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

WRITTEN BY:  Inyo Pools

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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


Step by Step


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.


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


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


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


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.


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Inyopools  Posted: 06/08/2017 23:14 PM  Inyo Product Specialist

Kevin C - When you replaced your pump's motor, you may have inadvertently reduced the size of the motor. If it is smaller than your old motor and you did not replace the pump's impeller, you may be asking the smaller motor to do the work of your old motor and you would be overloading your new motor. On the other hand, if everything is running fine, pool motors do run hot and you may be ok.

Kevin C.  Posted: 06/06/2017 19:55 PM 

Need some technical assistance please. Recently had to replace the motor on my Hayward Super II above ground pool pump. The problem was when I went to replace the motor the part of the label with the horsepower rating was unreadable. I believe the motor was not the original Hayward motor that I installed around 1984 but was the second one I replaced probably around 15 years ago. I took a guess since I knew the motor was rated for.75 kilowatts. Also the model number was unreadable as well. I ordered a 1 h.p. motor and replaced it and the seal. The unit is running fine and the P.S.I is between 15 and 20 psi. My concern is this: I notice that the outer casing is extremely hot to the touch. I don't have a voltage meter to check the amperage but I am figuring it must be high. My pool is 23,000 gallons with 3 returns and 2 skimmers and a main drain. I keep the main drain closed otherwise the skimmers don't pull. I remember that the old motor was always hot to the touch as well. But it lasted approx. 15 years in that condition. My filter is rated at 75 gpm. Is this 1 h.p. too small for my pool ???

Inyopools  Posted: 06/04/2017 23:12 PM  Inyo Product Specialist

Jim - Not sure how you calculated your numbers. As I said in this guide, we are reevaluating our head calculation process. Ours are currently too high. But most "experts" estimate feet of head for a typical pool your size to be 50 to 60 FOH. And we typically recommend a 1 1/2 HP pump for your size pool.

Kevin  Posted: 06/04/2017 21:48 PM 

I'm planning to replace my old "silver bullet" Sta-rite cartridge filter with a Pentair CCP420. My pool is approx 17,000 gallons running a 2" return and 2" from the bottom drain and 1 1/2" from the skimmer. What HP pump should I go with? Thanks.

Maria  Posted: 06/02/2017 20:04 PM 

I have a huge 80,000 gallon pool, no features. What pump and filter do you recommend?

Thank You

Anonymous  Posted: 06/02/2017 16:03 PM 

I have 3 above ground pools which circulated and filter back to an in ground pool. The total head is about 270 ft. as the pools are 10 ft to 14 ft higher than the in ground. Total gallons 3250 (they are reflection pools). With such a low volume but need for high pressure what kind of pump would you recommend?

Inyopools  Posted: 06/02/2017 12:34 PM  Inyo Product Specialist

lima - Two possible problems with a 2HP pump. First, it can easily generate water flow greater than the GPM that your current S220T is designed to handle (52 GPM). You would need an S244T or even an S270T. Second, assuming your piping system is 1 1/2", a 2 HP pump is too big for your piping. A 1 1/2 HP pump would be best for you pool size and configuration.

Jim  Posted: 06/02/2017 12:33 PM 

Ive done the calculations based off of your instructions and my total gallons are 21120 and after doing the dividing down my final number is 44 and I have approximatly 60 ft of head and if Im looking at the chart correctly would that be the 2610X15?

Thanks JIM

lima  Posted: 05/31/2017 23:44 PM 

Hi, can anyone help me? I need to know if it is compatible or is it recommended to use a 2 horsepower pump with a sand filter (s220t) for a 25,000 gallon pool? thank you.

Inyopools  Posted: 05/30/2017 16:55 PM  Inyo Product Specialist

Faziemarie - Your psi should be about 15 - 20 psi. 58 psi is way too high and dangerous - unless you have a bad pressure gauge. The pump is bigger than you need. 3/4 HP or 1 HP would be better for your ~7,000-gallon pool. I would guess that your filter is too small for your pump and the pump is trying to push more water through the filter than it is designed for. Also, your Harris pump/motor may be for an in-ground pool where you should be using an AG pump.

Faziemarie  Posted: 05/29/2017 13:28 PM 

I need some help I have a 17x52 round above ground pool I am using a hayward vari-flo sand filter and a 1.5 Harris motor the water pressure is running at 58 psi is that too high psi for this set up

Inyopools  Posted: 05/25/2017 23:29 PM  Inyo Product Specialist

anwalden - If you have no water features and your pool piping is 1 1/2" in diameter, I would go with the 1.5 Hp VS pump. If your pool piping is larger than 1 1/2" and you have or might have several water features, I would buy the later 2.5 Hp VS pump.

