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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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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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(1 to 40 of 603)

 Posted: 6/7/2022 

I have a 20 x 40 30,000 gallon in ground pool, 1.5" plumbing, 1 HP Hayward pump, and 3.1 sq. ft sand filter. Is this sufficient or should I have a larger pump and filter?
 Reply

InyoPools Product Specialist  Posted: 6/9/2022 

The equipment is already in the pool; you would be better able to tell me if the equipment is adequate for your pool. Is the water clear, does the water chemistry seem healthy, and are you happy with the operation of the equipment? With that basic information in the comment, I can say, "go with a bigger filter and pump," but I don't know if it is actually necessary.
 Reply

 Posted: 5/4/2022 

I have a 20 x 40 36,000 gallon in ground pool, 1.5" plumbing, 1 HP Pentair pump, and 3.1 sq. ft., 62 gpm flow rate Hayward sand filter. Also a pool heater. Need to replace pump and filter with variable pump and cartridge filter. Pump is 6' from edge of pool, which has 2 skimmers and 2 returns, not sure how deep the pipes run. Straight line from pump to center drain is about 16 feet. From pump, it's about 18 ft to skimmer 1, 60 ft to skimmer 2, 30 ft to return 1 and 45 ft to return 2, going around corners (not straight line). What's the best pump/filter combo to move the most water with the existing plumbing? Thanks!
 Reply

InyoPools Product Specialist  Posted: 5/11/2022 

What is the model number of your current pump? The information provided is excellent, but calculating feet of head blind is not a good way to do things. So, we'll use your current pump as a baseline. Also, "moving the most water" you can is not always the best way to do things.
 Reply

 Posted: 4/11/2022 

I have a 20k kidney shaped pool, 9' deep end 3' shallow end. My filter is a PLDE48 Mod 3. I estimated about 56 gpm in a 6 hour time and about 40 feet of head. With a clean filter running at about 10 psi.(2" piping) I was looking at getting the Jacuzzi 1.85 pump. Would this be to much or will that be sufficient? Thanks in Advance.
 Reply

InyoPools Product Specialist  Posted: 4/14/2022 

We don't carry any Jacuzzi 1.85 VS pool pump, so I am not sure which model you reference. Jacuzzi is a manufacturer that sells different lines of VS pumps, which would have varying flow rates. But it looks like you have all the information needed to look at the pump curve for the specific model.
 Reply

Anonymous  Posted: 4/9/2022 

I have a 16,000 gallon pool under construction. Tristar VS 950 pump. Will a C100S filter be sufficient?
 Reply

InyoPools Product Specialist  Posted: 4/14/2022 

Yes, it should be enough based on the information provided.
 Reply

 Posted: 3/25/2022 

I have a 40,000 gal inground pool, SD80 sand filter, 1HP pump, and 1.5 plumbing. I’m getting 50GPM flow measured coming out of the filter but my circulation and pressure is low on the 2 drains, 3 skimmers, and jets. Thinking about upgrading to 1.5 or 1.65 VS pump to increase circulation for my size pool but concerned about the plumbing size and if it would even help. But initially I was thinking the pump is undersized based on the size of my pool. Any thoughts? Thx
 Reply

InyoPools Product Specialist  Posted: 3/28/2022 

The 1HP pump is undersized for a 40,000-gallon pool, but the 1.5" plumbing restricts you from going higher than a 1.5-1.65HP. 2HP pump would cause cavitation, which is terrible for your system. The flowrate boost produced by switching to a 1.65VS pump should give you a noticeable difference.
 Reply

Anonymous  Posted: 10/12/2021 

Do you have formula calculations in metrics?
 Reply

InyoPools Product Specialist  Posted: 10/14/2021 

No, but you can use an online gallon to liter calculator to use the formula, and then convert the totals back to metric.
 Reply

 Posted: 10/4/2021 

I have an 25K gallon pool. I believe I have 60 feet of head (It's a 13x40, with the pump being at the far catty corner from the farthest skimmer, so does 60 sound about right?) My filter is a cartridge filter than can handle 140 gpm. The pump that died is 1.5 hp, however it was driving a sand filter for most of it's life. According to my calcs is 1.4 or 1.65 the best size waterway pump to get?
 Reply

InyoPools Product Specialist  Posted: 10/5/2021 

You've done some excellent work there. You are correct; the 1.65 would be my preferred choice. Of course, you could always go with a 1.65 or even a 2.7 HP VS to give you the top-end flow rate if you ever have to do a chemical treatment or heavy cleaning. But if you are going with a single speed, the 1.65 would be my choice.
 Reply

 Posted: 8/14/2021 

I have a inground pool 8 .000 gal heater getting a high limit fault have a3/4 hp about 45 feet from pool hot tub flow rate calls for 30 gpm I cleaned took cartridge filter out heater ran a little bit longer but still faulting so it's the heater or the pump I dont know do you think I need a 1 hp ??
 Reply

InyoPools Product Specialist  Posted: 8/17/2021 

This sounds like a heater issue, not a pump issue. The hi-limit switch is telling the heater the water is too hot, not that there is a low flow. The most likely fix would be to replace the hi-limit sensor.
 Reply

 Posted: 8/8/2021 

I am wanting to replace my pool pump with a Hayward VS pump. My current pump is a 2HP motor, can I replace it with a TriStar VS 1.85HP or do I need the 2.7HP pump? The pool size is approximately 16,000 gallons. I am also thinking about replacing the 1HP pump for the water features but am not sure if it is worth buying a variable speed pump for the water features.
 Reply

