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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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Step 1

Rectangular Pools

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.

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Step 2

Round Pools

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

Oval Pools

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

Kidney Shaped Pools

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 the required gallons per minute (GPM) you need to a pool 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 pool example, the GPH required is 20,250 gallons / 8 hours or 2531 GPH.

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 6

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 7

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 8

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 9

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 10

There are a couple of other considerations that should be mentioned in your selection of a swimming 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 Product Specialist  Posted: 3/30/2015 

kreiner - The 3/4 HP Pentair SuperFlo pump is a good choice for you size pool. Pentair is one of the top pump manufacturers and its SuperFlo pump is very efficient and quiet as well as economical. It is Pentair's motor of choice for small to medimun pools. These pumps are comparable to Hayward's popular Super Pumps.
 Reply

 Posted: 3/30/2015 

I have a 13,000 gal pool and 2 intakes with an average head of 35 ft. I also have a Pentair Clean & Clear catridge filter system rated for 320 gpm and a 3/4 HP booster pump for the Polaris 280 robot. The motor on my 3/4 HP StaRite pump went out and I would like to replace with single speed pump. I only run about 5 hrs per day and we are moving soon so don't want to spend the money on a high efficiency model. I was looking at the 3/4 HP Pentair SuperFlo 340037 or could you make a recommendation. Thanks.
 Reply

InyoPools Product Specialist  Posted: 3/26/2015 

4lil'swimmers - I would recommend a smaller 1.5 HP VS pump for your size pool. And a 24" Hayward Sand Filter is larger than you need but we generally recommend going bigger on filters. They will last longer and you won't have to backwash them as often.
 Reply

 Posted: 3/25/2015 

Hello Just moved into a home last summer with 34X17 rectangular grecian shaped pool. 24000gal, 1 skimmer, 3 returns, 1.5in. plumbing, no features, and use a kreepy to clean. I estimate the skimmer is about 40-45ft from the pump. The pump and filter are ancient pacfab, nautilus stuff. According to calc. I need a 1HP pump. Would like to get a full size VS or if I can use a smaller VS? and would a 24in Hayward Sand filter work with this size? - Thanks
 Reply

InyoPools Product Specialist  Posted: 3/20/2015 

meemers - If your pool is 7,500 gallons, that is a relatively small pool and 2.5 HP is considerable overkill. You do not need anything over 1 HP even with your cleaner.
 Reply

 Posted: 3/19/2015 

I have a 7,500 galloon 20 year old pool with a spa with spillover into the pool (4 jets). My current pump is 2.5 HP Hayward. I run a 2 wheel The Pool Cleaner vacuum from the skimmer intake. What pump should I be considering?
 Reply

InyoPools Product Specialist  Posted: 3/15/2015 

labergebnt - We usually recommend a 1 1/2 HP pump for your size pool. If you can afford the initial cost, we also recommend buying a 2-speed or variable speed pump for significant operating cost savings.
 Reply

InyoPools Product Specialist  Posted: 3/15/2015 

Chris - I would recommend buying a 3/4 HP or even 1 HP full rated dual speed pump for your pool configuration. As you point out, this would give you a low cost option for filtering your pool at low speed while insuring that you have enough flow for a pool cleaner at the higher speed.
 Reply

 Posted: 3/13/2015 

I have a 17' x 35' rectangular pool at 3' area and 8' deep area ( 5.5 )
I have 1 skimmer and 2 return. the pump is 30 feet away from the pool. as per calculation, it is a 51.13 gpm. Right now I have a 1hp and it seems so little. 2" pipe. Should I take a 2hp or 1.5 hp?
 Reply

