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


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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 Posted: 4/17/2019  Latest

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?

InyoPools Product Specialist  Posted: 4/17/2019  Latest

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

 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?

InyoPools Product Specialist  Posted: 3/21/2019 

No, it will not.

 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.

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.

 Posted: 1/18/2019 

I have 20 inch hayward sand filter and 1.5hp pump, each time i try to clean my pool all dirty sand goes back to the pool . Is the pump too powerfull for the filter ? Thank you

InyoPools Product Specialist  Posted: 1/21/2019 

Hello Hamz - your filter recycling dirty water could be a few things. First, you may have old grimy sand that clumping which causes channels through which dirt can return to your pool. Or there is a broken lateral; these laterals are water collect the water at the bottom of the tank after it filters through the sand.I would try a thorough backwashing and rinse of your filter. Try some clarifier and/or flocculant to help the filter catch the dirt. If the dirt and silt are still present after the treatment, open the filter, remove the sand and check the laterals for cracks

 Posted: 1/10/2019 

I am going to build a 10ft.x20ft. pool with a depth of 4ft.what pump and filter would you recommend?

InyoPools Product Specialist  Posted: 1/11/2019 

Hello Robert - In order to recommend the pump, we'll need to know your total feet of head. Check out our article titled "How Do I Calculate Feet of Head for My Pool".

 Posted: 1/3/2019 

15000gallons old motor overheating replaced with new still overheating has hot tub that motor is 1.25 old motor and new are . .95 not sure what old motor was . is it possible motor is to small

InyoPools Product Specialist  Posted: 1/3/2019 

Hello Doug - overheating is often a wiring issue, stemming from low voltage. Have you checked the voltage of the pump circuit? For info on how to check the pump motor's voltage: How To Use a Multimeter to Test a Pool Pump Motor - Voltage

 Posted: 12/10/2018 

have 16x32 pool 26,000 gal need to pump..want variable speed what size should i get have 1hp now should i get 1.5 or 3.0 vs no falls or other items on pool

InyoPools Product Specialist  Posted: 12/10/2018 

Hello Gil - I'd recommend the PureLine Prime model PL2605. The PL2605 is a 1.65 total HP variable speed pump.

 Posted: 12/9/2018 

My father passed away and I'm in charge of getting their old pool running again. It currently has an old Centurion 2HP pump that is leaking on the housing. I was told the pool is a little more than 15,000 and in reading, that seems like too large of a pump. The pool has no water features and does have a spa. I was thinking of stepping down to a 1.5HP pump. What is your opinion?

InyoPools Product Specialist  Posted: 12/10/2018 

Hello Shawn - It's possible that the pump is oversized. Do you happen to know the total feet of head? We'll need to know that before we can say for sure that you can go with a lower HP. Check out our article titled "How Do I Calculate Feet of Head".

 Posted: 11/22/2018 

The bearings in my pump are bad. (very loud) when pump is running. The label with all my info on the pump are worn off. I can't tell how many hp this pump is or any other info I need to replace it. My pool is 30'x15' 3' deep in shallow end and 6' deep in the deep end. Any recommendation on what pump I should be looking for. All I know is Marathon is the manufacturer on my pump now. Thanks

InyoPools Product Specialist  Posted: 11/27/2018 

Hello Ken - if you cannot find any identification numbers on the outside of the pump, I suggest looking on the pump's impeller. The impeller will at least identify the pump's horsepower, as well as narrow the field of suspects for identifying your pump. With that information in hand, we can suggest a group of similar pumps that can deliver similar flow rates.

 Posted: 10/23/2018 

Hi. I have a 10' x 22' pool with a constant depth of 60". One end of the pool on the 10' side has a negative/infinity edge. Can you recommend correct HP pump that should be used? Thanks!

InyoPools Product Specialist  Posted: 10/23/2018 

Hello Jeff - We'd recommend the PureLine Prime model PL2605. The PL2605 is a 1.65HP variable speed pump. 

 Posted: 10/3/2018 

Hello Bill - Solar panels will add to your total feet of head but it wouldn't triple. We'd recommend going with a variable speed pump. A variable speed pump will allow you to have a margin of error because you can dial in the correct speed to operate the solar panels.

 Posted: 10/3/2018 

How does a roof top solar heater affect the pool pump size? I am assuming that it will triple my head size.

 Posted: 10/2/2018 

Hello Patricia - as we mentioned in Step 7 & 8, a major component into figuring out flow rate for a system is the feet of head. The skimmers you brought up are a part of the equation but does not provide enough information for an educated guess. If you cannot provide the feet of head, it would be helpful to have the model number of the pump housing to at least get a benchmark of what your system needs.

 Posted: 10/1/2018 

Greetings, I am looking at your Pureline 1.65 hp variable speed pump for a replacement of my Century 1.5 hp pump. My freeform pool is about 31,000 gal with an estimated 65 gpm filter rate (by your dyi calculations). I have 2" pipes and a large Triton -140 sand filter with a flow rate of 106 gpm. I have three skimmers and a polaris vac powered by a booster pump. Will this replacement pump be adequate for my specs? Thanks in advance.

