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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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 Posted: 6/11/2014 

My grids seem to be broken but the mesh is still intact. I just replaced the grids last year. I have a De 4800 filter and a 1 1/2 hp pump. I went up a 1/2 hp from the old pump. Now it seems that de is blowing back in the pool. Why will the grids brake like this? The last grids I had lasted 9 years....will this happen again?
 Reply

InyoPools Product Specialist  Posted: 6/9/2014 

Charter Oak - It sounds like your pump is too large for your size pool. You can reduce the HP on your current new pump by 1/2 HP by replacing the impeller with one that is paired with a 1 HP motor of the same model. The pump's motor would then be driving water with a smaller impeller generating less GPM. A general note: a motor can always drive a smaller impeller; it cannot drive a larger impeller (one step up) without overheating. See our guide on "How To Replace A Pool Pump Impeller".
 Reply

 Posted: 6/9/2014 

I have a small in ground pool 15X27 12,000 gallons. I just installed a Hayward 2610 1.5hp motor. Thinking more is better, and the price wasn't much different. As the pump builds preasure it starts to click like its straining. I am wondering if this is back preasure and is there a fix. I was thinking about by passing the heater just to see if this improves the flow. Your thoughts would be greatly appreciated.
 Reply

InyoPools Product Specialist  Posted: 6/3/2014 

Stelly2 - I would stay with the 1 1/2 HP pump. You should have 2" piping to handle a 2 HP pump. Look for an energy efficient replacement pump. And I would try to get the main drain working to improve flow and keep your pool cleaner.
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 Posted: 6/2/2014 

Help! I have a 45x18 oval pool about 26,000 gallons. The main drain return has been disabled years ago, the only return is from the surface skimmer about 80' from the pump. It has a Hayward 1.5hp pump that's aging. It's harder to keep the water clear and after a lot of research I'm suspecting the pump (at least ten years old) needs to be replaced. Would a 2Hp pump be better and work in this configuration? All piping is 1.5". Thanks
 Reply

InyoPools Product Specialist  Posted: 6/2/2014 

JT - My apologies. I misread the size of your pool. And your analysis correct. A 3/4 HP motor would be fine for your size pool. As for the piping, if you could replace your 1 1/2" piping with 2" piping you would be able to decrease your feet of head. Much of the resistance to water flow is in the size of the piping. As you open up the piping, you decrease the resistance to flow. Also if you use a pair of 45 degree elbows to make a more gradual 90 degree turn, you will also decrease resistance to flow.
 Reply

 Posted: 6/2/2014 

I apologize if this shows up twice but it appears the first one didn't post.
I was looking at a 3/4 HP pump rated at 47gpm@40ft of head, 38@50, 27@60, 12@70. Since I only need about 22 gpm for 8 hour turnover shouldn't this be sufficient? Why would I need 1 HP. The pump and the pool both have 1 1/2" fittings. I believe a 1 1/2" pipe will only have a maximum flow of 43gpm. Wouldn't any thing above that be a waste of energy. Another question. If I put a 2" adapter on the fittings and run 2" pipe would my flow increase or due the smaller inlet and outlet not allow a higher flow regardless of the pipe size?
 Reply

InyoPools Product Specialist  Posted: 5/31/2014 

18,000K pool - When you replaced your pump, did your replace it with an equivalent size pump. To match a pump, you need to look at the pump's Total HP which is HP x SF [service factor]. A pump with HP=1 and SF=1.0 has a THP of 1.0. A pump with HP=1 and SF=1.5 has a THP of 1.5, half again as powerful as the first pump. If you did buy an equivalent pump, look at our guide on "How To Get Rid Of Algae In Your Swimming Pool" for further information.
 Reply

