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


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.

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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.

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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.

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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.

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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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(521 to 545 of 545)

 Posted: 6/20/2012 

Last year we had a new 1hp super pump installed for our 16x36 in ground pool. We have a green algae problem since this was installed (we have spent a small fortune on chemicals but it keeps returning) Could the pump be to small. Should we replace it with a 1-1/5 or 2 hp pump.

InyoPools Product Specialist  Posted: 6/20/2012 

bmccormack - For your size pool I would lean towards the larger 1 HP pump.

 Posted: 6/20/2012 

Our existing pump did today. I have been wanting to replace it with a Pentaire efficient pump. Now I HAVE to. Reading all the info is helpful yet still complex. I have an in-ground pool that holds 13,500 gallons of water4 and we use a Pool Pilot salt sy7stem for chlorine generation. From what I am reading I should get a 3/4 horse pump or 1 H/P. am I correct?

InyoPools Product Specialist  Posted: 6/16/2012 

Yes there is a difference in pump design between an in ground pool pump and an above ground pool pump. Because an above ground pool pumps is installed below the surface of the water, the water is fed into the pump with gravity. An in ground pool pump, on the other hand, is installed above the surface of the pool so is designed to suck water into the pool pump. You will need to get an in ground pool pump and for your size pool I would recommend getting a 1 1/2 HP pump.

Anonymous  Posted: 6/15/2012 

We moved into a house with a rectangular pool that holds 25,000 gallons of water. Recently, it seems that not much water is filtering through the system. When I went to check the pump it is a 1 hp pump that is supposed to be for an above ground pool. Is there a difference betweem pumps for an inground and above ground pool?

InyoPools Product Specialist  Posted: 6/3/2012 

atpool - If you have low flow your problem is more likely to be with the pump and suction side that the filter. If your impeller is sound and not clogged, check for leaks in the piping- especially at any unions and valves. Check for blockages in the piping. If you replace the motor, you might go to 1.5 for that size pool. With the new size motor you may need a larger impeller. Also you might consider also stepping up the filter one size to accommodate the larger motor.

InyoPools Product Specialist  Posted: 6/3/2012 

VB - If you are looking for the most energy efficient Hayward motor, we would recommend getting a Hayward EcoStar Variable Speed pump. They cost more initially but you will recover your initial buy costs in less than a year with significantly reduced (80-90%) operational costs. And it gives you the flexibility to adjust your water flow rate to meet your current problems. See our How to Guide on variable speed pumps for more information.

 Posted: 6/2/2012 

I have a sta rite system 3 pool filter old style installed in 1997 (PLM150 cartridge) with a 1 hp A.O. Smith pump. The last few years pool water quality has been mediocre not sparkly clear. This season I have had problems with low flow rate so I replaced all o rings and pump gasket. I bought a new cartridge but can't pump water through it only my old cartridge. Does anyone know if they changed cartridge material and would a larger pump say 1.5 hp solve the problem. I have an inground pool 22000 gal.

 Posted: 6/1/2012 

Hello, A really good article. I have a kidney shaped in ground pool with a spa attached (approximately 28000 gallons of water capacity). I also have the caretaker 5 port valve system for water circulation. We are using Hayward 1.5 HP pump at the moment. Every year when we open the pool, the pump does not start initially. a local pool guy told us that we may need to get 2HP pump because of the 5 port valve system. Is it true? If I am looking for a cost effective energy efficient pump, do you recommend Hayward tristar? if so, which one would you recommend?.

Thank you so much for the article and information you provided.



InyoPools Product Specialist  Posted: 5/25/2012 

Jagajak - 2 HP sounds like overkill for a 10,000 gallon pool unless your spa adds 5000 gallons to it. Even if your total gallons is 15K you could easily get by with a 1 1/2 HP motor or convert to a smart pump for circulation.

 Posted: 5/25/2012 

I have a 10K gallon pool with three pumps, one 2HP for water features (rarely used unless I am in the pool) a 2HP pump for general circulation, and a 1.5HP pump as a "blower" motor for the spa..when I use the spa, I use the main pump and the blower pump...its a small pool but an 8 person spa. Question is, to save $$, should I replace the 2HP main with a smart pump, or can I cut the time a day down to 4 hours circulating? I dont want to sacrafice spa force too much but maybe a 1.5 HP motor wouldn't do too much to impact the spa because i have the second motor? Ideas? My goal = reduce cost and maintain some sort of enjoyment of the spa when I want to. :)

InyoPools Product Specialist  Posted: 5/22/2012 

Poolman2 - Sounds like you could reduce your filtering time in half. I would try that and if your pool doesn't stay clean, raise filtering an hour.

