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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Video

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.


Comments

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(281 to 320 of 603)

 Posted: 6/3/2016 

So for variable speed pumps is the HP rating not important due to the ability to change speeds? I'd really like to go with a variable speed pump to get the most efficiency. We have a lot of trees around the pool so it's going to be running quite a bit.
 Reply

InyoPools Product Specialist  Posted: 6/2/2016 

Chris - For your size pool and water accessories, I would recommend one of two size pumps. If you have 1 1/2" piping, I'd go with a 1 1/2 HP pump. If 2" piping, go with a 2 HP pump. One pump should be able to handle your pool requirements. We have a series of new pumps we are recommending for quality and savings: the PureLine Pure Water Pool Pump 2 HP -PL3503 and the PureLine Pure Water Pool Pump 1.5 HP - PL3502. There is some confusion in the HP descriptions. The PL3503 is 2 HP not 1 HP, and the PL3502 is 1 HP not 3/4 HP.
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 Posted: 6/1/2016 

I've got a 20,000 gallon pool, with roughly 35' of head, a waterfall and a pool sweep. It has a Hayward C4025 filter and some really old pumps (made over 20 years ago). Can I use one pump? What size would be ideal? I can post photos if necessary.
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InyoPools Product Specialist  Posted: 5/31/2016 

6000-gallon pool - Here a link to a VS pump that would better suit your small pool needs: Hayward Super Pump VS Variable Speed Pump (115V) 0.85 HP. If your pool system needs a stronger pump for jets, water features, look at this new Prime Variable Speed Pool Pump 1.65 HP. Great quality and price.
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InyoPools Product Specialist  Posted: 5/31/2016 

BM11 - For your ~26,000-gallon pool, I would recommend a mid-size VS pump like the new PureLine Prime Variable Speed Pool Pump 1.65 HP.
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Anonymous  Posted: 5/30/2016 

Hi. I have a small 6000 gallon pool and I need to upgrade to a variable speed pump. I've been told that I can't just upgrade the motor but that it needs a whole new pump with pipes etc. He is recommending the Pentair Intelliflo 2 vst(?) which he says is the new one that they're selling. I'm trying to do some research and I'm seeing that the Intelliflo has a 3hp motor. Is this correct and if so why would he be suggesting such a large hp for my small pool? Does Pentair Intelliflo make a 1hp-1.5hp vs motor?
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InyoPools Product Specialist  Posted: 5/30/2016 

Susan - Susan - The humming you hear from your motor is most likely due to the start capacitor going bad. To check this capacitor and possibly replace it, see our guide on “How to Replace a Pool Pump Capacitor”. For you size pool, I would stick with the 1 hp pump/motor.
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InyoPools Product Specialist  Posted: 5/30/2016 

Motor humming – The humming you hear from your motor is most likely due to the start capacitor going bad. To check this capacitor and possibly replace it, see our guide on “How to Replace a Pool Pump Capacitor”.
 Reply

InyoPools Product Specialist  Posted: 5/30/2016 

Dave – Do you have diverter or ball valves to regulate water flow to your waterfall or are they taking the full pressure from your 2 HP pump. If the water flow is not being regulated and you replace the 2 HP pump with a 1 HP pump, will you be satisfied with half the waterfall length? If they are being regulated, how close are the valves to being fully open? Half way would be equivalent to a 1 HP motor.
 Reply

 Posted: 5/28/2016 

Hello , I have an 18x36 in ground rectangular pool. 3ft-8ft. What size HP variable speed pump should I buy?
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 Posted: 5/25/2016 

Have a 20000 gal inground rectangular pool - currently have a Hayward Super Pump HP 1. Currently hums and then blows breaker. Would a 3/4 hp 115 pump work for me?
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Anonymous  Posted: 5/25/2016 

I have a 20000 gallon rectangular pool with 1 skimmer. I currently have a Hayward Super Pump that just hums and then blows my breaker. Could this be a short in the pump?
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 Posted: 5/24/2016 

Thanks, Inyopool!

Second question. I have the same 2hp Northstar pump for my water feature, which is 2 - 18" slit waterfalls. I think that is overkill as well. Since it's only a feature with no cleaning or filtering duties, could I use a 1hp single speed? While I can look at the pressure gauges (filter and pump), not sure how I could determine the needed HP necessary so that I still have waterfalls and not just trickles, especially since I will most likely have to do some PVC re-routing for a smaller pump.

