How To Find The Right Size Filter For Your Pool

Your pool filter does the most important work in filtering the water in order to provide a clean, clear and safe swimming environment. The filter is an integral part of the pool filtration system and it will make it so much easier for you to maintain your pool if you know how filtration works and what parts are included in the system.

 

Suggested article: 8 Best Pool Filter Reviews 2017

What size filter for my pool” is often asked if you start noticing that there is something wrong and come to think that you filter is struggling with keeping your water clean.

However, there might be other reasons for that so make sure you check them first.

  • Are you running the pump long enough during the day?
  • Are the pump and skimmer baskets clean?
  • Is the water chemically balanced? 
  • Is the sanitizer level correct?

If all of the above is checked and the values are good, then proceed to ask yourself the next question: What size pool filter do I need for my pool? The same question should be asked if you’re setting up your pool for the first time or if the existing filter doesn’t seem to cut it.

Pump Size to Filter Size Requirements

With a big pool filter, you’ll have a peace of mind knowing that the pump will not be too large for the filter. If your pump looks undersized for a larger pool filter, don’t wreck your brain – it just looks that way.

When the pump is too strong, on the other hand, it can cause all sorts of trouble. Dirt will be pushed through the filter, internals can become damaged or cracked, and high pressure will make for slow, inefficient and potentially unsafe conditions.

Filter Size to Pool Size Logic

Always look for pool size in gallons. If you’ve never figured it out before, we’ll go into detailed calculations in just a little while.

What size pool filter do I need?

Know your pool size – A standard pool circulation rule is to filter all the water in your pool at least once each day. Pool owners generally run their single speed pump for an 8 hour period, so they need a pump and 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. The formula for calculating the total gallons in a rectangular pool is: Gallons = Length x Width x Average Depth x 7.5. The formula for calculating the total gallons in a round pool is: Gallons = Diameter x Diameter x Average Depth x 5.9. The formula for calculating the total gallons in an oval pool is: Gallons = Longest diameter x Shortest diameter x Average Depth x 6.7. 

The formulas are simple and once you calculate them, you’ll remember how many gallons of water your pool has and there will be no need to remember the formula.

For this article, let’s assume that you have a typical in-ground pool size of 20,000 gallons

Gallons Per Minute Requirements – After you figured out the number of gallons in your pool (20,000 gallons for this example), you will need to calculate the Gallons Per Minute (GPM) required to filter 20,000 gallons in 8 hours.

To calculate this flow rate we first need to divide the calculated gallons by 8 to get Gallons Per Hour (GPH). Then we’ll divide that number by 60 to get Gallons Per Minute (GPM).

For our example of 20,000 gallons pool: 20,000 / 8 hours or 2,500 GPH divided by 60 mins/hour and we get 42 GPM (41.66 rounded to 42 GPM).

Your Pool’s Feet of Head – As water flows through the piping between the pool and the pump, it meets the resistance to flow. This comes from system properties such as the length of the piping, the number of turns (elbows) and the diameter of the piping. This resistance is called “Feet of Head or simply “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.

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′ as a middle value for our purpose.

What about your pump size?

 

– Now we have determined your pool’s required GPM and its Feet of Head. This helps to select the pool’s pump size. Many pump manufacturers provide a chart on their specs page showing the HP required for your particular GPM and Foot of Head requirement.

For example, let’s say you are considering (or already have) 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 which 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.

Let’s determine the filter size needed

– 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 while meeting more resistance at the filter. This will eventually burn out the pump motor and your filtration will also be compromised. Generally accepted recommendation is over-sizing the filter to be absolutely certain it can handle the flow coming from the pump. So in our case, instead of getting a filter rated at exactly 42 GPM, we should select one that is a little higher – around 60 GPM would be perfect.

What other factors come into play?

– The above calculations are based on a basic pool configuration with no extra water features such as: waterfalls, fountains, spas, solar heating, and in floor cleaning systems. The extra features generally require a higher GPM rate, which dictates a higher HP pumps/filters.

In addition, if your pool requires more than 60 GPM, you might need at least 2″ diameter suction pipes. Suction pipes of 1 1/2″ have a physical limit of 60 GPM while 2″ pipes can handle up to 100 GPM.

Have in mind that it will need a regular maintenance. Learn how to clean a pool filter.

What size sand filter for inground pool

As we concluded, choosing a larger pool filter has many benefits:

  • Longer filter cycles, or time between backwashing
  • Longer lasting filter media
  • Less bypass of fine particles
  • Faster clean-up of water issues
  • Better filtration of the water

How are Sizes of Residential Pool Filters Expressed?

Filters are sized by the square footage of filtering surface area. The more sq. ft. of filter media you have, the better the filter will perform.

Apart from widely popular sand filters, there are also cartridge filters and DE filters.

They all trap dirt and debris in different ways and within each filter type, there are typically several sizes. 

The smallest are suitable for aboveground pools, the mid-range for small pools of 15,000 gallons and the big filters are perfect for pools of 25,000 gallons and more.

Sand Filters

For sand filters, on inground pools, go for 30 inch tank diameter, which is a 4.9 sq. ft. filter surface area, and holds 600 lbs of sand.

For small pools (under 20,000 gallons), 24 inch tank is the next best option, if your budget is tight. But do not go smaller than these filter sizes for an inground pool. 

Cartridge Filters

100 sq. ft. of filter surface per 10,000 gals of pool water will be recommended minimum. 400 sq. ft. models are recommended for inground pools above 25,000 gallons.

In-detail comparison article: Sand Filter vs Cartridge Filter – Everything You Need to Know

D.E. Filters

Because they offer the most superior type of filtration, don’t make a mistake and go for a smaller size: 48 sq. ft. filter wild be ideal for mid-sized pools and if your pool is over 25,000 gallons, get a 60 sq. ft. filter.

Aleks Stanic

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