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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.

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Step by Step

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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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InyoPools Product Specialist Dennis R. Posted: 5/22/2017

Jw - I would recommend buying an up-rated 1 HP pump for your size pool. And yes, people generally run the skimmer and sweep at the same time.
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InyoPools Product Specialist Dennis R. Posted: 5/22/2017

Palmer - The 115V VS pumps are either for above ground pools or are too small for your size pool. If you can convert to 220, the VS pumps offer great operational savings. When you cut the motor speed in half, your water flow also reduces to a half but your energy cost is reduced to 12.5%. So yes, during filtration, you would run your pump twice as long at half speed but for a fraction of the cost. On your other question, if you bought a 1.5 HP pump to replace your 1 HP pump, you would run it for 7 hours instead of 10. You may not be saving much, however, because the 1.5 HP pump requires mot amperage than your current 1 HP pump.
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Jw Posted: 5/21/2017

Kidney shaped 10500 gallon pool. 15'W 17'W 34'L 3'S 7.5'D. 1/2 hp pump now. Do I need more hp pump? Pool guy says I do. Pump works fine just air leak from valve. Do I run sweep and skimmer at same time? Please help getting hot days now
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palmer Posted: 5/20/2017

My single speed 1 hp pool pump for a 20 X 40 inground pool (depth from 3.5 ft to 9 ft) just broke. I was thinking about getting a variable speed, but I don't know much about it. I ran the single speed pump around 10 hours a day. Does it pay for me to purchase a 1.5 hp pool pump and should it be a variable speed? Does a variable speed pump require you to run the pump more to keep the pool clean? If I get a 1.5 hp pool pump single speed does that require me to run the pump a shorter period of time than a single speed 1 hp? Also does a variable speed pump run on 110 Voltage?
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Palmer Posted: 5/20/2017

Does a variable speed pump clean as well as single speed pump?
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InyoPools Product Specialist Dennis R. Posted: 5/19/2017

Sunnymick - Yes, the A.O. Smith 1 HP Round Flange Up Rate Motor - UST1102 is the correct 1 HP motor replacement for a Hayward SP2607X10 Super Pump. Be sure to also buy the GOKIT03 for the replacement gaskets and shaft seal.
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Sunnymick Posted: 5/18/2017

I have a kidney shaped pool 14Wx28L, 3x6 depth. I am looking to buy a replacement motor, but the label on my current motor is worn and not legible. I believe it is a Hayward SP2607X10 Super Pump 1-HP Max-Rated Single-Speed Pool Pump (1hp). Does that sound right? I am looking at this replacement motor: http://www.inyopools.com/Products/00200017001331.htm?Keywords=ust1102
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InyoPools Product Specialist Dennis R. Posted: 5/18/2017

pam - For your 24x5' abound ground pool, I would recommend a 1 HP AG pool pump. Make sure the filter is large enough to handle this pump.
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InyoPools Product Specialist Dennis R. Posted: 5/17/2017

brendarhope50 - If you have 1 1/2' piping, I would use a 1 HP or 1 1/2HP Hayward Super Pump. If you have 2" or higher piping, I would use a 2 HP Hayward Super pump. A better option for your size pool would be a variable speed (VS) pump. It could save you up to 80% on operational costs. A newer and more economical VS pump coming into the market is the PureLine Prime Variable Speed Pool Pump 1.65 HP.
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pam Posted: 5/17/2017

I have a round 24x5' deep pool what size pump do i need
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brendarhope50 Posted: 5/16/2017

I have a 20 x40 rectangle pool with drain at deep end which 8 ft deep shallow end is 4 ft deep, what HP would i use with the Hayward Inground super pool pump?
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InyoPools Product Specialist Dennis R. Posted: 5/15/2017

Gijs - The Pentair Intelliflo is much bigger than you need. The Pentair SuperFlo VS is a better fit for your size pool. You might also look at the newer PureLine Prime Variable Speed Pool Pump 1.65 HP. It is equivalent in design and quality to the Hayward and Pentair pump at a lower price.
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Gijs Posted: 5/14/2017

Hi! Would the Pentair Intelliflo be too much for a 14000 gallon pool? Does the Pentair SuperFlo VS has sufficient capacity? What would you recommend?
Thanks!
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InyoPools Product Specialist Dennis R. Posted: 5/8/2017