 Posted: 9/12/2018 

Hello Bud - We can't say for sure but I doubt the higher HP caused it to burn up after 3 years. Motors, today, often last 3-5 years. For your pool, we'd recommend something like the PureLine Prime PL2605. This is a 1.65HP variable speed pump. It is the most energy-efficient option. You would run the pump on a lower speed to circulate the water and only use the higher speeds to vacuum or backwash, if necessary.

 Posted: 9/8/2018 

Hello guys, I have a 33,000 gal inground pool with 1 1/2" piping. I had someone replace my motor 3 years ago and he went from from a 1hp motor to a 1.5hp. He didn't replace the pump just the motor. Now my pump seems to have died. My question is did the higher motor burn out my pump? The pump is 25 years old. When buying a new pump what hp should I get?

 Posted: 8/20/2018 

Hello Chris - The Pentair SuperFlo 342001 would be perfect for your situation. The Pentair Clean & Clear 150 sq ft can handle flow rates up to 150 GPM; so it can handle anything that VS pump can throw at it.

 Posted: 8/18/2018 

I need to replace my in ground pool pump. I currently have a 3/4 HP superflo for a 14000 gallon pool. The filter is a Pentair Cartridge (Model 160355: 150 sq ft?). Both are close to the pool. I would like to go with a VS pump and am considering the Pentair Superflo 342001 1.5 HP model. Would this be the right size for my pool. I don't have any features attached or a heater.

 Posted: 8/16/2018 

Hello Mem - the great thing about a variable speed is that is can be throttled for any type and size of an application. My main concern about whether the pump is overkill or not is dependent on whether you have 1.5" or 2" PVC pipe. With 2" pipe the IntelliFlo VS will do great, and you can use its full capacity. But if your system has 1.5" piping you would be unable to use the higher speeds of the Intelliflo as they could cause cavitation. For 1.5" PVC I suggest using the Pureline Prime 1.65 VS.

 Posted: 7/27/2018 

@Tom: Actually, Tom, I think the opposite is true. The greater the flow rate, the lower the outgoing water temperature and the greater the heat exchanger to water temperature differential. The greater this differential, the greater the heat flow into the water. The lower outgoing water temperature is compensated by the higher flow. At higher flow rates, more heat is transferred to the water and less is lost to the surrounding sir.

 Posted: 7/24/2018 

I have a Hayward Superpump 1Hp motor which is making a loud humming noise and not pumping so I assume is it burned out. I installed it 3 years ago to replace the previous Hayward Superpump which lasted 5-6 years. My pool is about 30-35,000 gallons. I’d 1 hp too pump is outside and gets rained and snowed on. Could that be the problem it isn’t lasting or is it undersized?

 Posted: 6/15/2018 

Hi Guys found your site thru a friend it is very informative. I would like to add a comment about pump sizing when you have a pool heater either gas or electric heat pump.i live in Florida where most have pool heaters for winter time use and spa heating.the big problem I am seeing is the flow in the systems far exceeds the design flow of the installed pool heater. This results in poor heat transfer equating to far longer run times. This is a big inefficiency that goggles up gas and electric and eats at your wallet

 Posted: 6/7/2018 

Hello Malinda - Lowering your pump HP is one option. Another option is to add a regulator to the cleaner hose. The regulator can be set to open up a little and take some suction away from the cleaner.

 Posted: 6/5/2018 

18 x 48 above ground coleman pool. The 1 hp is too strong for my pool cleaner. Should I with .75 hp pump ?

 Posted: 5/18/2018 

I have a 15K gallon includes a SPA and a small slide that has water running on it. My 1.5 hp pump broke, and now I'm quoted to get the Pentair 011018 IntelliFlo Variable Speed High Performance Pool Pump, 3 Horsepower. Do you think this is a little bit overkill for 3hp? thanks

 Posted: 5/8/2018 

Hello Hydrox - We'd recommend the Pureline Prime model PL2605. This is a 1.65 total HP variable speed pool pump with 2" ports. The variable speed pumps are the most energy-efficient option.

 Posted: 5/4/2018 

Looking to replace both Filter & Pump. Pool is around 25k Gal. Vinyl pool. Filter is Hayward 244T which handles 62GPM. Pump is very old Pentair housing but looks like the motor is an old 1.5hp Century? Maybe was replaced at some point? Think my filter is too small. 2 skimmers, 1 main, All 1-1/2" piping. Guessing 50'ish ft head? If I move up to the 270T Filter which rates for 72GPM or the 310 which rates at 100GPM. What would be a good pump?

 Posted: 4/25/2018 

Though, 1.5 HP does seem undersized for that pool size I do not believe that is the cause of the issue. Repeated pump failures is likely an environment issue. For example is the pump in an open area with no cover from the sun, or is it in a splash zone for sprinklers or rain runoff? These can cause wear and tear on the motor, shortening its lifespan. If your pool is saltwater, then it may be the water is compromising the OEM shaft seals. If you have saltwater, I would suggest upgrading to the saltwater shaft seal to ensure a longer life.

 Posted: 4/24/2018 

I had a pool installed in 2011. It's 44x22. I have replaced my pool pump 3 times in 7 years. It's a 1 1/2 HP Hayward and it's bad again. I was told by installing contractor that my pool held around 50k gallons. What is going on here?

 Posted: 4/23/2018 

Hello Mike - The 1.5 EE Superflo would be sufficient for your pool size and TDH. 2HP would be overkill unless you need additional flow for an attached spa.