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
gallons of water in pool

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
round swimming pool


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 swimming pool


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 swimming pool


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 268)

Add Comment

User: Inyopools

Gayle - If your pool was originally configured for a 1 1/2 HP pump, a 3/4 HP pump may not be big enough to overcome the resistance in your pool's piping system (feet of head). A 3/4 HP pump is operationally not the same as a 1 1/2 VS pump running at half speed. When a VS pump drops to half speed, it reduces flow which cuts the resistance in half. It also cuts the amperage to an eighth of the amperage at full speed. The VS pump would be the way to go to save energy.

User: Gayle

We need to replace our old pool pump. Our previous pump was a Pentair 1 1/2 hp. My husband wants to replace it with a 3/4 hp saying he will run it longer but will save us money because it uses less amps to run. I think buying a variable speed would be better, what would you suggest? Thanks!

User: Inyopools

pump replacement - The Pentair Challenger 2 HP 230V FR Pump would be a good replacement for your old Pac-Fab Challenger pump.


I am trying to determine what pump might be a good selection. I am replacing a 20 yr old Pac-Fab Challenger pump. Label on the side of the motor says Century Centurian and A. O. Smith. I cannot find a model number on the pump itself

CAT: B855
PART: 7-177897-22
HP: 2.0 SF 1.10 PH 1
HZ: 60 RPM 3450
VOLTS: 230
AMPS: SF 10.0
CODE: P (or might be F) INSUL CLASS: B AMB: 50 ºC

The pool, including spa, holds 30,000 gallons.
If my calculations are close, I am looking at about 35 feet of head.
Piping is 2”
Filter 60’ Nautilus Steel DE filter
Polaris 360 & booster pump

User: Inyopools

SteveP - There are two sizes of VS pumps on the market currently: 1 1/2 HP and 3 HP. For your size pool and pool features, I would recommend a 1 ½ HP VS pump. If you are partial to Pentair, here is a link to their Pentair 1 ½ HP VS Super Flo Pump with Timer.

User: SteveP

I am interested in replacing my Pentair 3/4 hp single speed pump with a VS pump but not sure what size or model I would need. My pool is just under 10,000 gallons. I have a gas heater and a separate dedicated pump for the Polaris. I see that there are few different models of VS pumps all with various features and benefits. So, I am trying to figure out which one I need and will it be strong enough to replace my two single speed pumps that are on my pool now.

User: Inyopools

Dave - For your size pool and stated features, we would recommend the 1.5 HP Pentair VS SuperFlo Pump. California does require a 2-speed or VS pump to conserve energy. The VS pump does cost a little more initially but you will recoup that cost in operational energy savings in a little more than a year. Pentair also offers a 3 HP VS pump but that would be overkill for you pool setup.

User: Dave

Hi guys, I need some help selecting a new pump. The pools old, about 25 years old and this may be the original pump. From the stickers looks like it's a Challenger Pac-Fab 2hp and on motor says Centurian for Pac-Fab. Service factor is blank, amps= SF 9.6-8.8, rpm= 3450. If my calculations are correct the pool is about 14,500 gallons, including spa with 16-18 feet of head (hopefully this is close). 2 inch piping with new Pentair clean/clear plus 420 cartridge filter. Has Polaris 360 sweep with Jandy valve for flow, water fall, on roof solar heating and I already mentioned the spa. I want a Pentair pump and was looking at 1.5 or 2 hp single speed but live in California and read something about Title 20 restrictions for pool pumps. Would like to keep the cost somewhat under control. Recommendations would be much appreciated.

User: Inyopools

Jrmags - I would have thought that normally the pump would be able to draw the water from a partially filled pool but you may be right. You may need the pressure of a fuller pool to help overcome the weight of a column of water going 65' up a 6' rise. Make sure you have no other suction pipes open or leaking.

User: Jrmags

We are currently installing an inground sports pool 40' x 18' with a 5.5' depth in the middle 1/3 of the pool. The pool has 42" depth on both shallow book ending the middle.

The water level is currently about 6 inches in each shallow end with the middle depth at about 2'.

At this point we wanted to test the main drains which has the 2 main drains tied together with single exit 1.5" pipe. The main drains are 65" away from the Hayward 1.5 hp super pump. It seems the pump is having a hard time pumping the water up the 65' @ a 6' rise in elevation.