 Posted: 3/12/2015 

I recently bought a foreclosure property that has a pool. I’m going to be reopening it this spring, but while it was vacant, the power end sub-assembly of the motor and accessories was stolen and so I am replacing everything on the pump except the volute, skimmer basket, and clamp top. The pool itself is a gunite 18’x36’ oval with a 3’ shallow and 7.5’ deep end for a total of approximately 20,000 gallons. It has only one skimmer, in line between the main drain and the pump/filter. The skimmer is 28’ away from the pipe and the main drain is 9’ farther away horizontally for an average total head of approximately 37 feet. The current filter is a Pentair Tagelus TA 60D, 1.5’’ PVC Pipes, and Pump was a Pentair Whisperflo WF-23, ¾ HP up-rated pump. I planned on buying the same ½ HP, single-speed, full rate motor sub-assembly as before to replace it. I would like to purchase a kreppy krauly or a vacuum similar to clean the pool sometimes. Will this motor be sufficient to clean and for everyday use or should I get a more powerful or dual speed motor? Thanks for the advice!
 Reply

InyoPools Product Specialist  Posted: 1/22/2015 

kt - When you replace a pump, you want to look at its "real" or Total HP (THP). THP is determined by multiplying the stated HP with the stated Service Factor (SF). I would guess that the THP of your old Northstar is close to 1.5 THP while the replacement pump you are looking at is 1 THP [1.0 Hp x 1.0 SF]. If you want to replace you existing pump with an equivalent one, look for a pump with 1.5 THP.
 Reply

InyoPools Product Specialist  Posted: 1/22/2015 

notso - I would go with the Pentair Whisperflo Dual Speed full rated 2 HP Pump. But take one more look at VS pumps. See our guide on "How to Save Money Using a Variable Speed Motor". Don't know what your negative comments were, but you will save more in operating costs with a VS pump.
 Reply

 Posted: 1/21/2015 

I have a 26k gallon pool with a waterfall. the waterfall is independently running from a 1hp hayward northstar pump which just started making a lot of noise. Can i replace it with a 1 hp pump which has sf 1.0 hmax15m qmax 79gpm? i rarely run the waterfall in summers. the new pump looks much smaller than Hayward northstar for some reason.
 Reply

 Posted: 1/18/2015 

26k gal. inground, 30ft head, 2" inlet/outlet lines, 4 returns, new 60 sf cart/DE. not very good reviews of vari speed pumps, so looking at 2 speed, prewired for 230. replacing 22yr. whisperflo 2hp
thank you.
 Reply

InyoPools Product Specialist  Posted: 1/15/2015 

USCARCH - The smaller VS pumps including these two have 1 1/2" ports, but they are easily converted with adapters to fit 2" piping without affecting performance.
 Reply

InyoPools Product Specialist  Posted: 1/15/2015 

USCARCH - For a 2 degree per hour rise in temperature in a 15,000 gal pool, you will need a heater with around 300 BTU output and because you live in CA, you have to buy a LowNox version. Two models that fit these requirements are: Hayward's Pool Heater Universal H-Series Low Nox 300K BTU NG and Raypak's LoNox Heater 335000 BTU NG Elec Ign. You set the max temperature that you want to stop heating. Also be sure to buy a pool cover to save the heat that you are putting into this pool.
 Reply

InyoPools Product Specialist  Posted: 1/15/2015 

Justin - For your size pool and the short amount of time you will need to filter your pool water, you will not save that much scaling down a VS pump vs running the 3/4 HP pump. I would go with the 3/4 HP single speed pump.
 Reply

 Posted: 1/14/2015 

One more question about the pump. All my lines are 2". Would one of the two pumps you suggest be appropriate for the 2, 2" suction and the 2, 2" return lines? Thanks
 Reply

 Posted: 1/14/2015 

Thanks for the info on the pump and filter. Both pumps qualify for the rebate. As far as the heater goes, I am looking at around 2 degrees for hour, max. 25 degree temp rise. I live below 2000' in Los Angeles, CA. I have natural gas plumbed to the location. What model / models would your recommend. Thanks
 Reply

InyoPools Product Specialist  Posted: 1/14/2015 

USCARCH – For your size pool and setup I would recommend the smaller 1.5 HP VS pump like the SuperFlo VS 1-1/2 HP Variable Speed Pump or the Hayward Super Pump VS Variable Speed Pump. For a DE Filter - Hayward Pro Grid DE Filter 60 Sq Ft w/o Valve or Jandy DEV60 60 Sq. Ft. DE Filter w/o Valve. I’m not sure what you mean by “max 25 degrees temperature rise”. Need to know over what period of time. I would guess you would want a heater that generates 300K BTUs for your size pool. For more information on selecting a heater, see our guide on “How To Select a Gas Heater For Your Pool or Spa”.
 Reply

