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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Inyopools  Posted: 04/27/2017 8:58 AM  Latest Inyo Product Specialist

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.

kplum  Posted: 04/26/2017 8:31 AM  Latest

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.

Inyopools  Posted: 04/18/2017 11:35 AM  Inyo Product Specialist

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.

Jacksonman  Posted: 04/17/2017 21:23 PM 

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.

Inyopools  Posted: 03/10/2017 15:00 PM  Inyo Product Specialist

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.

Inyopools  Posted: 03/10/2017 14:56 PM  Inyo Product Specialist

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.

Anonymous  Posted: 03/06/2017 14:20 PM 

What is the head loss from solar panels? I have 6 panels on the roof of a single story house.

Nic  Posted: 03/06/2017 13:47 PM 

i have a pool with 11000 gallon with 159 feet of head what pump would you recommend?

Inyopools  Posted: 02/28/2017 14:30 PM  Inyo Product Specialist

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.

bigdon  Posted: 02/27/2017 15:18 PM 

how to find incomming air leak in underground plumbing or repair

Inyopools  Posted: 02/26/2017 16:15 PM  Inyo Product Specialist

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".

Cowboyzzz  Posted: 02/23/2017 16:45 PM 

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

Inyopools  Posted: 01/24/2017 13:01 PM  Inyo Product Specialist

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.

SeanC  Posted: 01/20/2017 11:14 AM 

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?

Inyopools  Posted: 12/19/2016 14:08 PM  Inyo Product Specialist

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.

sagar kandel  Posted: 12/14/2016 11:14 AM 

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 .

sbaussie  Posted: 10/02/2016 13:00 PM 

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 .

Inyopools  Posted: 09/26/2016 13:31 PM  Inyo Product Specialist

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.

Inyopools  Posted: 09/26/2016 10:12 AM  Inyo Product Specialist

antman4hg - If your pool piping is 1 1/2", I'd suggest going to a 1 1/2 HP pump with a 120 GPM filter. If your piping is 2" or more , I would go to a 2 HP and 150 GPM filter. In both cases, I would recommend going to a VS pump for better operational costs.

Bftigger  Posted: 09/25/2016 22:48 PM 

We just bought a house with a pool. The guy who was "maintaining" it for the prior owner did a crap job (IMO, long story).
Anyways... We are going to replace the pump and filter sometime soon, but not sure what to go to. Would love some input.

Pool is 12,000 gallons (has 1 wall skimmer & 2 floor drains, all in the deep end)
Pump is 3/4hp (looks old as the pump filter is inline separate from the pump)
Sand Filter is 200 lbs
Salt chlorinator cell (T-15)

What is the largest pump and filter we can go to? Which filter is more efficient and user friendly (was considering switching to a cartridge or DE)?

Antman4hg  Posted: 09/22/2016 17:13 PM 

I have a pool 20x30 3ft to 8ft
I have a Hayward setup 1hp motor
And a sand filter. 75gpm. Do I need a bigger setup for this size pool.
It's about 28000 g

Inyopools  Posted: 09/07/2016 11:14 AM  Inyo Product Specialist

Blues - I would recommend replacing your motor with a 1.65 HP variable speed motor. It cost a little more initially, but can pay more itself in reduced operational costs in just over a year. Here's the link to variable speed motors.

Blues  Posted: 09/05/2016 16:07 PM 

I have a 13500 Gal pool. The main pump motor is currently a 1 HP Marathon with a SF=1.25 which has become very noisy (screech) and should be replaced. I added a passive solar heating system last spring, left all the other equipment unchanged. Should I consider upgrading the pump and motor to 1.5HP or larger to accommodate the new solar system? Is a larger HP pump for the passive solar system even necessary? System currently runs 8 hrs/day, approximately 5-6 hrs/ day with solar heat.

The filter is a Tagelus TA-50 (50GPM) sand filter. Head pressure per your video instructions yield 17ft. Other option would be to install a VS pump, but not sure if extra expense is worth it. Cheap option is to just replace the motor.

Your thoughts please.

Inyopools  Posted: 08/24/2016 9:42 AM  Inyo Product Specialist

WTF - Those are typos. They should read GPM. Thanks for the catch.

WTF  Posted: 08/23/2016 18:24 PM 

I understand GPM (reference Step 10), but what is GFM as referenced in Step 11?

Inyopools  Posted: 08/23/2016 12:28 PM  Inyo Product Specialist

Slamingsam - I'm not sure why your 2 HP motor is not enough to power your cleaner. In fact, if your motor's Service Factor (SF) is 1.3 or 1.5, your pump is larger. Actual or total HP is the product of HP and SF. Try partially closing your main drain valve to divert more water flow to the suction port. Did you downsize the impeller (and maybe diffuser) when you changed the motor? If you are still considering a larger pump, I would recommend a 3 HP variable speed pump or motor. They cost a little more upfront but offer considerable savings in operational costs.

Slamingsam  Posted: 08/22/2016 22:30 PM 

I have a20x40 with 71/2 @ the deep end. just bought a 2hp Haywood motor. Had a Haywood"King Ray" that stopped working. Bought a "Extreme Power" it's like a Kreepy. The new 2hp. motor hardly moves the new "Extreme" cleaner. I did have a 3or 31/2 hp motor. The 2hp just doesn't cut it. Any ideas on what to do. Looking going to 3hp motor. I hate to lose the money spent on the 2hp motor

Inyopools  Posted: 08/13/2016 10:44 AM  Inyo Product Specialist

Hawt in Texas - For your size pool, I would recommend a larger pump. If you can cover the initial cost, I would get one of the larger 3 HP variable speed pumps then run it at what works best for your system. You will save a high percentage of your operating costs.

