung cancer. Yes DE filters are even more effective than cartridge filters at removing find debris from the water, and yes DE filters also typically have high maximum design flow rates similar to cartridge filters, however I do not use or endorse DE filters for pools when cartridge filters are a perfectly acceptable and effective option without the constant use and exposure to things toxic to humans.
Want to see the maximum design flow rates for every brand, make, model and size of pool sand filters and cartridge filters? Pool Filter Flow Rates This buyers guide that I put together lists the maximum design flow rates and filtration area for every filter you should be looking at with the exception of the new Hayward C150S that I mentioned.
Pool filter reviews
More info on pool filter sizing
How much power does a pool pump use?
How much can you save with a variable speed pump?
Flow rates for common pool pipe sizes
Variable speed pool pump reviews
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