TimR - Is your filter pressure gauge reading high? If so the resistance to water flow (feet of head) in your pool may be too high for a 3/4 HP pump. You might consider going to a 1 HP Pump. Pentair's WhisperFlo pumps are quieter than most.
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InyoPools Product Specialist Dennis R. Posted: 5/8/2017

GPCowan - I would recommend getting the 1.67 HP VS pump to be able to run your waterfalls at full power when desired yet be able to run at half speed or less during most of the time for filtration. Running your VS pump at 1/2 speed only uses 1/8 of the energy (cost) of running at full speed.
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GPCowan Posted: 5/6/2017

We planning a small 7000 gal pool (no spa) with 3 waterfall features. Max depth 4.5', 2" piping and flat lot. I am getting recommendations of 2 HHP variable speed pump which just seemed large as normal 8 hour full circulation is 17 GPM. I don't want to undersize but even 1.5 HHP may be too large. Your thoughts...???
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TimR Posted: 5/5/2017

On our 4th pump since 2003, all professionally chosen/installed. 24k gallons, one intake, one return. Current 3/4 HP AO Smith with FNS Plus 36 filter. Need to do it myself this time. Want one that is quiet. No additional equipment but may add gas heater later. What do you recommend?
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InyoPools Product Specialist Dennis R. Posted: 5/2/2017

southernmom121 - If you have an older pool that was designed with 1 1/2" piping, I would not go over a full-rated 1 HP pump. If you think you have larger pipes, you could go with a 1 1/2 HP full-rated pump. If you invest in a pump with a variable speed motor, you don't have to know your system piping. You can set the motor speed to what works best operationally.
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InyoPools Product Specialist Dennis R. Posted: 5/1/2017

Chrisv - A standard 2.7 pump is too big for your size pool. However, the beauty of a VS pump is that you don't have to use the full power of the pump. You can use it at 2/3 speed or so for the spa then back it down to half or below for the pool.
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southernmom121 Posted: 5/1/2017

I have a 20x40 pool - 37,000 gal - what pump size would you recommend? I dont have the pipe measurements. Im just trying to get a general idea.
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Chrisv Posted: 4/29/2017

Hi guys, I just purchased a house with a pool (my first pool) and i used your info to calculate that my pool is just under 13k gallons INCLUDING the built in spillover spa. Before I found your site I got a great deal on a Hayward Tri-Star VS 950 pump that has 2.7hp that is not yet installed. The current set up is a 1hp pump for the pool and another pump that is for the spa. I'm planning going from the current 2 pump set up down to just this 1 new hayward pump. My question is, is my new pump going to work? Is it too powerful or is it ok since it will be doing the job of what 2 pumps used to do and it's variable speed. thanks!
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InyoPools Product Specialist Dennis R. Posted: 4/27/2017

kplum - Your current 3HP Pentair pump is much too big for a pool with 1 1/2' piping. It is generally used with 2 1/2" and 3" piping. I would use a 1HP full rated pump for your pool.
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kplum Posted: 4/26/2017

Hi. We bought a house that has a 38,000 gallon pool. It has 1.5" piping and a 3HP Pentair pump. I believe we got air in the line and the pump ran for quite some time with no water. It seemed to have overheated and melted some of the pie around it, which of course caused more air. We had someone come out to fix it and said we need to remove the 3HP pump and get a 3/4HP pump. is that correct? Rectangular pool, 38k Gallon, 1 skimmer, no main drain. Trying to see if I am being taken or if we should just fix the current melted connector.
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InyoPools Product Specialist Dennis R. Posted: 4/18/2017

Jacksonman - We are currently in the process of modifying this guide and accompanying blog for estimating feet of head. It greatly oversimplifies a complex process. You are correct when you say your current estimate based on rough numbers is too high. Professionals would estimate the feet of head for your size pool to be more in the neighborhood of 60 to 80.
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Jacksonman Posted: 4/17/2017

The feet of head calculation has got me down. Our pool is roughly 28,000 gallons, 2 skimmers, a deep end drain. Seems to me I'd have at least 17 90 degree elbows. These elbows alone seem to add 129 feet of head! Add this to the roughly 260 feet of 1.5 inch piping for skimmers and drains, and add a furnace and sand filter, and I don't know if my 1.5 hp pump motor is the answer. Help! The pool was built in 2005.
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InyoPools Product Specialist Dennis R. Posted: 3/10/2017

head for solar panels - We do not have the numbers for added feet of head for solar panels mounted on a roof. I would suggest adding another 1/2 HP to your standard HP requirement for a pool without elevated solar panels.
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InyoPools Product Specialist Dennis R. Posted: 3/10/2017

