How to Size a Pool Pump for Your In-Ground Pool

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When purchasing a new in-ground swimming pool, you need to determine what size pool pump is required. There is a tendency to purchase a bigger pump than what is necessary, because people think bigger is better. However, not only does this lead to higher operating costs, but you may also be overpowering your filter system. As a general rule you should have a pump that filters all the water in a pool in an eight-hour period. This page will show you how to select a pump that filters all the water in your pool in eight hours.

Things You'll Need

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Step by Step

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Step 1

Your first step is to determine the number of gallons of water in your pool. The formulas for calculating the gallons depend on the shape of your pool.

For a RECTANGULAR POOL:

Measure the length (ft), the width (ft) and the average depth. The average depth is determined by adding the depth at the shallow end to the depth at the deep end and dividing by two.

The formula for calculating the total gallons in a rectangular pool is: Gallons = Length x Width x Average Depth x 7.5.

For example (see picture): your pool is 30 ft long and 15 ft wide. The pool's shallow end is 4 ft and its deep end is 8 ft. So, the pool's average depth is 4 plus 8 = 12 divided by 2. This gives you 6 ft. The pool's capacity is 30 ft x 15 ft x 6 ft x 7.5 = 20,250 gallons. Go to Step 5.

Step 2

For a ROUND SWIMMING POOL:

Determine the number of gallons of water in your round pool, measure the diameter of the pool and its average depth. The average depth is determined by adding the depth at the shallowest part to the depth at the deepest part and dividing by two.

The formula for calculating the total gallons in a round pool is: Gallons = Diameter x Diameter x Average Depth x 5.9.

For example (see picture): your pool is 25 ft in diameter, the pool's shallow end is 3 ft and its deep end is 7 ft. So the pool's average depth is 3 plus 7 = 10 divided by 2. This gives you 5 ft. The pool's capacity is 25 ft x 25 ft x 5 ft x 5.9 = 18,428 gallons. Go to Step 5.

Step 3

For an OVAL SWIMMING POOL:

To determine the number of gallons of water in your oval pool, measure the longest diameter, the shortest diameter and the average depth. The average depth is determined by adding the depth at the shallow end to the depth at the deep end and dividing by two.

The formula for calculating the total gallons in an oval pool is: Gallons = Longest diameter x Shortest diameter x Average depth x 6.7.

For example (see picture): Your pool's longest diameter is 25 ft, shortest diameter is 15 ft and the pool's average depth is (3 + 7) / 2 = 5 ft. The pool's capacity is 25 x 15 x 5 x 6.7 = 12,563 gallons. Go to Step 5.

Step 4

For a KIDNEY-SHAPED SWIMMING POOL:

To determine the number of gallons of water in your kidney-shaped pool, measure the largest width, the smallest width and the average depth. The average depth is determined by adding the depth at the shallow end to the depth at the deep end and dividing by two.

The formula for calculating the total gallons in a kidney-shaped pool is: Gallons = (Longest width + Shortest width) x Length x Average depth x 3.38.

For example (see picture): Your pool's length is 25 ft, longest width is 15 ft, shortest width is 10 ft and the pool's average depth is (3 + 7) / 2 = 5 ft. The pool's capacity is (15 + 10) x 25 x 5 x 3.38 = 10,563 gallons.

Step 5

Now that you have calculated the number of gallons in your swimming pool, you want to determine how many gallons per hour (GPH) you need to pump to clean all the water in your pool in eight hours. To come up with this flow rate, simply divide your calculated gallons by eight. For the RECTANGULAR swimming pool example the GPH required is 20,250 gallons / 8 hours or 2531 GPH.

Step 6

Most pool pump specifications are expressed in gallons per minute (GPM). So, to convert from GPH to GPM, divide your GPH by 60 minutes - 2531 GPH / 60 = 42.2 GPM.

Step 7

Having calculated your required GPM, you next have to figure out the average Feet of Head for your pool pump. Check out our blog on How to Calculate Feet of Head. PLEASE NOTE, WE ARE CURRENTLY REVIEWING OUR PROCEDURE FOR ESTIMATING FEET OF HEAD. CURRENT ESTIMATES ARE TOO HIGH. 

Step 8

You now have the information required to select the size of your pool pump. 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. 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 calculated above for a typical RECTANGULAR pool, we are looking for a pump that will handle 42GPM with a 47 Feet of Head. According to the chart for 50 Feet of Head (closest to 47'), we need a pump between 3/4 HP (31 GPM) and 1 HP (50 GPM). Since we always go for the higher GPM, we would select the 1 HP pump.

Step 9

The full performance page for the Hayward Super Pump can be found at this link, Hayward Super Pump. For the location of performance pages for other pump models, contact an Inyopools sales representative at 1-877-372-6038.

Step 10

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 encounters resistance at the filter. This will eventually burn out the pump motor and your filtration will also be compromised. We recommend selecting the filter so that it is oversized 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.

