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

WRITTEN BY:  Inyo Pools

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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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Keith  Posted: 07/27/2018 14:09 PM 

@Tom: Actually, Tom, I think the opposite is true. The greater the flow rate, the lower the outgoing water temperature and the greater the heat exchanger to water temperature differential. The greater this differential, the greater the heat flow into the water. The lower outgoing water temperature is compensated by the higher flow. At higher flow rates, more heat is transferred to the water and less is lost to the surrounding sir.

Tom  Posted: 06/15/2018 9:11 AM 

Hi Guys found your site thru a friend it is very informative. I would like to add a comment about pump sizing when you have a pool heater either gas or electric heat pump.i live in Florida where most have pool heaters for winter time use and spa heating.the big problem I am seeing is the flow in the systems far exceeds the design flow of the installed pool heater. This results in poor heat transfer equating to far longer run times. This is a big inefficiency that goggles up gas and electric and eats at your wallet

Inyopools  Posted: 06/07/2018 13:44 PM  Inyo Product Specialist

Hello Malinda - Lowering your pump HP is one option. Another option is to add a regulator to the cleaner hose. The regulator can be set to open up a little and take some suction away from the cleaner.

Malinda  Posted: 06/05/2018 23:06 PM 

18 x 48 above ground coleman pool. The 1 hp is too strong for my pool cleaner. Should I with .75 hp pump ?

Inyopools  Posted: 05/08/2018 14:44 PM  Inyo Product Specialist

Hello Hydrox - We'd recommend the Pureline Prime model PL2605. This is a 1.65 total HP variable speed pool pump with 2" ports. The variable speed pumps are the most energy-efficient option.

Hydrox  Posted: 05/04/2018 18:23 PM 

Looking to replace both Filter & Pump. Pool is around 25k Gal. Vinyl pool. Filter is Hayward 244T which handles 62GPM. Pump is very old Pentair housing but looks like the motor is an old 1.5hp Century? Maybe was replaced at some point? Think my filter is too small. 2 skimmers, 1 main, All 1-1/2" piping. Guessing 50'ish ft head? If I move up to the 270T Filter which rates for 72GPM or the 310 which rates at 100GPM. What would be a good pump?

INYOPools  Posted: 04/25/2018 10:39 AM  Inyo Product Specialist

Though, 1.5 HP does seem undersized for that pool size I do not believe that is the cause of the issue. Repeated pump failures is likely an environment issue. For example is the pump in an open area with no cover from the sun, or is it in a splash zone for sprinklers or rain runoff? These can cause wear and tear on the motor, shortening its lifespan. If your pool is saltwater, then it may be the water is compromising the OEM shaft seals. If you have saltwater, I would suggest upgrading to the saltwater shaft seal to ensure a longer life.

Ryan  Posted: 04/24/2018 18:09 PM 

I had a pool installed in 2011. It's 44x22. I have replaced my pool pump 3 times in 7 years. It's a 1 1/2 HP Hayward and it's bad again. I was told by installing contractor that my pool held around 50k gallons. What is going on here?

Inyopools  Posted: 04/23/2018 17:00 PM  Inyo Product Specialist

Hello Mike - The 1.5 EE Superflo would be sufficient for your pool size and TDH. 2HP would be overkill unless you need additional flow for an attached spa.

Inyopools  Posted: 04/23/2018 15:38 PM  Inyo Product Specialist

The lines on the return side will need to be added to calculate the friction loss.

mike  Posted: 04/23/2018 12:25 PM 

superflo 1.5hp standard gave out. considering replacing with 2hp ee superflo. 25,000 gal. sand dollar 80 filter. with what I can only roughly estimate according to your instructions and some guessing about 50' +/- of head. plumbed with 1-1/2" piping. thoughts? or is 1.5hp enough to effectively clean at 52gpm? pressure at filter @20psi with current setup?

Pete  Posted: 04/21/2018 9:03 AM 

When figuring feet of head do I include lines to jets?

Derek Dewitt  Posted: 03/28/2018 12:16 PM 

My wife and I recently got a pool installed, so thanks for the help selecting a pump for it. I like your point about how you can find the number of gallons in an oval pool using the Longest diameter x Shortest diameter x Average depth formula. I'll be sure to measure this out so I can find a pump that supports that many gallons of water.

Inyopools  Posted: 03/26/2018 12:56 PM  Inyo Product Specialist

Since we do not know your feet of head, it's hard to say for sure what you'll need. With that being said, you could go with a variable speed pump. Variable speed pumps are the most energy-efficient option. You can use a higher speed when necessary (vacuuming, backwashing, etc). Then it could be run at a low speed for regular circulation. For your pool, something like the Pureline Prime model PL2605 should be sufficient. This is a 1.65 total HP variable speed pump.

william miller  Posted: 03/24/2018 14:01 PM 

I have a 15000 gal pool with a solar system. what size pump would you recommend?