 Posted: 1/13/2015 

Thanks Inyopools! So given the small pool size and short head runs, a variable speed would not save much on the energy bill over the 3/4 HP uprated motor? I usually only run the motor 8hr/day. Forgot to mention that the pool has a very small attached hot tub with heater. Thanks
 Reply

 Posted: 1/13/2015 

I have a 15,000 gallon (~800 Sq. ft. surface area) in-ground pool with 2 return lines, 1 skimmer and 1 dedicated vacuum port. Max, 20' of head. My Vacuum is the Baracuda G3 Residential Advanced Suction Side Automatic Pool Cleaner. Need to replace pump with a Variable speed pump to get an SCE rebate. What would you recommend? Also need to replace by DE filter, Would like to stay with DE. What would you recommend?. Would also like to add a heater. Max 25 degree temperature rise Gas line is already in place. What would you recommend. Thanks
 Reply

InyoPools Product Specialist  Posted: 1/13/2015 

Justin - For your size pool, I would recommend a 3/4 HP pump with a Uprated motor (Service Factor (SF) of less than 1.15 - like this Hayward 3/4 HP Super Pump. Note: I am assuming this is a small in-ground pool. If it is above ground, you will need an above ground pump.
 Reply

 Posted: 1/12/2015 

I'm looking to replace an overkill 2HP Pentair Whisperflo WFE-8 on my 8,000 gallon pool with 1 skimmer and 1 drain with an average 13 feet of head all returning through magnasweeps. What would you recommend?
 Reply

InyoPools Product Specialist  Posted: 1/9/2015 

Jc - For your size pool and its associated operational costs, I would recommend getting a large 3 HP Variable Speed (VS) pump. That way you could run it most of the time at a lower speed to circulated the water and then run it at full speed when you want to use the spa. It will save you up to 75% on operational costs over using a single speed 2 HP pump. See our guide on "How to Save Money Using a Variable Speed Motor" for more information. This VS pump would take care of all the features you listed. You would not need an additional pump.
 Reply

 Posted: 1/8/2015 

I am building a very large pool 60x25 with spa 15x7, cascade and slide. I figured around 56,175 gallons of water with ur calculations. Do i need a 2hp motor? And if so seperate motors for spa and cascade?
 Reply

InyoPools Product Specialist  Posted: 12/30/2014 

S2BEIT - I would recommend a 1 HP above ground pump for your setup. Two factors to consider when replacing your pump: Total Horse Power (THP) and type of pump. The actual or THP of a pump is determined by multiplying its HP by its Service Factor (SF) - both stated on the label on the pump's motor. The Hayward PowerFlo has a SF of 1.0 so your old pump's motor had a THP of 1.0 x 1.0 or 1.0 THP. The Pentair SuperFlo pump motors have a SF of 1.65 so for the 3/4 HP motor, its actual THP is .75 HP x 1.65 or 1.15 THP. The acutal THP of 1.0 HP motor is 1.0 x 1.65 or 1.65. In this case the 3/4 HP motor would have been the better match. However, factor two, the Pentair SuperFlo pumps are designed for inground pool which work through suction (pulling water to the pump). Your Hayward PowerFlo pump was designed for above ground pools and work off of gravity - pump is lower than the pool water surface and water drops into the pump. I would go back to the 1.0 HP Hayward PowerFLo or similar above ground pool pump.
 Reply

 Posted: 12/29/2014 

HELLO,

I have an above ground pool 18x30oval around 46inch depth. What size pump do I need. My existing 1hp hayward Powerflo burned out. I was considering a 3/4 pentair supeflo or maybe 1 hp?
 Reply

InyoPools Product Specialist  Posted: 12/21/2014 

Capt Jack - I would go with a 1.5 VS pump for your size pool. 2 or 3 HP VS pumps sounds like overkill, especially with they have Service Factor (SF) of over 1.0. Total or real HP is the product of HP and SF. If a 2 HP pump has a SF of 1.5, your total HP (THP) is 3 HP. I would be looking for a VS pump of 1.5 to 2 THP.
 Reply

