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

Written by:  Danny Rhodehamel

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.

Tips & Warnings


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. A good estimate is to take the average number 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 three 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 lengths and divide by three. Thus, 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. 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 GFM rates which equate to a higher HP pump. 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 GFM. 2" pipes can handle up to 100 GFM.

Comments (1 to 40 of 341)

Add Comment

User: Inyopools

TBtooks - Yes, you can use a 1.5 HP pump for your pool. I would hedge a little and buy an uprated 1.5 HP pump rather than a full rated pump. The actual or Total HP (THP) will be closer to 1.5 HP than the full rated pump which can be 2 HP or greater. THP = the listed HP times the motor Service Factor (SF) listed on the motor label. See our guide on "How To Understand True Pump Horsepower - Up Rated vs Full Rated".

User: TBrooks

Hello, We are trying to figure out if we can use a 1.5 horse power pump on our pool. The cooper pipe that connects to the pump is 1.5. Is this large enough to use a 1.5 HP pump? Right now we have a 1 HP pump but our pool is large and it doesn't seem to circulate the water very well. Any information you can provide would be appreciated. Thank you.

User: Inyopools

DurValFarm - The C5030 is definitely overkill for your system. But you can never have a filter too large. It will last longer and not require as frequent cleaning. Does your system still have good flow? If not, your problem may be somewhere else in the system like a clogged pump impeller. See our guide on "How to Correct Low Water Pressure in Your Pool System".

User: DurValFarm

We have a MaxFlo 1 hp pump and just got a Hayward C5030 cartridge filter installed. Our pressure is now virtually non-existent. Could the filter be too large for the pump or needs of the pool? Our pool is IG 15,000 gal.

User: Inyopools

pump stopped - See our guide on "How To Troubleshoot a Pool Pump Motor - Motor Overheated" for possible causes of a motor failing.

User: Inyopools

Sam - The PHPM is the newer version of the PHPU. I don't believe it is a drop-in. Some plumbing adjustments may be required. If your pump wet end is still good, you might consider just replacing the pump motor. See our guide on "How To Replace the Motor on Your Pool Pump".

User: Inyopools

wahoo - Yes, we generally recommend a 1 HP above ground pump for an 18 ft round AG pool.


i have a 25000 gal pool the hayward pump just stopped working. had it replaced two years a go.

User: Sam

I have a Jandy PHPU 1.5 pump that needs replacing. I don't see that model in your catalog. Can you recommend one like it? Thanks.

User: wahoo

We have an 18ft round above ground pool with a 1.5 hp (70gpm) Hayward pump that came with the house we just purchased. The pump is loud and we are guessing it's not suppose to be. we are considering replacing the pump. My question is if it would be OK to replace with the 1 HP version of the pump. The pump sits about 10 feet from the skimmer below the water level.

User: Inyopools

adpinc - Your filter size is more than adequate for your 1.5 HP pump. Most pumps operate with some bubble in the top of the strainer basket. As long as you aren't seeing bubbles continuously coming out of the return ports, you should be alright there. If you haven't changed your sand in 5 years, you might change it now. Lastly, tap your gauge slightly to see if it adjusts. Sometimes they go bad.

User: adpinc

Hi - I think my pool pump is oversized. My filter pressure after backwash is about 20psi. My head run is about 35' average. There are a lot of bubbles in the basket of my Hayward 1.5hp Super pump, but there doesn't appear to be a leak anywhere on the suction side. I can't seem to figure it out. My filter is pretty large, it is a sand filter with 3.14sq ft of area, and can handle 20gpm/ft^2 or around 60gpm. Is my pressure ok in the filter?

User: Inyopools

Elevated equipment - I'm not sure on the impact, but it sounds like you are adding 8 feet of head onto your calculations. I would get a full rated motor to be on the high side of the listed HP. Or you might consider buying a VS pump.


My pool equipment is 8 ft higher in elevation than my skimmer and main drain. How does that factor into the equation to determine filter pump size and pipe diameter?

User: Inyopools

Jbizzle - If you are looking to save on operational costs, I would recommend getting a 1.5 VS pump like this PureLine 1.5 H.P Variable Speed Pool Pump. It is currently on sale and can save you up to 80% on operational costs. For more information on VS pumps see our guide on "How to Save Money Using a Variable Speed Motor".

User: Jbizzle

Correction 1.5 HP Pentair Challenger

User: Jbizzle

I have about a 14k gallon in-ground pool with 1 skimmer about 8 ft from the pump. A Pentair FNSP 48 DE filter, 3 return lines all feeding from 1 pipe that runs down the side of the pool. Pipe length about 35 feet. The current pump looks to be a Pentair 1.5 HP Dynamo. I'm looking to replace it due to the 150 dollar a month cost to run it for 12 hours a day. Any suggestions? The only thing that runs on it is a Zodiac G3 suction side cleaner. Any help appreciated.

User: Inyopools

Chris - If you are replacing your pump, you would want to get a VS pump that has the same upper HP so that you can drive your spa and water accessories at full power.

User: Chris

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.

User: Inyopools

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.

User: Chris

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.

User: Inyopools

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.

User: Inyopools

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.


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?

User: Inyopools

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.

User: Inyopools

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”.

User: Inyopools

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.

User: BM11

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

User: Susan

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?


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?

User: Dave

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.

User: Inyopools

john mac - For your size pool with spa, I would definitely go with the larger 3.0 HP VS motor.

User: john mac

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?

User: inyopools

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.

User: Dave

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?

User: Inyopools

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

User: Inyopools

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.

User: Inyopools

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".

User: Steve

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?

User: Homerun

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 ??

  1     2     3     4     5     6     7     8     9    

Back To Top

Please Note:

Inyo Pool Products is not responsible for any injury or damaged equipment
while using our guides. Using our guides is doing so at your own risk.
These guides are suggested use of your pool or spa equipment and may vary
depending on which product you are using.