InyoPools Product Specialist  Posted: 5/31/2014 

JT - We generally recommend a 1 HP pump for that size pool. Also, if it is placed at or slightly below the pool surface level, you will need to buy an in-ground suction pump rather than an above ground pump. The AG pumps rely on gravity to pull water from the pool. On the question of head, variable speed pump work below 20 ft of head because as you decrease speed and water flow [GPM], you significantly decrease resistance to flow [head] in the pipes. Your 20 ft of head is only 20' at full speed.
 Reply

Anonymous  Posted: 5/31/2014 

I have an in ground liner pool, 32 X 15, 18,000 gallons. I have a problem with the pool turning green all the time. Had the pump changed but I am not sure it is the correct size. In the past it took 24hrs or so for the color to change to blue. Now it is hard to keep clear. It is green or cloudy blue. The chems are fine... I have changed the pump, the sand in the filter, changed the water. I seems like the filter is not working right or the pump is the wrong size. What do you suggest?
 Reply

 Posted: 5/30/2014 

I am installing a 11,000 gal 48" deep pool. The pump will be located about 40ft from the pool. The plumbing will run under ground and the pump would sit at approximately the same height as the pool or slightly lower. To turnover in 8hrs I figure I need about a 20 GPM flow at 40 feet of head. Do you agree? Also can a variable speed pump at low speed circulate any water. The stats for most pumps show no flow rating after about 20ft of head.
 Reply

InyoPools Product Specialist  Posted: 5/27/2014 

Birch - The Pentair Whisperflo is a very popular pump. For your size pool, I would stay with a 1 1/2 HP full rated pump. And to save energy I would go with an Energy Efficient like the Pentair Whisperflo Energy Efficient 1.5 HP Full Rate Pump. It will save you 20% in operating costs.
 Reply

 Posted: 5/26/2014 

I have an 18x32 rectangle pool. The skimmers are are about 50' and 60' feet from pump and drain is about 70'. I have a jacuzzi magnum pump that I am looking to replace. I was looking at a 2hp pentair whisperflo. Would the 2hp be too much for my 1 1/2 piping? Any feelings about this pump?
Cheers!
 Reply

InyoPools Product Specialist  Posted: 5/25/2014 

Sriley5 - You would not have that great a lost going from a 1 1/2" port to your 2" piping. The greater lost would be in the friction pushing water through a 1 1/2 pipe. You can get adapters to go from a 1 1/2" port to 2" piping. Another option would be to go to a Hayward Super II EE Pump 1 1/2 HP Single Speed. It costs more initially, but it has 2' ports and you will get your money back with 20% reduced operational costs.
 Reply

InyoPools Product Specialist  Posted: 5/25/2014 

dave - I would recommend a 1 1/2 HP FULL RATED pump for your size pool.
 Reply

InyoPools Product Specialist  Posted: 5/25/2014 

mike - You can save money by just replacing the motor of you pump. For an equivalent Energy Efficient (EE) motor use CT1102, Energy Efficient A.O. Smith 1 HP Full Rate Motor- save about 20% in operating costs. Or for a 50% savings use B975T,1 HP Full Rate Two Speed Motor W/ Timer. See our guide on "How to Replace a Single Speed Motor with a 2Green Motor" for replacement instructions.
 Reply

 Posted: 5/24/2014 

I have a free form shaped pool about 28,000 gallons with 2 skimmers and main drain (all with 2in pipe). Current setup is waterways SVL56 1.5 and 60ft filter. I'm looking to replace it with a Hayward Super pump 1.5hp but noticed it only has 1.5in inlet/outlet... This would reduce the over all flow rate to below 60GPM (that's all 1.5 in can handle) correct? Or is it as simple as using a 1.5 - 2in union at the pump?
 Reply

 Posted: 5/23/2014 

I have a 29000 gallon 18'x36' pool.
61 gpm
20' head
Can you please recommend a pump size?

thanks
 Reply

 Posted: 5/22/2014 

I have a 36x18 ft, roughly 32 gallon pool..I have a 1 HP E-Z Flo pump that is very noise. Probably can make it thru this season with it but wondering if I should change for cost savings and better efficiency. Any recomendations?
 Reply