Nic - Unless your pool system has very log runs of piping with an extreme number of turns and elevations, I would question your value of 159 feet of head for an 11,000-gallon pool. Feet of head for a typical pool of that size run from 40 to maybe 90 ft. A typical 11,000 gallon pool uses a 1 to 1 1/2 HP pump.
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Anonymous Posted: 3/6/2017

What is the head loss from solar panels? I have 6 panels on the roof of a single story house.
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Nic Posted: 3/6/2017

i have a pool with 11000 gallon with 159 feet of head what pump would you recommend?
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InyoPools Product Specialist Dennis R. Posted: 2/28/2017

bigdon - You may have to have someone come in and do a pressure test on the input lines of your system. They close off the end of a section of pipe and put pressure in the section. If it section loses pressure quickly, you have a leak. If you have an underground leak, you may have to dig up the pipe.
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bigdon Posted: 2/27/2017

how to find incomming air leak in underground plumbing or repair
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InyoPools Product Specialist Dennis R. Posted: 2/26/2017

Cowboyzzz - Generally speaking, your pump size depends on the size of your pool. For pools 5,000 gallons and less, you could stay with a 3/4 HP pump. Up to 10,000, use 1 HP. Above 15,000, go with a 1 1/2 HP. When you compare pumps, make sure you are looking at the total HP (THP). THP is the product of listed HP and Service Factor (SF). See our guide on "How To Understand True Pump Horsepower - Up Rated vs Full Rated".
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Cowboyzzz Posted: 2/23/2017

I have a 3/4hp pump in pool that took a crap. Should I replace with a 3/4 or can I go to a 1hp or 1 1/2 hp
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InyoPools Product Specialist Dennis R. Posted: 1/24/2017

SeanC - I'd recommend replacing your Pentair SD-60 filter with a Pentair SD-80 filter. It will give you about 10 GPM more filter capacity than your current filter. That should be sufficient and it is about as high as you can go if you have 1 1/2" piping.
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SeanC Posted: 1/20/2017

I have a problem. I have an inground swimming pool, with an attached jacuzzi that waterfalls into the pool and a rock waterfall that drops water into the pool. The jacuzzi jets have a separate pump. I have a Pentair SF-N1-2A that supplies the water to the jacuzzi, rock waterfall, and pool. The problem is the filter. I have the Sand Dollar SD-60 filter. Two years after building, I am now told that I need to replace the filter as it is the wrong one. One option is to downsize the pump, but if I do that I think that I will lose the waterfall effects. If I replace the filter, what size should I get?
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InyoPools Product Specialist Dennis R. Posted: 12/19/2016

sagar kandel - The answer to your question is more involved than can be answered here. Please give us a call at 407.834.2200 and discuss this with one of our service reps.
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sagar kandel Posted: 12/14/2016

l want build olympic pool , pool volumes 2500cubic meter
so which size of filter I can use and how many .
which power pump and how many pump we can use . and indoor olympic pool heater how many kw we used please kindly send me idea thank you .
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sbaussie Posted: 10/2/2016

I need to replace a pool pump ( motor cuts out when warm on high speed) . pool is a funny Tshape with total gallons around 29000. I have a relatively new 60 Sq Ft Pro-Grid De Filter (1 year) Really old Whisperflow Pump (probably original 20 years ago) that I have replaced many parts on over the years (pump seal. backing plate . gaskets etc). Motor is 2 speed (also replaced from original )I cannot get enough suction to move great White pool cleaner unless I cover water intake on bottom of pool . Same applies if trying to vacuum with regular old skimmer box style vacuum. Gauge on Filter reads 20 psi after cleaning filter . There is a pool / Spa heater inline on return side .
Thanks
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InyoPools Product Specialist Dennis R. Posted: 9/26/2016

Bftigger - For your size pools, I would recommend a 1HP or 1 1/2HP pump. You are right on the edge. See our Pureline Pool Pumps. For the filter, I would recommend something like a Hayward Star Clear Plus 120 Sq Ft. Filter 1.5" Ports. We always recommend a little larger than needed on the filter. It will last longer and require less maintenance. I also think cartridge filters are the easiest to maintain.
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