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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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.

 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?

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.

 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!


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.

 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

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.

 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????

InyoPools Product Specialist  Posted: 4/11/2014 

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

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.

 Posted: 4/8/2014 

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

Anonymous  Posted: 4/7/2014 

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

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.

 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?

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"

 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

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.

Anonymous  Posted: 1/3/2014 

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

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.

 Posted: 9/7/2013 

I have a pool 12'x34', shallow end 3.5', deep end 8.5'. I have a heyward DE4800, old school. after calculating everything here, I think my pool holds about 18400 gallons of water. my DE4800 is shot and I need to replace it. a neighbor is giving me a Sta-Rite system 2 PLM150 filter system. will this work. the pump I have now is 3/4 HP.

InyoPools Product Specialist  Posted: 9/3/2013 

Frog - $70 a year seems very low for a VS pump savings. See our guide on "How to Save Money Using a Variable Speed Motor" for more information. As to the size of VS pump to purchase, they currently come in two sizes, about 1.6 HP and 3.0 + HP. I would go with the larger HP for your size pool. Since they are self-adjusting, you can scale the HP down to what's actually needed.

 Posted: 9/1/2013 

I have an inground irregular/kidney shaped pool with 16,000 gallons. It is a saltwater pool with solar heating (2 story house) and infloor sweepers. I have 2 drains ~3 ft apart and 1 skimmer with an avg. ft head of 48 ft. I have a DE filter. My pump size is 2HP A.O. Smith MOD K48N2PA105C4, Volts 230 with a Pentair WhisperFLo Mod WFE-8 2 HP/ Service factor of 1.3 pump. Should I change to a VS pump with less HP and if so which size would be best? I have used energy savings calculators and they say I will only save $70 a year, seems low. 50% of our energy use is running the pool pump ~5 tp 6 hrs a day. Thanks for your help!

 Posted: 8/27/2013 

Thank you!!

InyoPools Product Specialist  Posted: 8/26/2013 

MDF - You are correct. For a single speed motor you want to run it as short a time as required. For a VS motor you want to reduce the speed as much as you can and run it for as long as you can at that lower rate. Remember if you cut your speed from 3450 RPM to 1725, you will reduce your energy costs to 1/8 over the same period of time. If a SS pump costs $240 to run a month at 8 hours a day, a VS pump will cost $60 to run at 1725 for 16 hours a day. Both will turn over the same volume of water.

 Posted: 8/26/2013 

Thanks for the fast response! Ok, I thought I was required to get all of the pool water through in 8 hours. We run our pump continuously, so does that mean we could spread it over 24 hours and plan for bursts of turning it in 8?

InyoPools Product Specialist  Posted: 8/26/2013 

MDF - Your requirement for 198 GPM is based on running your 95,000 gallons of water through your filter in 8 hours. If you ran the pump 10 hours instead of 8, you could use a pump that had a flow of 160 GPM. There are at least two VS pumps that will generate 160 GPM: the Hayward EcoStar Variable Speed Pump and the Pentair IntelliFlo Variable Speed. Both motors are over 3.75 Total HP. If you ran these motors at half speed [and half flow] for 20 hours, you would reduce your energy cost to 1/8 of your full speed energy cost. See our guide on "How to Save Money Using a Variable Speed Motor".

 Posted: 8/25/2013 

We have a 25'x 78' rectangular pool with a deep end of 10' and a shallow end of 3'. I calculated this to be about 95,000 gallons based on the formula given in step 1. This leads me to 11,875 GPH or 198 GPM.

We have 4 skimmers and 2 main drains, but no other suction features.
I calculated the average feet of head to be 90'.

We had a single speed 3HP pump running 24x7 that just froze up. So we need to replace it.

Question 1. Can we get a variable speed pump?
Question 2. None of the pumps listed in the Hayward table seem to meet these specs. Can you tell me the name of another brand that might meet these specs?


InyoPools Product Specialist  Posted: 8/23/2013 

lance - The model number of a pump is sometimes stamped into the shoulder of the pump near the discharge port. If not, try looking on the underside of the strainer cover. A part number is usually stamped there that can be crosschecked to the pump on a parts list.

 Posted: 8/22/2013 

Thank you, very helpful !!!

 Posted: 8/22/2013 

Hi, sorry me again. I think the pool pump is a sta rite however, I cannot find a model number on it at all. Any clue on where it is located? This is actually my mother pool and bought the house a couple of years ago and she didn't get any manuals or information on the pool. I don't know the age of it and I am assuming it's a sta rite based on a picture search I did.

InyoPools Product Specialist  Posted: 8/14/2013 

lance - You are correct. The pump shaft seal will depend on the type of pump.