InyoPools Product Specialist  Posted: 5/14/2014 

MicheleD - I would agree that a 1.5 HP motor is a better fit for your size pool and configuration. If you are at the stage of having to buy a new pump, consider buying a 2-speed or variable speed motor. Both provide substantial savings in operational costs. Here's our guide on "How to Save Money Using a Variable Speed Motor".
 Reply

 Posted: 5/14/2014 

I have an approximately 12,000 gallon pool with a waterfall. It has 2 main drains that are approximately 50 ft from the pump and 1 skimmer that is approximately 25 ft from the pump. I have a 2hp pump on there now with a cracked mounting bracket. I was advised to replace the entire pump since it is 10 yrs old and the part itself is costly to replace. I was told a 1.5 hp is a more appropriate size for my pool. Any suggestions?
 Reply

InyoPools Product Specialist  Posted: 4/30/2014 

myst - This is a fairly small heater but if you are just trying to heat your spa, it's probably all right. You really don't want to heat your whole pool with an electric heater. It would cost too much.
 Reply

 Posted: 4/29/2014 

Thank you for your response. After much headache and research, I came to the same conclusion. The pool and spa dealer who installed it wouldn't back up any of their work, so it's back to chlorine for us. There is only one local pool store in town, and I can't say any good about them at this point, so your expertise here was greatly appreciated. Thank you! One more question- Is the heater that we have sufficient for the size pool?
 Reply

InyoPools Product Specialist  Posted: 4/29/2014 

myst - Your SP2800X5 is a 1/2 HP motor. For your size pool, we generally recommend a 3/4 or even 1 HP pump. Your salt chlorine generation (SCG) may be damaging your heater elements. When you had your salt chlorine generation (SCG) system installed, it should have been placed as the last unit in the return line after the heater, and you should have had a check valve put in between the SCG and your heater to keep the concentrated chlorine from backing up into the heater coils when the pump is shut off.
 Reply

 Posted: 4/26/2014 

I have a 10,260 gallon indoor rectangular pool, (dimensions- 36'x8'x4.75')there's one 2" intake line that's about 35' from the pump. we could say 40' to be safe, and it has a Hayward C751 filter (rated with a 75' surface area and 73 gpm max flow rate. There is an adjustable valve to reduce flow between the pump and the filter. I need to replace the pump, the pump that was there is a Hayward SP2800X5 (the label says 1 HP, but when I that pump model it shows it as a .5 HP pump) What pumps would you recommend?
Additionally, the heater is a Ray-Pak Spa-Pak 900615 Rev. 5 but the elements kept burning out. It was a salt water system and the suspicion is that the salt corroded the elements. But the reducer was also put in as an after thought to control flow and maybe that was the cause of the element failure. The company that installed the system messed up in several places and I question the choice of heater. I'm going back to a chlorine system as the salt water system they installed was faulty. As far as the heater is concerned, is it the right size and what else would cause the elements to burn out so often? Thanks in advance for the information!
 Reply

InyoPools Product Specialist  Posted: 4/25/2014 

Gstewart – I would stay with the 1 ½ HP pump for your size pool. If you are interested in saving on operating costs, I would consider a Variable Speed motor or Pump. They cost more initially, but you will get your money back in just over a year in significantly lower operating costs. Here’s a guide with information on Variable Speed pumps and motors. Note: Many of the prices listed have been lowered since this guide was published. Check our site for current prices and sizes of VS pumps.
 Reply

 Posted: 4/25/2014 

I have a 16x38 fiberglass pool that holds roughly 20,500 gallons. My 1 1/2 hp Hayward superpump went out last night. People are telling me that a 1hp would work just fine, but others are saying put the 1 1/2hp back on. What should I get? 300LB sand filter. Pump is about 25-35 ft away
 Reply

