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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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Comments

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(1 to 40 of 551)

 Posted: 4/5/2020 

Hi, I would like some help please with sizing pump and filter? I have a pool that is 5o ft x 26 ft and between 4 ft x 6 ft deep, I wanted to know my flow rate. I figures 93 GPM also there is 2 1/2 piping a 3 hp pump and a Hayward filter W3C4030 {425 sq ft } and 3 heater in parallel would like to know if the pump is too big or keep pump and increase filter. at present motor and pump are new but current draw in high. it may be causing drag through the filter. thank you
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InyoPools Product Specialist  Posted: 4/7/2020 

Is this a commercial or residential installation?
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 Posted: 4/7/2020 

It is condo association.
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InyoPools Product Specialist  Posted: 4/7/2020 

That would probably fall under commercial, which has different local ordinances and codes that vary depending on the area. You'd do best to contact a local contractor or inspector to have them guide you.
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Anonymous  Posted: 3/30/2020 

I have a 20x40 unground pool with a 10ft deep end 30,000 gallons of water. I have a 1 1/2 hp motor but I just got a lager filter and my water flow has greatly decreased. Should I get a 2 or 2 1/2hp?
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InyoPools Product Specialist  Posted: 3/31/2020 

The addition of the filter shouldn't have changed the flowrate of the system. it may have affected the PSI reading on your filter, but flow shouldn't be an issue. PSI does not necessarily equal the flow rate. The larger filter allows the water to sift through quicker, keeping PSI lower but the flowrate should stay unaffected; if not increase Take a look at this article: How Do I Correct Low Water Pressure?; for an idea of why you may have lower flow.
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 Posted: 12/13/2019 

Great article. Too bad there are so many lazy people who don't even take the time to use the formulas. "I have a 10,000 gallon round pool. I"m too dense to figure out my pump size. DUUUUUUUUH"
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 Posted: 12/6/2019 

We have a kidney shape in-ground fiberglass pool 10000 gallons. Pump Waterway smf-110 just blew. What pump should we purchase for replacement?
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InyoPools Product Specialist  Posted: 12/9/2019 

You may install another Waterway SMF 1 HP Max Rate Single Speed Pump or you can use a different model like the Pureline Prime 1HP or Prime 1.65 VS
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 Posted: 8/28/2019 

I just have a 13 x 8 round above ground pool please what size pump
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InyoPools Product Specialist  Posted: 9/4/2019 

A 1/2 or 3/4 HP pump would be sufficient for your pool.
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 Posted: 9/9/2019 

You stated 13 x 8 round. I assume you are saying 13' diameter? Or is this an oval 13x8?
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 Posted: 8/13/2019 

What size pump should I use for my 24ft round pool?
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InyoPools Product Specialist  Posted: 8/13/2019 

Because you have an above ground pool, this guide would be more helpful: How To Select Above Ground Pool PumpsFor a 24 ft round pool, a 1 or 1.5 horsepower pump would be sufficient.
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Anonymous  Posted: 7/28/2019 

Pool is 50.000 gallons. Has a Pentair 60 filter. In a pool house. No need for quiet. Need a new pump. Also have a heater, propane. Short run to deep end.
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InyoPools Product Specialist  Posted: 7/30/2019 

Your post is missing information we mention in the guide required to determine the right pump, but generally the IntelliFlo Variable Speed Pump - 011018  would work for a residential installation.
 Reply

 Posted: 7/15/2019 

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 

I would just buy a sump pump, since that's what it is meant to do instead of trying to repurpose a pool pump.
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 Posted: 7/12/2019 

I just have a 20 x 48 round above ground pool please what size pump
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InyoPools Product Specialist  Posted: 7/16/2019 

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
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 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.
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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
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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!
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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.
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 Posted: 6/5/2019 

I have an above ground pool that is 53”x30’ what size filter and pump would you recommend?
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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
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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?
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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.?
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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.
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 Posted: 5/8/2019 

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

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