Size Matters! 10 Ways to Know the Right Filter Size for the Pool

Is your swimming pool water getting gross?

Do you notice yourself changing or cleaning your filter way more often than you’d like?

Do you feel as though it takes forever to move water through your pool filter?

If so, it might be time for a new filter system!

Filter systems are a very important part of any swimming pool setup. Whether you choose to go with a traditional cartridge filter or you want something a little more upscale, like a sand filter, you need to have a filter installed to keep your pool functioning properly and ensure that the water is safe for you and your family to use at all times.

In this article, you’ll learn everything you need to know about how to choose the right filter size for pool needs, no matter what type of pool you have and how big it might be. You’ll learn why filters are important, what they can do to improve your pool’s water quality, and how to select the perfect system. Pretty soon, you’ll be shopping for a pool filter like a pro.

So what are you waiting for? Let’s get started.

What are Pool Filters?

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Pool filters are the systems that are installed as part of your swimming pool to help clean up the water and keep it free from physical debris like dirt and leaves as well as from bacterial contaminants. While most people use pool water treatments such as chlorine in conjunction with a good quality filter, it is possible to operate your pool with a filter only, depending on how safe the water is where you live. The cartridge portion of your filter may be installed on the side of your swimming pool, but it’s more likely that it will sit next to the pool and be attached to the pump that keeps it functioning instead. Sometimes, these items are clamped to the pool itself, and other times, they’ll need some sort of base—like a section of patio or another flat area—to be set up on.

What do pool filters do?

In short, pool filters do a lot! You might think of your filter as a part of your pool you don’t really understand, or even one you find a little bit irritating. After all, you probably have to clean it pretty often, and if you’re looking into replacing it, yours might not be working very well. It’s understandable if you feel frustrated with your filter, but remember that they have a lot of important functions and they’re very necessary. Check out this list of the things pool filters do for you.

  • Pool filters remove debris from your swimming pool. Dirt, leaves, and twigs can get into your pool easily, but with a filter in place, you won’t notice them floating in your water nearly as often.
  • Filters keep the water flowing, which reduces the risk of insects and parasites breeding in the pool. Mosquitos especially like to lay eggs in stagnant water. When you have a filter, the water is kept moving too much for mosquitos to want to breed there.
  • Filters keep algae from growing on your water. Algae also needs still water to grow, and it needs certain water conditions. With a filter, it’s much less likely to form in your pool water.
  • Filters can keep bacteria out of your water. Although they can’t get rid of all bacteria, they can make a huge difference in keeping you and your family healthy and safe from potentially polluted or contaminated water.

Why do I need a pool filter?

Now that you know what pool filters do, you probably understand more about why you need one in your pool. However, if you’re still not convinced, remember these few tips about why you need a filter as part of your swimming pool’s setup.

pool filter uses

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  • You and your family will be safer. You won’t risk catching waterborne illnesses as often, and you won’t be potentially injured by physical contaminants in the water.
  • Many pools won’t operate without a filter. This is for your safety as well as for the life of the pool.
  • Some regions require pool filters by law. Depending on where you live, there might be laws in place about filtration as well as the strength of the filter required. Be sure to check with your municipality for details.

Why does the size make a difference?

Okay, so now you know why you need a filter and how it can help. But does the size of the filter really make that big of a difference? Pool filters are all pretty much the same, right? Actually, they’re not! Read up on these tips about why pool filter sizes make such a big difference.

  • Smaller filters won’t get the job done. They simply can’t handle the number of unwanted contaminants present in a large pool, so you’ll have to change the filter media very often and may be cleaning much more frequently than you’d like. For cartridge filters, for example, size D pool filters are usually a good place to start.
  • Smaller filters may get fried trying to work for a large pool. They might stop working altogether, and if the pump is too small as well, you might have a piece of expensive broken equipment on your hands in no time.
  • Filters that are much too large can be dangerous. On the other side of the filter size issue, if you choose one that’s too big, it might have a high electrical current or just move water too quickly and powerfully to be safe for your pool.

10 Ways to Choose the Right Size Pool Filter

Sizing a pool filter doesn’t have to be too complicated. The tips in this section can help you figure out the best way to determine the right size pool filter for your needs and finally answer the question: is bigger really better?