Inyopools  Posted: 03/21/2018 11:17 AM  Inyo Product Specialist

Hello Larry - We'll need to know the feet of head in order to recommend a pump. If you are unsure, check out our blog titled "How Do I Calculate Feet of Head for My Pool?"

LARRY  Posted: 03/12/2018 18:42 PM 

I have a 16x32 4ft shallow 4ft deep 4ft rectangular. What size sand filter and pump by hayward do i need. 1 skimmer. 1 bottom drain. I cleaner outlet 2 regular outlets and 2 controlable outlets in 4x8 steps

Inyopools  Posted: 03/07/2018 11:04 AM  Inyo Product Specialist

Cliff - Thank you for the clarification. We would recommend a 1 HP AG pump an matching filter for your AG 24' AG pool. Typical sets are:19" Above Ground Pool Sand Filter System W/ 1 H.P Pump and Hayward Pump & Filter S180T Filter w/ 1 HP Matrix Pump. Note, these systems cannot be used with soft sided pools.

Cliff  Posted: 03/06/2018 19:55 PM 

The pool is an above ground circular pool that is 24’ x 4’ deep (13,500 gallons). Which sand filter/pump would you recommend.

Cliff  Posted: 03/04/2018 21:22 PM 

What sand pump/filter would you recommend for a 24’ x 48’ (13,500 gallons) pool?

Inyopools  Posted: 02/10/2018 23:42 PM  Inyo Product Specialist

Randy - We haven't got a good handle on calculating feet of head yet. It's not a simple process even if you could see all the underground piping. Many experts suggest estimating it for 40' for small to medium size pools and 50' for larger ones.

Randy  Posted: 02/09/2018 10:47 AM 

All my pipes are underground, how on earth is anyone able to figure head pressure when you can't see any fittings/pipe ?? I need to replace my pump and it has never had the oomph it should have.

Inyopools  Posted: 01/29/2018 14:29 PM  Inyo Product Specialist

Suzy - Your proposed pool and spa would hold approx 13,000 to 14,000 gallons. For that size pool, I would suggest either a Hayward Super Pump 1.5 HP SINGLE Speed W/ S310T2 30" SAND Filter & Valve or a Hayward Super Pump 1.5 HP SINGLE Speed W/ C17502 175 Sq. Ft. CARTRIDGE Filter depending on which filter system you prefer. For the heat pump, I would recommend buying the largest you can afford. A couple of good mid-range heat pumps are Raypak Standard 119K BTU Digital Heat Pump and Hayward Heat Pro Heat Pump 110,000 BTU. For sanitation, I would recommend going with a salt chlorine system like Pool Salt System 40,000 Gallons. It's always better to go with a larger than needed system on these. The cell lasts longer.

Inyopools  Posted: 01/29/2018 13:06 PM  Inyo Product Specialist

Efficiency - You do not say how the 1.5 HP pump is used, but most systems with your size pool and limited water features get by with just one pump. If you are using a 2.6 HP single speed pump now, you can keep the wet end of the pump and just replace its motor with a 2.7 HP variable speed motor.

Suzy  Posted: 01/28/2018 2:14 AM 

Intend to install a 12m x 3m x 1.3 lap pool plus a 2.5m x 2.5m spa. Can you advise on what equipment we will need. we need to heat both the pool and spa using a heat pump. Would appreciate your help.

Efficiency  Posted: 01/26/2018 0:22 AM 

I have a ~12,000-14,000 gallon pool. I calculate ~40 ft of head (give or take 5 ft). I have a 60 sq ft DE filter, 2.6 hp single speed motor and what appears to be 1.5 hp pump and single speed based on serial number on the strainer based. I am not sure if the impeller has been switched out since the motor is higher hp. Everything works fine but I want to reduce my elec cost since it consumer ~500 kwh each month. The local pool company says replace the whole unit and get a variable speed pump to match the motor. The cost is nearly $1500 for a 2.5 hp motor. The pump casing etc 2009 but seems to be in good shape. What do you recommend that I do to get the best bang for the buck. Could I get away with just replacing it with a 2.7 hp variable speed motor and leave the pump alone. Most of the time the pump will be running to clean the filter and occasionally I may run the waterfall.

Inyopools  Posted: 12/11/2017 13:29 PM  Inyo Product Specialist

Marc - Generally, with filters, bigger is better. People oversize their filters to avoid any chance of blowing out the filter and for lower maintenance. A larger filter can go longer between cleanings. However, the larger filter cartridges cost more. I would go with the 150 as a good compromise.

Inyopools  Posted: 12/11/2017 13:17 PM  Inyo Product Specialist

Dan - Generally. a pool function is different than that of a pool - maintenance, chlorine levels. I would tend to keep their operations separate with two pumps.