 Posted: 8/13/2013 

Thanks so much! Now...the pump shaft seal will depend on the type of pump I have right?

InyoPools Product Specialist  Posted: 8/12/2013 

lance - For a 20K gallon pool, people will generally use a 1 1/2 HP motor. You actually have a 1.67 HP motor. A pump's Total HP (THP) is measured by multiplying the pump's stated HP by its Service Factor (SF). If you look on the label of your B848 you will see that it has a HP 1.0 and a SF of 1.67. The product of the two numbers is 1.67 which is your pump's THP. I would replace your current motor with the same motor. Remember to replace your pump's shaft seal when you change out the motor.

 Posted: 8/11/2013 

I have a 20k gallon pool with a 1.0 hp SF 165 AO SMITH B848 Pump motor that has died. The pool has 2 returns, 1 skimmer, and 1 drain. My thinking is to replace it with a 1.5 hp AO Smith B2854. Can I do that or should I stay with 1.0 hp? If I replace with the 1.5 hp will I need upgrade the impeller, diffuser and eye seal?

InyoPools Product Specialist  Posted: 8/1/2013 

Jon - Sounds like your bearings are going again. They may have been misaligned when installed. If you didn't replace the shaft seal, it may be spraying onto the motor. You could definitely go to a 1 HP motor for your size pool and setup. And, if you can afford the initial cost, you should consider a variable speed pump. See our guide on "How to Save Money Using a Variable Speed Motor". They are just coming out with smaller 1 1/2 HP VS pump that would work well for your setup.

 Posted: 7/28/2013 

Excellent information so far, just a couple of questions. I have 16000 gallon inground with no fountains, just suction vacuum and solar heating as well as gas heater. Currently I have 3/4 hp Hayward super pump that I just changed bearings however its starting to make a squeaking noise from the motor. First question, are my new bearings going again or do you think motor is just dying? Also wondering if i should upgrade to 1hp? I have to keep my pump running 24/7 should look into variable speed or stick to single ? Thanks in advance

 Posted: 7/28/2013 

We have a 1000 gallon, in ground spa that we cannot keep from getting green from one Saturday to the next. Usually, by Wednesday or Thursday, it is green. We have been trying to bring the phosphates under control. We shocked it yesterday and today the phosphates were at 300. A month ago, we drained the entire spa and filled it with new water. We are constantly needing to shock it because the chlorine barely registers even though there are tablets in the dispenser and we have been diligent about keeping the Soda Ash level in range. We are going nuts. The GPM rate is 16.7 and the head is 10. I took a picture of the pump to see what the pump was and found that it is a Magnetek Century 8-77064-03 Pool and Spa Motor. 1081 Pump Duty. the
HP is 2.o - .25 The filter is a Hayward, Star Clear Plus,__ __175.

We are at our wits end. It is in our rental and we maintain it but we can't.

Can you make suggestions as to what the problem can be. It acts like the water goes through the inlet and right back out the outlet and never is filtering because the pressure never changes at all. Yesterday, we engineered to parts that close the cartridge filter hold so it fits tightly rather than leaving a space in the center. It seems to have changed the pressure from 28 - 30 after filtering out most of the green algae. It didn't seem like the water was ever going through the filter so we forced it to go through the filter.

HELP, Please....


InyoPools Product Specialist  Posted: 7/23/2013 

oyster56 - I don't see a motor replacement at 1.25 THP, but based on your information, you could probably use a motor with 1.1 or 1.0 THP. The 1.1 THP motor is a standard uprated motor, UST1102. The 1.0 THP motor is an Energy Efficient (EE) motor, UCT1102. The EE motor is $40 more but would save you 20% on operating costs. Also since these both are slightly smaller motors than your old one, you will have to buy a smaller impeller. And, for any motor replacements, you should buy a new shaft seal.

 Posted: 7/23/2013 

We have a pool that just about exactly matches your average pool but it has only one drain and one skimmer, which are an average of 30 ft. away from the pool pump. We currently have a A.O. Smith Century Centurion 1 HP motor with an SF factor of 1.25, which has reached the end of its life. This pump motor is on a Jacuzzi Magnum 1000 pump. Is a replacement with THP of 1.25 sufficient or over or under our needs? There are no additional features such as waterfalls using the pool pump's capacity. Although I favour a VS pump for the quiet and the lower environmental impact, given our modest electricity costs and a short swimming season of less than three months, we will probably stay with a single speed pump motor. Thoughts? Suggestions? Thanks.

InyoPools Product Specialist  Posted: 7/18/2013 

Mike - Your pool holds about 10,000 gallons of water. A 1 1/2 HP pump should be sufficient to handle circulation for this size pool and your waterfalls.