InyoPools Product Specialist  Posted: 8/12/2021 

What is the model number of your pump housing? There is a wide range of pool pumps, I would need to know what you have to accurately compare the two.
 Reply

 Posted: 8/12/2021 

If I am reading it correctly it is B855, part # 0-177897-22.
 Reply

InyoPools Product Specialist  Posted: 8/13/2021 

That is the motor number. You need to look on the pump housing for the sticker, and not the motor case. You can see the difference between the two, here - Pool Parts 101: The Difference Between a Pump and a Motor 
 Reply

 Posted: 8/13/2021 

Unfortunately there are no tags on the pool case. Only on the motor itself.
 Reply

InyoPools Product Specialist  Posted: 8/17/2021 

You should be able to use the TriStar 1.85 as a replacement for most pool pumps. The TriStar is Hayward's high-end residential pump, so it pushes more water than your standard Hayward Super II or Pentair WhisperFlo.
 Reply

 Posted: 7/19/2021 

I just replaced my Haywood Super pump that was a 2hp with a1.5hp. I have a small kidney shaped pool with 21,000 gallons. Free installation, my Haywood heat pump is showing a P5 error and will not go on. Any thoughts?
 Reply

InyoPools Product Specialist  Posted: 7/20/2021 

Ok, you replaced a 2 HP pump with a smaller 1.5 HP; and now your heat pump is reading a PS code? If you reference the HeatPro manual it shows "PS" is referencing a low-pressure switch. I would take a look at the HeatPro Manual on page 18 for a list of solutions.
 Reply

 Posted: 7/19/2021 

I have a 26300 gal pool which is 55ft from the pump and filter using 1.5 pipe. I have calculated a 180 head. The pool was installed by the previous owner about 25 years ago with a 1HP Hayward, I replaced the pump with a 1 HP SuperFlo High performance Pentair. Could I up to a 1.5 Hayward?
 Reply

InyoPools Product Specialist  Posted: 7/23/2021 

I think your calculation of feet of head is incorrect. A 1 HP pump would have trouble getting good flow through 90 feet of head, let alone 180 ft. You may be able to go up in HP, depending on the size of your filter. What is the model number of your filter?
 Reply

 Posted: 5/7/2021 

What size pump do I need to run three 8ft. cascades? Using 2" pipe that reduces to 1.5" in three inlets on each 8ft. cascade. Anyone?
 Reply

InyoPools Product Specialist  Posted: 5/13/2021 

If this is a new install, does the cascade waterfall manual list a required/optimal flow rate? If this is an old install and you are just replacing the pump, what is the model number of the old pump?
 Reply

 Posted: 5/4/2021 

I have a 13'8" x 30' 12,000 gallon pool and our Jandy Model#PMPM,1.5 Serial#K07FC0389 went out after 12 years. We purchased a Hayward Super Pool Pump 115/230V 1.5 HP (suggested by a pool company) but it seems so much smaller. I just want to make sure it will work without overworking my pool before installing. Any advice you can share is greatly appreciated!
 Reply

InyoPools Product Specialist  Posted: 5/13/2021 

The 1.5 HP Hayward Super Pump should be well suited for a pole that size. I'd go ahead and install.
 Reply

 Posted: 5/13/2021 

We did and so far it is working great! Thanks for the feedback!
 Reply

 Posted: 3/24/2021 

Hello, I have a 28,000 in-ground pool. if the pool originally had 2 pump motor can i change that to just one motor?
 Reply

InyoPools Product Specialist  Posted: 3/26/2021 

What do you mean by a 2 pump motor? Do you mean the pool had two pumps? Regardless of the answer, I would need to know the pumps' make and model to get an idea of what might need to replace them. Does the pool have a spa, water features like laminar or deck jets, or does it have a solar heater?
 Reply

 Posted: 3/1/2021 

The pool is small. 9000 gal. 5 1/2 ' deep. I have absolutely no idea how to calculate filter size for this. The motor is 1 1/2 hp. The present sand filter is quite old fiberglass and our current record freeze damaged it. It is 30" high that sits on a 6" high base and circumference is 6 1/2 '. It's a Triton (part #140210 (?)). It has a drain valve on the bottom. My search has not found a match for this. Can you help??
 Reply

InyoPools Product Specialist  Posted: 3/1/2021 

What is the model number of your pump housing? Pump flow rates can vary widely depending on the pump model.
 Reply

 Posted: 3/3/2021 

Pentair pump. Model # DYNII -1 - 1 1/2 Part # 340106 1.5 hp 230/115V 60Hz 1PH KW 3450 RPM MaxAmps 8.0/16.0 Again the damaged sand filter is a Pentair Triton II Part # 140210 fiber glass. 30" high on a 6" base. 6 1/2' circu
 Reply

InyoPools Product Specialist  Posted: 3/4/2021 

Is this an inground or above ground pool? Because the DYN is a Pentair Dynamo pump that is used mainly on above ground pools. I did a search for both of the model numbers you listed, and the models came up on our site: 140210 and 340106
 Reply

 Posted: 3/5/2021 

It's an in ground pool. 20x30. 9000 gal. 5 1/2 deep. Think it's suppose to be called a sports pool. (?)
 Reply

InyoPools Product Specialist  Posted: 3/8/2021 

You can go with the same sized sand filter (140210), but if you are on a budget and you are sticking with the same pump, you can use a 21-inch tank Triton II TR50 Filter Side Mount 21 Inch Tank - 140249.
 Reply

 Posted: 1/13/2021 

I have a 200k Gallons in ground pool , what size pump would i need and also what filter would be suitable ? Thanks
 Reply