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InyoPools Product Specialist  Posted: 5/23/2016 

john mac - For your size pool with spa, I would definitely go with the larger 3.0 HP VS motor.
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 Posted: 5/22/2016 

I currently have a starite maxi glass II, 2hp pump . 22000 gal pool with attached spa... I want to replace with variable speed motor. Looking at eco tech motor . Should I get 1.5 hp or 3.0 hp?
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InyoPools Product Specialist  Posted: 5/10/2016 

Dave - Generally a 2 HP pump is larger than you need for a 13,000 gallon pool. I would go with the 1.5 HP VS pump.
 Reply

 Posted: 5/10/2016 

I have a 13,000 gal salt pool with attached spa. Drain, skimmer and pool sweep. Currently running 2hp Hayward Northstar. Want to get a variable speed to try and save on electricity. Do I need a 2hp, or would a 1 or 1.5 suffice?
 Reply

InyoPools Product Specialist  Posted: 5/9/2016 

Steve - No, you do not need to make any other system upgrades to convert to a VS pump. It is a relatively easy process.
 Reply

InyoPools Product Specialist  Posted: 5/9/2016 

Homerun - If you downsized your motor from 1 HP to 3/4 HP without changing the impeller, you may be overloading your motor severely. The impeller is trying to move as much water as it did with a smaller motor. Check if you need a smaller impeller, and if so, replace it. Or, for your size pool, a 1 HP motor is generally what's required to move that much water. You might consider going to a 1 HP motor.
 Reply

InyoPools Product Specialist  Posted: 5/9/2016 

Willis258 - Sound like you have a 30,000 gallon pool. I would use 2" or 2 1/2" pipe where you can. Also, for your size pool, I would consider a large variable speed pump. You can save up to 80% on operation costs. Here is a guide on what you might consider for the circulation function of your pool - "How To Set Up In Ground Pool Equipment - Part 1 - Circulation".
 Reply

 Posted: 5/8/2016 

I currently have a single speed pump. Can I replace it with a variable speed pump or do I need to make addidional upgrades to the system?
Thanks,

 Reply

 Posted: 5/8/2016 

I have a rectangle pool 14w30long from 3ft to 9ft 3/4 motor was just put in ,,But the fanblade was not changed so some times it runs strong & other times very sluggish,,there is a small leaf floating around for 4 days it don't have enough thrust to move it to the skimmer so what do i do ??
 Reply

 Posted: 5/7/2016 

What size pump should I use on a 20x40 with 6 returns aND 3 step jets, also should I use 2 inch pipe for the skimmers I have a 50ft and 70fy run on the skimmers to the pump.
 Reply

InyoPools Product Specialist  Posted: 5/4/2016 

Windy Miller - Sounds like your proposed pool size is just over 100,000 gallons. So you are going to need as large a pump and filter as you can get. And you may want to split your pool functions into two systems: one for basic circulation and one for water features. See our guide on "How To Set Up In Ground Pool Equipment - Part 0 - Overview". This series of guides was based on a 30,000 gallon system so you may have to get larger equipment or run it longer. I would get the largest Variable Speed Pump you can find like the Sta-Rite INTELLIPRO VARIABLE SPEED ULTRA ENERGY-EFFICIENT POOL PUMP. On the cartridge filter, look for maximum sq. ft. filtration and GPM flow like the Jandy 4-cartridge system shown in the guide.
 Reply

 Posted: 5/2/2016 

I am building a pool 82 x 33 x 5 ft - from the formula it comes out to a flow rate of ~211 GPM to filter water in 8 hours. What size of pump, filter(s) and pipe do you recommend? I would very much appreciate some advice. Thanks Windy
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InyoPools Product Specialist  Posted: 4/19/2016 

les gideons - Here's a link to our best priced 2 HP in ground pool pump - PureLine Prime Pool Pump 2 HP. Shipping is free. Total cost is $399.
 Reply

 Posted: 4/19/2016 

price of a 2 horse in ground pool shipping price included. thanks
 Reply

InyoPools Product Specialist  Posted: 4/14/2016 

lovesbiddy - I would replace your current pump with one that has the same HP and SF. See our guide on "How To Understand True Pump Horsepower - Up Rated vs Full Rated". Pool heaters will shut off at 20 to 40 GPM depending on the make and model of heater.
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 Posted: 4/13/2016 