1. Know the size of your pool first.

size of pool

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This is the surface area or the physical size of the pool itself—not the water inside. Don’t worry! If you’ve just had your pool installed you probably have some paperwork that states this number somewhere, but if not, you can calculate it easily. If your pool is a rectangle or square, measure the length and then the width. Multiply those together to get the area (A = L x W). If your pool is round, measure the distance across the pool, multiply it by itself, and then multiply that by 3.14 (A = 3.14 x r2).

2. Determine the volume of your pool.

Many filters are sold based on the overall volume of the swimming pool. Measuring the volume of your pool can be much more challenging than finding the area, so it’s a good idea to look for any paperwork you have that might give you this information. If you can’t find the volume but do know the brand name and model of your pool, you can usually look up its volume online. However, to calculate it yourself, you can use this formula: V = L x W x Average Depth x 7.5 for a rectangular pool. For a round pool, change 7.5 to 5.9.

3. Think about whether you have an above-ground pool, an in-ground pool, or a pop-up pool. 

Note that most pop-up pools must be used with the included filter, because many other varieties of filters have electrical currents that are too high to be used with a flimsier pool frame. Above-ground pools must be used with smaller filters than in-ground pools. Conversely, a small filter usually won’t offer enough power to filter even a portion of your in-ground pool, unless it is very unusually small.

4. Figure out the turnover rate of your swimming pool.

This refers to how long it takes your pool to cycle water through its current filter. Depending on where you live, this rate might be determined by local laws. Many places require a pool filter to cycle completely at least twice in a 24-hour period. However, it’s much healthier to choose a turnover rate of once every 8 to 10 hours. Some filters allow you to set this amount of time, while others come pre-fixed.

5. Think about the resistance against the flow of water in your pool.

flow of water

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The type of piping you have in your pool can make a difference when it comes to the resistance, or “feet of head,” you have against the flow of your water. It’s common for in-ground pools to have around 45 feet of head, but other types of pools may have more or less. Pool filters will specify how many feet of head they can handle, and your pool paperwork should give you the resistance number for your model.

6. Consider any water features you might have now or plan on installing in the future.

Waterfalls and fountains add to the resistance and turnover rate of your swimming pool, while floor cleaning systems can make a difference too. If you have solar heating or a hot tub that is partially attached to your pool (and will be sharing the same filter), these are also considerations you need to keep in mind.

7. Bigger is almost always better! This isn’t to say you should go out and buy a massive

commercial-size pool filter for a small square backyard swimming pool, but it does mean you can always stand to go up a size or two in terms of your filter. Calculate the numbers listed above and figure out which pool filters are recommended for your specific size first. From there, you can take a look at sizes that are slightly larger for the best possible results. If you choose a filter that’s too small, you run the risk of having very dirty water more often than not.

large pool filter

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8. Decide between sand, cartridge, and DE filters.

Each type of filter has its own pros and cons, and the size you choose will depend entirely on which type of filter media you go with. Sand filters, for example, are much preferred for in-ground pools than they are for above-ground ones. You should never buy a small sand filter because it simply won’t get the job done. DE filters can offer the highest level of filtration, so always choose at least one size larger than recommended for your pool. Cartridge filters will need to be changed much less frequently when you size up, too. Pool filter size D is a good choice for most above-ground smaller cartridge filters.

9. Buy the pump separately from the filter if you’re going bigger.

Many times, you can do a lot of damage if your pump has too much power for your pool. However, you might want the filter section that comes with these larger and more powerful pumps. It’s a good idea to make these purchases separately and then connect them with hoses or PVC pipes to give yourself the best of both worlds.

10. Think about how much money you want to spend up front versus over time.

 While it’s very true that smaller filters and those that come with their pumps already included may be much cheaper to set up, you might find yourself having to buy a replacement filter pretty shortly because the one you chose to begin with isn’t strong enough to get the job done. It’s also important to consider potential future repairs, and if you’re buying a cartridge filter, think about how often you might have to change your filter media if you choose one that’s too small for your pool’s needs.

Conclusion

Are you ready to head out and purchase your pool filter system now? With all this information, you should be prepared to make the right decision to help keep your pool operating cleanly and safely for years to come. Remember that, even though it might cost more up front, it’s always better to choose a larger and more powerful pool filter to begin with, so you don’t have to worry about replacing it later on.

Remember, too, that your pool technician can be a valuable asset when it comes time to choose the right filter. Don’t be afraid to ask for help measuring and sizing your pool or finding information about the volume or feet of head your specific model has.

With the right numbers to help you choose, picking your pool filter will be a snap!

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