Hawt in Texas  Posted: 08/12/2016 5:08 AM 

Hi. My spa level dropped 6 inches although I did nothing. Upon further investigation, my pump motor was trying to turn-on for 10 seconds every 5 mins. I cut the power, and determined it is not the capacitor nor the booster capacitor. I'm afraid it is the pump. I have a 1.5 hp Whisperflo feeding into a cartridge only filter (System3, Modular Media, Model #S8M150) that has Maximum Flow Rate of 125 gallons per minute. The pool is 41,500 gallons with an attached spa of 4,500 gallon. No water features. 2 skimmers, 1 main drain. The average Feet of Head is 47. It has always been LOUD, but it worked. So, I never replaced it. Probably looking at 95 GPM. I just want to make sure the 1.5 hp will do the trick or if I should go with a stronger motor?

Inyopools  Posted: 08/10/2016 11:26 AM  Inyo Product Specialist

theprofessor - I'd go with the next size up on your pump, the Hayward Power-Flo II 3/4 HP Single Speed Pump and match that with a Star Clear Plus 75 Sq Ft. Filter 1 1/2" Ports.

theprofessor  Posted: 08/10/2016 9:31 AM 

hello! i have a 3K gallon in-ground swim-spa with 25 feet of suction head and 15 feet of line to rooftop solar (1.5in lines). we're using a 50sqft cartridge filter and a SCG. I have one skimmer only and would like to add a vac-sweep line (+25 more feet of head) when we replace the liner in a few months. I'm also in need of a new pump -- presently it's a self-priming 1/2HP pump (SP1750), but considering the added solar and to-be-added vac-line, we'll likely need an upgrade. could you suggest a range to investigate? A few models/combos? we might need to replace the cart-filter, too, because of flow-rates (OK if we do). variable speed might be nice, but they seem like they are a bit too large flow-wise for my application. The present combo seems to work, but seems a bit underpowered with the solar addition. thanks for any insight and suggestions you can lend.

Inyopools  Posted: 07/11/2016 10:11 AM  Inyo Product Specialist

jellybean - Sand in pools usually comes through cracks in the piping inside your filter. You may have to remove the sand in your filter to check for cracks in the laterals, the hub that the laterals are attached to, and the vertical standpipe. Also, some standpipes have an O-ring at the top of the pipe. See the parts list for your filter. If applicable, check that O-ring for damage.

jellybean  Posted: 07/09/2016 10:06 AM 

I have a 12000 gallon pool... part in ground part above ground.. we have a 1.5hp pump and a 24" sand filter... should I have sand in my pool?

Inyopools  Posted: 07/01/2016 11:17 AM  Inyo Product Specialist

Mark V - If you currently have a 1.5 HP pump, the Hayward EcoStar at 2.7 HP might be somewhat larger than you need. Unless you are planning to add addition features, you might consider something like the PureLine Prime Variable Speed Pool Pump 1.65 HP that will be out in July.

Inyopools  Posted: 07/01/2016 10:54 AM  Inyo Product Specialist

Zalde - For a pool your size and expense, I would have a pool builder do a complete analysis of your pool requirements including the total length of piping, pipe sizing, number of elbows, water features, etc. There are many factors involved that we only estimate for the average pool system.

Mark V  Posted: 06/29/2016 21:39 PM 

Using your sizing guide, I determined my GPM = 57 and Avg Ft of Head = 45' and I have 1.5" lines. I have an old 1.5 hp Jacuzzi pump and would like to upgrade to a variable speed. Would the Haywood EcoStar work for me? If I read their charts right, I could run efficiently at 2400 rpm. Thanks!

Zalde  Posted: 06/29/2016 20:19 PM 

Guys, I really need your help. There's this special someone of mine, who asked for my help. This is the situation, they're planning to build an Olympic Pool ( Length 50m, Width 25m, Depth 2m, Volume= 2 500 000 Liters). How can we choose the correct pump for its filtration system? PLEASE HELP. THANKS

Inyopools  Posted: 06/28/2016 11:31 AM  Inyo Product Specialist

Quincyrocks - For your size pool and piping, I would recommend upgrading to a 1 HP pump. Look for a pump with a 1 HP total HP (THP) rating. See our guide on "How To Understand True Pump Horsepower - Up Rated vs Full Rated".

Inyopools  Posted: 06/27/2016 15:16 PM  Inyo Product Specialist

Waterboy - You should be alright with a 1 1/2 HP pump for your size pool and 1 1/2" pipes. 2 HP would be too large for your pipe system. If you go to a 1 1/2 HP pump, make sure you select a pump with 1 1/2 Total HP (THP). See our guide on "How To Understand True Pump Horsepower - Up Rated vs Full Rated". Also, make that your current filter can handle the increase in GPM generated by a 1 1/2 HP pump.

Quincyrocks  Posted: 06/27/2016 8:04 AM 

I need to replace my pump which is currently a 3/4 HP Hayward super. I'm wondering if I should go up to a 1 HP, as I recently added a Hayward 350btu heater.
The pool is in ground, 10,000 gallons with a pro grid DE36 filter and the aforementioned heater. The skimmer is 36 ft from the pump and the drain is 53 ft. I had to install a bypass to run the pool without the heater when necessary, which made for some fancy piping. So there are about 8 90 degree turns between all the equipment, if that matters. Also, the lines to the pool are 1.5 inch but the piping between the equipment is 2 inch. Thanks.