How to Size a Pool Pump for Your In-Ground Pool

Written by:  Danny Rhodehamel
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 (3.81 OUT OF 5 STARS ON 26 RATINGS)

When purchasing a new in-ground swimming pool, you need to determine what size pool pump is required. There is tendency to purchase a bigger pump than is necessary thinking bigger is best. 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 8 hour period. This page will show you how to select a pump that filters all the water in your pool in 8 hours.

Tips & Warnings

Things You'll Need

Step by Step

Step 1
gallons of water in pool

Your first step is to determine the number of gallons of water in you pool. The formulas for calculating gallons depend on the shape of your pool. For a RECTANGULAR POOL, measure the length (ft), the width (ft) and the average depth. Average depth is determined by adding the depth at the shallow end to the depth at the deep end and dividing by 2. The formula for calculating total gallons in a rectangular pool is: Gallons = Length x Width x Average Depth x 7.5 Example (see picture): your pool is 30 ft long and 15 feet 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 = 6 ft. Pool's capacity is 30 ft x 15 ft x 6 ft x 7.5 = 20,250 gallons. Go to Step 5

Step 2
round swimming pool

ROUND SWIMMING POOL - To determine the number of gallons of water in your round pool, measure the diameter of the pool and its average depth. Average depth is determined by adding the depth at the shallowest part to the depth at the deepest part and dividing by 2. The formula for calculating total gallons in an oval pool is: Gallons = Diameter x Diameter x Average Depth x 5.9. 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 = 5 ft. 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 swimming pool

OVAL SWIMMING POOL - To determine the number of gallons of water in your oval pool, measure the long diameter, the short diameter and the average depth. Average depth is determined by adding the depth at the shallow end to the depth at the deep end and dividing by 2. The formula for calculating total gallons in an oval pool is: Gallons = Long diameter x Short diameter x Length x Average depth x 5.9. Example (see picture): Your pool's long diameter is 25 ft, Short diameter is 15 ft and the pool's average depth is (3 + 7) / 2 = 5 ft. Pool's capacity is 25 x 15 x 5 x 5.9 = 11,063 gallons. Go to Step 5

Step 4
kidney swimming pool

KIDNEY SWIMMING POOL - To determine the number of gallons of water in your kidney pool, measure the largest width, the smallest width and the average depth. Average depth is determined by adding the depth at the shallow end to the depth at the deep end and dividing by 2. The formula for calculating total gallons in an kidney pool is: Gallons = (Longest width + Shortest width) x Length x Average depth x 3.38. 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. 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 8 hours. To come up with this flow rate simply divide your calculated gallons by 8. 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. A good estimate is to take the average amount of feet from where your suction lines are (skimmers or main drain) back to where your pool pump will be located. The picture at the right provides an example of how the average Feet of Head would be calculated for a pool with 3 suction returns, two skimmers and one main drain. The lengths of each line are: Skimmer 1 - 5'+15'+25'+15'+10' = 70'; Skimmer 2 - 15'+10' = 25'; and Main Drain - 25'+20' = 45'. To get the average Feet of Head take the three suction line lenghts and divide by three. 70' + 25' + 45' = 140' / 3 = 47 Average Feet of Head

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 requirement. 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 42 GPM 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 to 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, Harward 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 meets resistance at the filter. This will eventually burn out the pump motor and your filtration will also be compromised. We recommend over-sizing the filter 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: fountains, spas, waterfalls, solar heating, and infloor cleaning systems. The features generally require higher GFM rates which equates to a higher HP pumps. Also if your pool requires greater than 60 GFM 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.

Comments (1 to 40 of 96)

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User: Inyopools

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.

User: LDore

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????

User: Inyopools

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

User: Inyopools

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.

User: zman

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

User: 

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

User: Inyopools

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.

User: lucyandpaco

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?

User: Inyopools

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"

User: hugo

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

User: Inyopools

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.

User: 

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

User: Inyopools

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.

User: carlos805

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.

User: Inyopools

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.

User: Frog

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!

User: MDF

Thank you!!

User: Inyopools

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.

User: MDF

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?

User: Inyopools

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".

User: MDF

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?

User: Inyopools

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.

User: alex

Thank you, very helpful !!!

User: lance

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.

User: Inyopools

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

User: lance

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

User: Inyopools

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.

User: lance

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?

User: Inyopools

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.

User: Jon

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

User: Oreshans

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....

User: Inyopools

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.

User: oyster56

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.

User: Inyopools

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.

User: Mike

I am having a 27x12 fiberglass pool installed. The deep end being 5'. I am also having a three tier waterfall with 1 shear decent in the middle and 2 18" shear decents on either side. I am having a 1 1/2 hp pump installed. My question is should I get a second pump to run the waterfalls. The pump will be located 15' from the skimmer.

User: Inyopools

MM - For a head of 11' you would need a 1/2 HP Hayward Super Pump that would provide 55 GPM. This is overkill but this is the smallest pump we sell. For a head of 67' the charts show you would need a 1 HP Hayward Super II Pump (different class of pump) which provides 35 GPM. The next lower pump, 3/4 HP, is right on the edge of providing 19 GPM for 67' of head and the manufacturer recommends going up to the next level.

User: Inyopools

Bjtex - Yes, you could use a 1 1/2 HP pump but you're on the edge. You could hedge your choice a little by getting a 1 1/2 HP pump with a SF of 1.10 or 1.25 to get a slightly higher THP. THP = HP x SF. If you are concerned about operating cost and can afford the initial pump cost, you should look at buying an Energy Efficient (EE) pump or a 2 speed or variable speed pump.

User: MM

I am building a small inground splash pool of size 13' X 8' with 3' depth. This will contain 2300 gallons of water. I have two options for placing the pump ; one nearby with head of 11' and the other at a distance with head of 67'. What capacity of pump and also the pump size is recommended in each of the two cases with flow of 19 GPM? Appreciate urgent help. Thanks


User: Bjtex

I have a 30K In-ground pool with 40 ft of head and 2 in water lines Could I use a 1.5 hp pump. I currently have a 10 yr. old 2 hp pump that's very costly to run...

Thanks

User: Inyopools

volts/amps/watts - A pool motor's volts/amps/watts are generally defined by the HP of the motor. With little exception, pool motors use either 115V or 230V. That is defined by the power available at the house. Amps (and Watts) are directly related to HP. The more HP a motor has, the more Amps/Watts it will use. Some EE motor are designed to be more energy efficient than their stand counterparts and might use 20% less Amps/ Watts for the same HP.

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