Dan  Posted: 12/08/2017 14:46 PM 

I'm planning to make a pool/pond. Each body of water is 24,000 gallons. Should I have 2 pumps?

Marc  Posted: 12/06/2017 23:31 PM 

I have a 10,000 gal in-ground pool that I need to buy a new filter. I want to switch from a DE to a cartridge filter. The pump is old but works good and is a 1 HP. My question is I was looking to get the Pentair clean and clear RP filter but should I get the 100 SQ ft, the 150 Sq Ft, or the 200 sq ft? I was told to always get the largest filter I can. Thanks

Inyopools  Posted: 11/13/2017 12:26 PM  Inyo Product Specialist

Anonymous (pump/filter size) - For you 27x6x6 (7,300 gallon) pool, I would suggest something like a Hayward Super Pump 3/4 HP Single Speed with a Hayward Sand Filter with Top Mount Valve 22 Inch Tank.

Anonymous  Posted: 11/07/2017 19:24 PM 

what sizes pump and sand filter I will need for a rectangular pool 27x6x6?

Inyopools  Posted: 10/22/2017 23:21 PM  Inyo Product Specialist

ANA - Your 19 x 45 x 6.5 contains over 40,000 gallons of water, so you will want to buy the larger 2.7 HP Variable like the Speed pump like the PureLine Prime 2.7 HP Variable Speed Pool Pump - PL2606.

ANA  Posted: 10/19/2017 6:55 AM 

I have a 19 x 45 roman pool with an average depth of 6.5 feet. I am looking to get a variable speed pump so I can keep it running 24 hours. What pool pump size would you recommend please?

Inyopools  Posted: 10/04/2017 11:48 AM  Inyo Product Specialist

Mr, Vinegar - We would recommend the larger PureLine Prime 2.7 HP Variable Speed Pool Pump.

Mr. Vinegar  Posted: 09/29/2017 12:45 PM 

Engineering Specs for our 25900 gallon Florida pool (two skimmers)specifies a 105GPM flow rate at a 50 TDM using a 2HP pump. Inspector says flow rates can be calculated three ways with the most stringent being the one used.In our case the most stringent calculation comes from multiplying the number of skimmers times 50GPM so according to him we need a 100GPM flow rate. We have been told that one can not get a 100GPM with a 2HP pump and that a variable speed pump is the answer. Can you suggest a variable speed pump to do this. Thanks

Inyopools  Posted: 09/15/2017 12:00 PM  Inyo Product Specialist

Lu - Your pool would hold about 7200 gallons of water. For that size pool, we would recommend a cartridge pump/filter set like the "Hayward Super Pump 1 HP SINGLE Speed W/ C1200 120 Sq. Ft. Cartridge Filter' or a sand pump/filter set like the "Hayward Super Pump 1 HP Single Speed W/ S244T 24" Sand Filter & Valve".

Lu  Posted: 09/10/2017 17:46 PM 

Good day Guy's, I am about to have a 12 ft x 20 ft pool construct, with a debt of 2-6 ft. What size of pump and filter do I need.

Thanking you,


Inyopools  Posted: 08/20/2017 12:38 PM  Inyo Product Specialist

Jynxy - The more I read on calculating head, the more I'm convinced that they are too many variables in the calculation to come up with anything more than a good guess on your system's head. Many experts say to use 40' to 50' as a good approximation and leave it at that. We are still looking into it, but don't have any better rationale than that at this time. That being said, for your size pool, I would recommend installing 2" pipes and buying the smaller Hayward Tristar VSP 1.85 HP pump. I would also recommend buying a larger filter in the range of 100 GPM to lessen the chance of blowing out the filter. Most pool designers recommend getting a filter that is 25% larger than your pump's capacity. This pump can operate at 80-100 GPM at full speed. You would probably be running it at lower speeds most of the time, so this would give you a good filter cushion. And, a sand filter is probably the easiest to clean/maintain until you have to change the sand every 5-7 years. The cartridge filter filters better than sand but you have to lift out and clean the cartridge every month or so which is a little more time-consuming.

Jynxy  Posted: 08/14/2017 22:41 PM 

My pool is 20' x 34' Mountain Pond (Shortest Width = 16') with 17,700 gallons of water and 518 sf area per my pool specs. Using the article instructions, I get 47 GPM for 8 hour cleaning. I am having trouble guesstimating feet of head as the pool installer does not tell you pipe measurements + # of connections! etched. My estimate is that pool equipment will be approx. 50ft. from the pool deep end. Can someone help me ballpark feet of head. I'm considering the Hayward Tristar VSP 1.85 HP or 2.67HP. I'm telling pool installer I want 2" pipe. I'm getting AquaCal SQ145 Heater. For filter, get one rated for 60GPM?? Still wondering if Sand or cartridge is best. Do not want to deal with complicated time consuming upkeep and needs to be cost effective. Any help is appreciated!! Thank you!