Inyopools  Posted: 05/25/2017 22:03 PM  Inyo Product Specialist

abenicig - Would you consider just replacing the motor on your pump? If so, the replacement motor is 1 H.P 115V Thru Bolt Motor - BN25. You can replace the whole pump with a Pentair Dynamo Pump 1 HP w/ STD Cord.

anwalden  Posted: 05/24/2017 10:32 AM 

I have a hayward single speed pump of 1hp right now for a 23,000 pool. Looking to get a variable speed pump. I am trying to figure out if a need a 1.5 hp or a 2.5 hp. Any answers on this and why?

abenicig  Posted: 05/23/2017 20:26 PM 

Hi, We need to change the pump that was installed with our 16*32*4 ft. above ground pool. We currently have a Sta-rite JWPA5E7L-2A1 1hp 115volt horizontal discharge pump which is no longer made. Any advice on what to replace it with? Thank you!

Inyopools  Posted: 05/22/2017 23:20 PM  Inyo Product Specialist

Jw - I would recommend buying an up-rated 1 HP pump for your size pool. And yes, people generally run the skimmer and sweep at the same time.

Inyopools  Posted: 05/22/2017 16:03 PM  Inyo Product Specialist

Palmer - The 115V VS pumps are either for above ground pools or are too small for your size pool. If you can convert to 220, the VS pumps offer great operational savings. When you cut the motor speed in half, your water flow also reduces to a half but your energy cost is reduced to 12.5%. So yes, during filtration, you would run your pump twice as long at half speed but for a fraction of the cost. On your other question, if you bought a 1.5 HP pump to replace your 1 HP pump, you would run it for 7 hours instead of 10. You may not be saving much, however, because the 1.5 HP pump requires mot amperage than your current 1 HP pump.

Jw  Posted: 05/21/2017 10:32 AM 

Kidney shaped 10500 gallon pool. 15'W 17'W 34'L 3'S 7.5'D. 1/2 hp pump now. Do I need more hp pump? Pool guy says I do. Pump works fine just air leak from valve. Do I run sweep and skimmer at same time? Please help getting hot days now

palmer  Posted: 05/20/2017 20:00 PM 

My single speed 1 hp pool pump for a 20 X 40 inground pool (depth from 3.5 ft to 9 ft) just broke. I was thinking about getting a variable speed, but I don't know much about it. I ran the single speed pump around 10 hours a day. Does it pay for me to purchase a 1.5 hp pool pump and should it be a variable speed? Does a variable speed pump require you to run the pump more to keep the pool clean? If I get a 1.5 hp pool pump single speed does that require me to run the pump a shorter period of time than a single speed 1 hp? Also does a variable speed pump run on 110 Voltage?

Palmer  Posted: 05/20/2017 19:36 PM 

Does a variable speed pump clean as well as single speed pump?

Inyopools  Posted: 05/19/2017 12:11 PM  Inyo Product Specialist

Sunnymick - Yes, the A.O. Smith 1 HP Round Flange Up Rate Motor - UST1102 is the correct 1 HP motor replacement for a Hayward SP2607X10 Super Pump. Be sure to also buy the GOKIT03 for the replacement gaskets and shaft seal.

Sunnymick  Posted: 05/18/2017 11:45 AM 

I have a kidney shaped pool 14Wx28L, 3x6 depth. I am looking to buy a replacement motor, but the label on my current motor is worn and not legible. I believe it is a Hayward SP2607X10 Super Pump 1-HP Max-Rated Single-Speed Pool Pump (1hp). Does that sound right? I am looking at this replacement motor:

Inyopools  Posted: 05/18/2017 10:31 AM  Inyo Product Specialist

pam - For your 24x5' abound ground pool, I would recommend a 1 HP AG pool pump. Make sure the filter is large enough to handle this pump.

Inyopools  Posted: 05/17/2017 11:10 AM  Inyo Product Specialist

brendarhope50 - If you have 1 1/2' piping, I would use a 1 HP or 1 1/2HP Hayward Super Pump. If you have 2" or higher piping, I would use a 2 HP Hayward Super pump. A better option for your size pool would be a variable speed (VS) pump. It could save you up to 80% on operational costs. A newer and more economical VS pump coming into the market is the PureLine Prime Variable Speed Pool Pump 1.65 HP.

pam  Posted: 05/17/2017 9:26 AM 

I have a round 24x5' deep pool what size pump do i need

brendarhope50  Posted: 05/16/2017 12:24 PM 

I have a 20 x40 rectangle pool with drain at deep end which 8 ft deep shallow end is 4 ft deep, what HP would i use with the Hayward Inground super pool pump?

Inyopools  Posted: 05/15/2017 13:33 PM  Inyo Product Specialist

Gijs - The Pentair Intelliflo is much bigger than you need. The Pentair SuperFlo VS is a better fit for your size pool. You might also look at the newer PureLine Prime Variable Speed Pool Pump 1.65 HP. It is equivalent in design and quality to the Hayward and Pentair pump at a lower price.