Step 11

There are a couple of other considerations that should be mentioned in your selection of a pool pump. The above calculations are based on a basic pool configuration with no extra water features like: fountains, spas, waterfalls, solar heating, and in-floor cleaning systems. These features generally require higher GPM rates which equate to a higher HP pump. 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.

Comments

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(1 to 40 of 535)

 Posted: 7/15/2019  Latest

I want to buy a pump to use as a waste out pump to clean dead algae from the bottom of my gunite pool rather than using my filtration system. I want to use a above ground pool pump that I can plug in. Is 1 HP enough?
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InyoPools Product Specialist  Posted: 7/15/2019  Latest

I would just buy a sump pump, since that's what it is meant to do instead of trying to repurpose a pool pump.
 Reply

 Posted: 7/12/2019  Latest

I just have a 20 x 48 round above ground pool please what size pump
 Reply

InyoPools Product Specialist  Posted: 7/16/2019  Latest

For a pool that size, you would need at least a 1 horsepower pump. This guide goes into more details about sizing speciically for above ground pools: How To Size an Above Ground Pool Pump and Filter System
 Reply

 Posted: 7/4/2019 

Trying to decide between PureLine 1.65 or 2.7 hp variable speed pumps. 25000 gal, rectangular, waterfall, spa on separate 2hp pentair pump, pentair filter 520 gpm. Equipment 60-65 feet from drain. Going to add UV sanitizer. Appreciate the help.
 Reply

InyoPools Product Specialist  Posted: 7/9/2019 

I would go with the 2.7 HP variable speed to have the extra filtering rate just in case.
 Reply

 Posted: 6/27/2019 

Hi , what pump would you recommend for my rectangle 23 x 13 with a 4 ft depth pool please? Many thanks
 Reply

InyoPools Product Specialist  Posted: 7/3/2019 

I would need more of the criteria we mention in the article to make an adequate guess.
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 Posted: 6/15/2019 

I have a 12,000 gallon kidney shaped in ground. I calculated GPM as 25. I don’t know the head rate. I want to update my pump which is currently a Sta-Rite Model MPRA6D-146L with a 3/4 HP SQ1072 Motor (which stopped working). The filter is Sta-Rite Model T-240BP-1. What pump would be a good replacement for energy efficiency and power? Should I stay with 3/4 HP? Thanks!
 Reply

InyoPools Product Specialist  Posted: 6/20/2019 

You can stay with the 3/4 HP or you can upgrade to a variable speed which will provide maximal utility savings. The PureLine Prime Variable Speed Pool Pump 1.65 HP can be throttled to match your desired flow rate for any need.
 Reply

 Posted: 6/5/2019 

I have an above ground pool that is 53”x30’ what size filter and pump would you recommend?
 Reply

InyoPools Product Specialist  Posted: 6/6/2019 

Great question; the answer can be found here: How To Size an Above Ground Pool Pump and Filter System
 Reply

 Posted: 6/29/2019 

my pool is 40 x 20 with a L shape step of 5 x 4 the depth is 9 ft. at one end and 3 ft. at the other my best guess on the piping is 12 elbows and 150 ft. run of pipe I need the right size cartridge filter and pump numbers and size for each
 Reply

InyoPools Product Specialist  Posted: 7/9/2019 

The Pureline Prime 2.7 HP Variable Speed, and the Star Clear Plus 175 Sq Ft. 2 Inch Ports.
 Reply

 Posted: 5/27/2019 

I have an 18000 gallon kidney shaped pool. With 23 ft of head. What would your recommendation be for the best pump?
 Reply

InyoPools Product Specialist  Posted: 5/30/2019 

As step 10 mentions, the size of the filter is an important thing to know when sizing a pump.
 Reply

 Posted: 5/7/2019 

Hello, I have a 22x42 kidney shaped pool, just installed a 72 square DE filter. Have 1.5” lines going to pump what is the best of the best and highest flow variable speed pump i can install ? Can you recommend a top of the line model.?
 Reply

InyoPools Product Specialist  Posted: 5/8/2019 

What is the model number of the pump currently on the pool? This may help us determine if your system is compatible with the higher horsepower variable speeds.
 Reply

 Posted: 5/8/2019 

Matt yes it’s a sta rite super max vs 1.5 hp
 Reply

InyoPools Product Specialist  Posted: 5/9/2019 

In that case, I would go with a similar horsepower variable speed like the PureLine Prime 1.65 HP. The Prime will have enough oomph so that you can run it on lower RPMs for energy savings. If you go with the larger 3-HP variable speeds, there is a danger of cavitation on the higher speeds. So you would basically be paying a bunch of money for speeds you couldn't use.
 Reply

 Posted: 5/7/2019 

I have a 27,000 gallon pool with a solar heater. Will I benefit from a VS pump or do I have to run the VS pump on high in order to run water through the solar heater.
 Reply