 Posted: 12/19/2014 

HI, I have a 16,000 gal in-ground pool with 1 drain and 1 skimmer. Currently have a 1.5HP Challenger single speed pump that I want to switch to VS. This pump serves the cartridge filter (PacFab Seahorse 300 sq ft), pool cleaner (Polaris 360) and my roof top solar heaters. I have been getting varying HP recommendations locally (anywhere from 2 to 3 HP) and I am wondering what your thoughts are.
 Reply

InyoPools Product Specialist  Posted: 12/15/2014 

Madche – I would go with a 1.5 VS pump. You will see a considerable savings in operational costs even with your smaller pool. See our guide on “How to Save Money Using a Variable Speed Motor” for more information.
 Reply

 Posted: 12/15/2014 

It is an in ground pool.
Thanks for any advice
 Reply

 Posted: 12/15/2014 

I have 10,000 gallon pool with some spa features, a fountain and solar heating on the roof.
The Hayward Northstar pump motor died. It was 2hp, 1.1sf. What size replacement do you recommend?
Single, dual variable speed. I understand that variable is most economical in the long run but someone told me for my small pool it won't matter.
 Reply

InyoPools Product Specialist  Posted: 12/1/2014 

t-lu - 30,000 is a lot of water to filter but since you only have two lines into the pump (skimmer and dedicated suction) and two return lines, your GPM flow is somewhat limited. A 1.5 HP motor is a good choice. You might also consider a 2 speed or variable speed pump for a large saving in operational costs.
 Reply

 Posted: 11/29/2014 

hi, I have a little more than 30,000gallon inground, I have one skimmer, 2 return lines and a dedicated line for a Polaris 360, all lines I had upgraded to 2". I use a jandy valve to control water flow to Polaris and jets. my pump just went out, my question is what size pump should I replace with, im thinking a 1.5hp. I generally run 24hrs a day in summer
 Reply

InyoPools Product Specialist  Posted: 11/14/2014 

tdeist - For your size pool and based on the size of your old pump, I would recommend one of the smaller VS pump like the Hayward Max-Flo 1.5 HP Variable Speed Pump. The larger VS pumps would be overkill. For a DE filter I would recommend something like the Hayward Pro Grid DE Filter 60 Sq Ft. This is a step larger than what you need but most people buy a larger size to extend the maintenance period. You will also need to buy the multiport valve that goes with this filter - Vari-Flo Valve 2" for DE.
 Reply

 Posted: 11/12/2014 

16x34 foot rectangular pool, average depth of 4', about 17,000 gallons. Existing pump is a pacfab challenger 1 1/2 hp, pump motor is a centurion 1 1/2 hp. existing filter is a hayward c1750 single cartridge filter. seems i am constantly having to clean the filter! I want to upgrade the existing pump and filter. would be interested in purchasing a variable speed pump. I want to upgrade the filter to a DE filter. what do you recommend for both the pump and the filter? thanks!!
 Reply

InyoPools Product Specialist  Posted: 11/4/2014 

Ellen - Hayward is one of our best manufacturers. They sell this set that would meet your stated requirements - Hayward Super Pump 1 HP Single Speed W/ S244T 24" Sand Filter & Valve.
 Reply

InyoPools Product Specialist  Posted: 11/4/2014 

Archtec - Sorry for the late response. I would go with the smaller VS pump for your size pool. You can adjust the speed for exactly what you need for the salt system. Pump companies built the smaller pumps to handle these smaller pools. The larger VS pumps are overkill.
 Reply

 Posted: 11/1/2014 

Hello, I have an 18,000 gal pool and the filter and now pump need to be replaced. There is currently 2" piping and I am loooking into a 24" filter with 1 HP pump. Is there a brand you recommend? The pool is considered a play pool and is 5' deep max with 2 returns, 1 skimmer & 1 drain, the filter/pump is about 2' away from the pool. thank you
 Reply