InyoPools Product Specialist  Posted: 4/15/2014 

L.Dore - You don't need a pump larger than 1.5 HP for your size pool. You should hook this pump up to an automatic timer [like T101P3] and only run the pump 8 hours a day. I would stay with a cartridge filter for convenience and size.
 Reply

 Posted: 4/14/2014 

I have a 27 foot round above ground pool. The pump that came with the pool was a 2 speed 2.5 HP pump. It lasted 5 years and now needs replaced. From what I read it doesn't seem that a pump this size is required for my pool. We tended to turn it on high and leave it on all summer. What can I purchase to accommodate my pool size. Considering buying a automatic timer to not have to run the pool 24 hours a day. Is a variable speed needed? We have a cartridge filter, but also debating switching to sand????
 Reply

InyoPools Product Specialist  Posted: 4/11/2014 

zman - A 3/4 HP pump would be a good match for your size pool.
 Reply

InyoPools Product Specialist  Posted: 4/9/2014 

Suction for cleaner – If your HP is borderline, you may have to shut down your main drain a little to get enough pressure to operate the cleaner.
 Reply

 Posted: 4/8/2014 

I have 288 sq. ft. pool 24x10 and will a3/4hp be good or not . thank you
 Reply

Anonymous  Posted: 4/7/2014 

How does action of a pool sweep with hose attached to skimmer affect pump horsepower requirements?
 Reply

InyoPools Product Specialist  Posted: 3/27/2014 

lucyandpaco - When you replace a pump, you want to look at the pump’s total HP (THP) which is a product of HP x Service Factor (SF). The values should be listed on your pump's motor label. Our specs show that a Speck 90-II has a THP of 1.0 [HP-1, SF-1). So you will be looking for a pump with a THP of at least 1. For your application, I would recommend an Energy Efficient pump like the Hayward Super II EE. For an equivalent size with a 20% savings in operational cost, I would get the Hayward Super II Energy Efficient (EE) 3/4 HP pump, model SP3007EEAZ. With a HP of 3/4 and a SF of 1.46, its THP is 1.14 slightly greater that your current Speck pump.
 Reply

 Posted: 3/25/2014 

My pool is 16x30 inground with a 9 foot deep end. I have 11 Fafco solar panels on the roof. My pump which is noisy is a 1hp Speck 90-II. Its time to replace it. What do you recommend?
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InyoPools Product Specialist  Posted: 2/26/2014 

hugo- For your 14,000 gallon pool, we would recommend a cartridge filter for convenience and a 1.5 pump with at least a 2-speed motor rather than a 1-speed. If you can afford the initially cost, buy a variable speed pump. You will recoup your initial purchase cost in 1-2 years of savings in operational costs. See our guide "How to Save Money Using a Variable Speed Motor". If you have 2" piping, you might purchase the "Hayward Star Clear Plus 120 Sq Ft. Filter 2" Ports". For 1 1/2" piping, buy " Star Clear Plus 120 Sq Ft. Filter 1.5" Ports"
 Reply

 Posted: 2/25/2014 

i am building amn inground gunite pool 16'x32' 3'-6' dp with 3 scuppers 6 returns 2 maindrains, 1400 galons not shure what pump size and filter to buy 1 hp ? 1 1/2hp? 2 speed? single speed? cartridge or sand
 Reply

InyoPools Product Specialist  Posted: 1/6/2014 

Cu ft to gals – There are 7.5 gallons in a cubic foot. Length x width x average depth gives you cubic feet. Multiplying by 7.5 gives you gallons.
 Reply

Anonymous  Posted: 1/3/2014 

Why are you multiplying 7.5 with the length, width and average depth for the rectangular-sized pool?
 Reply

InyoPools Product Specialist  Posted: 9/7/2013 

carlos805 - The Sta-Rite system 2 PLM150 filter system should work fine. You need 40 GPM flow to turn over your 18,400 gallons in 8 hours. The PLM150 has a GPM capacity of 50-120 GPM. It doesn't hurt to have a larger filter than needed. I would increase the pump to 1 HP for your size pool.
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