InyoPools Product Specialist  Posted: 5/8/2019 

The variable speed can still be cost-effective, but just not as much as it would be minus the solar panels. The VS gives you the ability to throttle its motor to the minimal RPMs needed to run your panels. Now, the particular speed required to run your specific pool is not something I could calculate off hand. You may have better luck contacting the manufacturer for that.
 Reply

 Posted: 4/25/2019 

I have a 1hp single speed pump, 50' of head, 62 GPM sand filter, 27460 gallon pool and 1 1/2" pipes. I want to purchase Pureline variable speed pump. What would you recommend?
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InyoPools Product Specialist  Posted: 4/26/2019 

Your best pump choice would be the PureLine Prime 2.7 HP Variable Speed Pool Pump - PL2606.
 Reply

 Posted: 4/25/2019 

210000 gals Size of motors 220 volts and pumps needed and have 4 inch piping . 573-883-0859
 Reply

InyoPools Product Specialist  Posted: 4/29/2019 

It sounds like you have a very large commercial pool. The guidelines mentioned in this article may not all apply to you. I suggest you call our support line 407-834-2200 or email upload@inyopools.com to contact our commercial division.
 Reply

 Posted: 4/24/2019 

I have a 16'x32' above ground pool that is 4' deep but does go down to about 6'. I have a 1.5 hp pump now but it seems like it is over powering my pool cleaner. The one it came with was a single hp. Should I go downtown to the single or remain with the 1.5?
 Reply

InyoPools Product Specialist  Posted: 4/25/2019 

Did the cleaner only start to malfunction after installing the new pump? Also, what do you mean it is overpowering the cleaner, can you explain what is happening?
 Reply

 Posted: 4/23/2019 

Have a 45k pool with a 30 inch sand filter , 2 1/2 piping need to know what size pump there is currently a 1hp on it but there’s no water circulation?????
 Reply

InyoPools Product Specialist  Posted: 4/24/2019 

Hello Russell - We'd recommend the PureLine Prime model PL2606. The PL2606 is a 2.7 variable speed pump.
 Reply

 Posted: 4/23/2019 

I have an in-ground rectangular pool housing approx. between 60,000 - 80,000 litres of water. What size pool pump should I be looking at? Thank you.
 Reply

InyoPools Product Specialist  Posted: 4/24/2019 

Ok, have you read the guide above which covers this topic? 
 Reply

 Posted: 4/20/2019 

Hello, I have been pouring many hours into trying to determine the best replacement pool pump & motor for my in ground pool. It is a Lazy L estimated at 33,000 Gallons. I calculated my THD a couple different times by utilizing the vacuum and pressure calcs and my THD came out to be between 40.3 and 46. The 2 biggest constraints I see with my system is that I have 120 out at the pool area, and not ready to do an electrical upgrade, and my plumbing is 1.5" PVC. Am I stuck replacing it with the .75 HP Hayward Super Pump that was on it when I bought the house? I am trying to balance turn-over with energy efficiency. Any help would be greatly appreciated.
 Reply

InyoPools Product Specialist  Posted: 4/24/2019 

Hello Terin - if you are restricted to 120 volts and 1.5" PVC, you are capped to a 1-1/2 horsepower single speed motors. Dual speed motors above 3/4 HP require 230 volts.A suitable pump would be the Pentair WhisperFlo 1-1/2 HP Energy Efficient.
 Reply

 Posted: 4/17/2019 

I have a 27' round pool (4' deep) and the filter sits about 3' away on the ground. If I go with a sand filter, what size motor would I need?
 Reply

InyoPools Product Specialist  Posted: 4/17/2019 

Hello Jack - We'd recommend a 1.5HP pump and a sand filter that is 19" or larger. 
 Reply

 Posted: 3/20/2019 

Hello, When running the backwash or waste I need to move water 60 ft from the exit point of the sand filter due to the close proximity of other houses. I have attached piping (I think 2 inch in diameter) that runs from sand filter away from houses at 6 ft. Will the pump burn out faster?
 Reply

InyoPools Product Specialist  Posted: 3/21/2019 

No, it will not.
 Reply

 Posted: 1/30/2019 

Hi, possibly a dumb question, but I am unable to find this out using various Google searches or your website. Can you tell me what is the recommended number of pool volumes that should be circulated daily through the filter? I need to start with this volume to figure out pump rates and times of operation. Thanks in advance.
 Reply

InyoPools Product Specialist  Posted: 1/30/2019 

Hello Kevin - The goal is to turn the water over once a day. If your pool is 20k gallons, all 20k gallons should run through the filter once a day. In the guide above, steps 1-4 explain how to calculate the water volume of the pool. In step 5 this guide, we have an example of turning over a body of water in 8 hours. The time frame can change but the formula will remain the same.
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