We have a pretty old pool but it is new to us as we just purchased this home. It does not have a bottom drain, only has 2 jets and 1 skimmer. We would like to replace the pump on it and by the calculations you have shown we need one with 51 GPM and just one skimmer at approx. 45 ft of head. Does the fact that it doesn't have a bottom drain create any extra issues? What about the pool heater, does it need to have the same flow requirements like the filter?
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InyoPools Product Specialist  Posted: 4/13/2016 

LuvMahPool - It sounds like most of your piping is 1 1/2" so I wouldn't go with a much larger pump. Variable speed is good. You can recoup most of your initial cost with an up to 80% reduction in operating cost. For a great price on great quality, I would recommend the newer "PureLine Prime Variable Speed Pool Pump 1.65 HP".
 Reply

 Posted: 4/11/2016 

I recently bought a house with a pool. I'm a first time pool owner. The pool is 36,000 gal. I have two skimmers connected in series through one 1 1/2"(65' long) pipe to valve where it connects to the drain 1 1/2" (38' long) line. These are joined to one 2" (2' long) line to the pump. All the rest of the plumbing is 2" line until it get to the ground to return to the pool via the three jets. There, I split the 2" line to use the existing 1 1/2" (38' long) pipe to the three jets and also use the 1 1/2" (30' long) line to the Polaris (no longer used). I am now using this line as a fourth jet. My pump is a 1 1/2hp single speed pump and my flowmeter shows 70gpm, filter pressure is 20psi. Would I benefit from a larger pump on a variable drive or would the savings be negligible?
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InyoPools Product Specialist  Posted: 3/18/2016 

scpoolguy - I would recommend a 1 HP pump with 2" piping.
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 Posted: 3/15/2016 

I am looking to build a 14x28 rectangle, around 12,500 gallons, 1 skimmer, main drain,3 returns, 2 step jets. Based on wind direction and where I would like to locate my pump and filter I am looking at about 70 feet of pipe from the pump to the skimmer and about 65 feet of pipe to the main drain.

My questions are=
1. What size pump should I use?
2. What size piping should I use?

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InyoPools Product Specialist  Posted: 3/7/2016 

Wai Kane - Glad to help. If you have a rectangular pool, you multiply the pool's length times it's width times its average depth. That will give you your pool's cubic feet. There are 7.5 gallons of water in a cubic foot so multiply your pool's cubic feet by 7.5 to determine its total gallons. See step 1 of this guide for an example of this calculation. GPM is Gallons Per Minute. You get this number by dividing Gallons Per Hour (GPH) by 60. See Steps 5 and 6. Also, if you would like to see what steps a builder goes through to build a pool, see our guide on "How To Build an In-Ground Pool".
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InyoPools Product Specialist  Posted: 3/7/2016 

wahoo - You may well have a leak around you skimmer. As an older pool shifts in the ground over time, it may cause a crack in your skimmer around the piping. Installing a new motor and pump will not fix this problem if that is what you are suggesting, but it sounds like you are due for a new pump. The energy usage for your pool is the same with 110V or 220V.
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 Posted: 3/6/2016 

Thank you for posting how to calculate the amounts of gallons for a pool.
I'm trying to understand how to build a pool, not that I will build one, but just so I have some education on building one when I hire someone.
My question is how do you get the last calculating factor to figure out the amount of gallons?
LxWxDx? = GPH
Also, what does GFM mean?
Thank you once again for your educating me.

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 Posted: 3/5/2016 

20 year old motor,50 year old brass pump.

110 volt,1 1/2 pipe,hayward DE filter.

Might have leak in skimmer. When I install kreepy K hose and plate, It creates a air bubble under plate,even though it is under water.

Can i install just a new motor and pump?
Does it make sense to go to 22o volts?

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InyoPools Product Specialist  Posted: 3/4/2016 

ekh443 - A 1 1/2 HP VS pump should be adequate for your size pool if you don't have a lot of extra features.
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 Posted: 3/2/2016 

Hello, I just bought a home with a 18,500 gallon pool. It currently runs a Hayward Super II, 2HP single speed pump. I want to switch it out to a VS pump. Do I really need a 2HP?
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InyoPools Product Specialist  Posted: 3/1/2016 

faceache - Your pump is the right size for your system. Your suction cleaner should work at around 2400 RPM and the Spa should run well at full RPM, 3450. It sounds like you have a low water flow problem. See our guide on "How to Correct Low Water Pressure in Your Pool System". A clogged impeller is a common problem.
 Reply