 Posted: 5/22/2012 

I live in Florida, I have a small pool (8,000 gal), with a 1hp Hayward super pump. I've been running it 6 hours a day for almost 5 years now. I process the water throught the filter approx 3 times a day, what if I just run the pump half the time, wouldn't that be a 50% savings electrically speaking? By your calculations, you feel that the water only needs to be filtered once a day? Correct?

InyoPools Product Specialist  Posted: 5/10/2012 

tailgate - The typical TDH for most pools your size is 45 to 60 feet. 20 feet sounds way low. Does any of your suction piping run around the perimeter of your pool before going to the pump. Sounds like you have minimal suctions piping, but I would go for a 3/4 HP to be safe.

 Posted: 5/8/2012 

My rectangular pool is 16'x34' and is 22,350 gallons. I have calculated that I need a 46.5 GPM pump initially. My pool has one skimmer and the main drain feeds into the bottom of the skimmer box. From there I have another hole under the basket which provides the intake to my pump which is 16-20 ft away. Do I have only 20 ft of head to overcome? I feel like I can get away with a 1/2 HP pump and a filter rated for 50 GPM.... am I calculating this out properly?

InyoPools Product Specialist  Posted: 3/19/2012 

NADB - The cost of a 2 HP pump on-line is $500-$600 depending on your pump model, so $700 installed doesn't sound that out of line. If you are trying to save money and since your pool is only 15,000 gallons you could replace your motor with a 1 HP or 1 1/2 HP motor - especially if you don't intend to use the Spa. Get a price on-line for both options and shop around for a couple of more quotes for peace of mind.

 Posted: 3/17/2012 

My local Leslie's supply co quoted me $700 for a new pool pump. When I bought the house 4 yrs. ago I got a 2 HP (15000 gal rectangular pool with spa/waterfall whose spa I don't use as the heater is broken) for $200. Could pool pumps have really gone up that much. After telling the guy I'm a single mom & can't afford that he told me he'd put a new motor and some other part in mine for $500 & that should be fine as long as I don't use the spa. Am I being taken for a ride & am I better off buying one on line & hiring a local handyman to install it? Thanks!

InyoPools Product Specialist  Posted: 2/27/2012 

Carpaul5 – Your pump’s total HP (THP) is a product of its HP times its service factor – “SF” on your pump motor label. If your motor’s HP is 2.0 and SF is 1.25, your pump motor is rated as 2.5 THP. If the SF is 1.0, the motor is rated as 2.0 THP. So if your old 2 HP motor has the same SF as your new motor, they are the same. 2-speed pumps run at full speed (3450 RPM) or half speed (1725 RPM). So you might run your filtration system at 1725 RPM for 10 hours and your suction cleaner at 3450 for 2 hours. A variable speed pumps can be programmed for up to 8 speeds with 8 different time slots at any speed between typically 800 – 3450 RPM - for example filtration at 1000 for 8 hours, cleaner at 2500 for 3 hours, spa at 3450 for 2 hours etc. Now the savings: A basic law of physics says if you reduce the RPM by one half from 3450 to 1725, you reduce energy cost to 12.5%. If it costs you $60 a month to run your single speed pump, it would cost you something like $15 a month running a 2-speed and maybe $10 a month with the variable speed. The variable speed is less because you can run the pump at a speed below half for a major portion of your time.

 Posted: 2/23/2012 

Re: pool pump sizing considering solar panels: you say if I am currently at 1 hp, go to 1.5, but stay at 1.5 if that's what I have. I currently have a 2 hp Magnatek Century Centurion pushing an old Purex Ah series pump. Would a new, energy efficient 2HP motor/pump combo be overkill, or is old 2HP = new 2HP?
Also, please explain how variable speed and 2-speed pumps differ and how they achieve the energyy savings over a single speed. (I want to make sure my particular configuration and use patterns would produce savings) - Thanks


InyoPools Product Specialist  Posted: 2/7/2012 

Carpaul5 - Roughly speaking, If your current pump is 1HP I would go to 1 1/2. If you are at 1 1/2 now and have an average size pool, I would stay at 1 1/2.

 Posted: 2/5/2012 

How do I calculate how much extra HP I need for spa, and solar heating panels?




 Posted: 1/2/2012 

Thanks for this information. My pool pump is running just fine now.

InyoPools Product Specialist  Posted: 6/6/2011 

We give a 5-7 day turn around for most of our pool products sold in June busy season. Pool Pumps typically ship out a little quicker.

 Posted: 6/5/2011 

This site - info - and pricing is phenominal!!!! Congrats. to the person(s) who designed and loaded the info onto this site with such clarity!!!
For my info, how long would it take to get a Hayward SP2607x10 shipped and received into MA??


 Posted: 5/23/2011 

great. it's help a lot