Gijs  Posted: 05/14/2017 14:34 PM 

Hi! Would the Pentair Intelliflo be too much for a 14000 gallon pool? Does the Pentair SuperFlo VS has sufficient capacity? What would you recommend?

Inyopools  Posted: 05/08/2017 13:54 PM  Inyo Product Specialist

TimR - Is your filter pressure gauge reading high? If so the resistance to water flow (feet of head) in your pool may be too high for a 3/4 HP pump. You might consider going to a 1 HP Pump. Pentair's WhisperFlo pumps are quieter than most.

Inyopools  Posted: 05/08/2017 11:36 AM  Inyo Product Specialist

GPCowan - I would recommend getting the 1.67 HP VS pump to be able to run your waterfalls at full power when desired yet be able to run at half speed or less during most of the time for filtration. Running your VS pump at 1/2 speed only uses 1/8 of the energy (cost) of running at full speed.

GPCowan  Posted: 05/06/2017 19:29 PM 

We planning a small 7000 gal pool (no spa) with 3 waterfall features. Max depth 4.5', 2" piping and flat lot. I am getting recommendations of 2 HHP variable speed pump which just seemed large as normal 8 hour full circulation is 17 GPM. I don't want to undersize but even 1.5 HHP may be too large. Your thoughts...???

TimR  Posted: 05/05/2017 9:20 AM 

On our 4th pump since 2003, all professionally chosen/installed. 24k gallons, one intake, one return. Current 3/4 HP AO Smith with FNS Plus 36 filter. Need to do it myself this time. Want one that is quiet. No additional equipment but may add gas heater later. What do you recommend?

Inyopools  Posted: 05/02/2017 15:46 PM  Inyo Product Specialist

southernmom121 - If you have an older pool that was designed with 1 1/2" piping, I would not go over a full-rated 1 HP pump. If you think you have larger pipes, you could go with a 1 1/2 HP full-rated pump. If you invest in a pump with a variable speed motor, you don't have to know your system piping. You can set the motor speed to what works best operationally.

Inyopools  Posted: 05/01/2017 16:11 PM  Inyo Product Specialist

Chrisv - A standard 2.7 pump is too big for your size pool. However, the beauty of a VS pump is that you don't have to use the full power of the pump. You can use it at 2/3 speed or so for the spa then back it down to half or below for the pool.

southernmom121  Posted: 05/01/2017 10:31 AM 

I have a 20x40 pool - 37,000 gal - what pump size would you recommend? I dont have the pipe measurements. Im just trying to get a general idea.

Chrisv  Posted: 04/29/2017 11:32 AM 

Hi guys, I just purchased a house with a pool (my first pool) and i used your info to calculate that my pool is just under 13k gallons INCLUDING the built in spillover spa. Before I found your site I got a great deal on a Hayward Tri-Star VS 950 pump that has 2.7hp that is not yet installed. The current set up is a 1hp pump for the pool and another pump that is for the spa. I'm planning going from the current 2 pump set up down to just this 1 new hayward pump. My question is, is my new pump going to work? Is it too powerful or is it ok since it will be doing the job of what 2 pumps used to do and it's variable speed. thanks!

Inyopools  Posted: 04/27/2017 8:58 AM  Inyo Product Specialist

kplum - Your current 3HP Pentair pump is much too big for a pool with 1 1/2' piping. It is generally used with 2 1/2" and 3" piping. I would use a 1HP full rated pump for your pool.

kplum  Posted: 04/26/2017 8:31 AM 

Hi. We bought a house that has a 38,000 gallon pool. It has 1.5" piping and a 3HP Pentair pump. I believe we got air in the line and the pump ran for quite some time with no water. It seemed to have overheated and melted some of the pie around it, which of course caused more air. We had someone come out to fix it and said we need to remove the 3HP pump and get a 3/4HP pump. is that correct? Rectangular pool, 38k Gallon, 1 skimmer, no main drain. Trying to see if I am being taken or if we should just fix the current melted connector.

Inyopools  Posted: 04/18/2017 11:35 AM  Inyo Product Specialist

Jacksonman - We are currently in the process of modifying this guide and accompanying blog for estimating feet of head. It greatly oversimplifies a complex process. You are correct when you say your current estimate based on rough numbers is too high. Professionals would estimate the feet of head for your size pool to be more in the neighborhood of 60 to 80.

Jacksonman  Posted: 04/17/2017 21:23 PM 

The feet of head calculation has got me down. Our pool is roughly 28,000 gallons, 2 skimmers, a deep end drain. Seems to me I'd have at least 17 90 degree elbows. These elbows alone seem to add 129 feet of head! Add this to the roughly 260 feet of 1.5 inch piping for skimmers and drains, and add a furnace and sand filter, and I don't know if my 1.5 hp pump motor is the answer. Help! The pool was built in 2005.