The pump pumbling is tight, but the water seems to only make it 2/3 of the way to the pump then stalls with no water ever reaching the pump. We primed the pump by filling the basket and pipe, but a lot of the water drains into the pool from the pipe before we can start the pump. The basket is full and when the pump is turned on a portion of that water is expelled (the valve is set on Waste since no other suction or return lines are in use).

I'm thinking the pool water level may not be high enough to assist the pump in its effort to pump the water 65 feet @ 6' rise.

What do you think?

Tomorrow I will check the suction line for an air leak.

User: Inyopools

GL Pool Services - Give one of our tech staff a call at 877-372-6038. They will be able to set up this order for you.

User: GL Pool Services

I need quote for quantity products is necessary for pool 160,000 gallons. When balance is.
Cloro 0
Ph 8.2
Alkaline 40
Acid cianuric 0

User: Inyopools

Jeff R - Your 1 HP pump should be adequate for your size pool and other features especially with 2" piping. If your flow is good now, I would stay with the current size motor. If you increase the size of your pump, you will probably also have to increase the size of your filter to match the GPM flow rate of the larger pump.

User: Jeff R

I have a 9,500 gallon rectangular pool with 2" pipes for both skimmer and drain. I also have a 5 panel solar heater with 2" pipes, along with a 2" piped waterfall at the far end of the pool. I have a short Hayward cartridge filter (not sure of size). The current pump is 1 HP USQ1102. Should I replace pump with same model or increase my HP on account of the solar and waterfall?


Central Florida

User: Inyopools

GreenPoolBlues - I would tend to agree with Craigz. Your 3/4 HP is probably too small to run your Serpent suction pool cleaner especially if your pump motor's Service Factor (SF) is 1.0. Depending on your system piping (1 ½’ or 2”) and the size of your pool, you should probably have a full rated 1.0HP or 1.5 HP pump to generate the GFM water flow required for this cleaner. Unfortunately, when you upgrade to this larger pump, you will also need to replace your filter to handle the higher GFM generated by the pump. I also agree with Craigz that your best option is to buy a Variable Speed (VS) pump. It will cost more initially but will pay for itself in a little less than a year in up to 80% less operational costs. And you will see significant savings after that year.

User: Craigz

Greenpoolblues. Just a quick comment on your pump situation. I'm not to familiar with what your dealing with since I have a variable pump but I know when I set mine to .75hp it's not enough to do the suction vacuum. I've also read other online comments about how 3/4hp isn't really enough for some suction vacuums.
Someone correct me if I'm wrong but I think you should probably have a 2 speed 1.5hp so you have plenty of power for cleaning.
Personally tho I would go variable. Mine paid for itself in the first year.

User: GreenPoolBlues

I'm at my wit's end! Bought house last year. Pump threw a ball bearing so replaced it with 3/4 Pentair on reco of pool company to go with a Hayward Pro Series S210T sand filter (20 GPM filtration rate; 44 GPM filtration and backwash design flow rate). I've had a green pool ever since -- and my fear is that my filter is now undersized for the pump. Am I right? Recommendations? I also bought a Serpent pool vacuum (plugs into skimmer) but not getting enough suction to run the darn thing -- and think it's b/c of the wonky pump/filter balance. Stuck between rock and hard place. Any advice is VERY welcome and appreciated. Thank you.

User: Inyopools

Frankie - Yes that combination will work for your size pool.

User: Frankie

Hi, so I have a 28,000 gallon pool with a sta-rite 2hp motor and a tr100 sand filter all 2"plumbed with a multiport valve. Is this going to work?

User: Inyopools

Shin - Your pump and filter have to be matched to work properly. If your pump is pushing around 70 GPM and your filter is designed for 32 GPM, your filter is way too small for your system. We generally recommend that you get a filter one size larger that required [over 70 GPM] so you are sure it will handle whatever the pump generates. It's all right to have a filter larger than needed but it can't be smaller.

User: Shin

I am having a problem and hoping someone here could give me a bit of advice or at least where to look for the correct answer:

Im a fairly new Pool owner and I have an Above ground pool: it is 22X12 (oval) with 4 feet of max depth (all around: 48 inches deep).

I bought a Hayward Motor which is 1.5 HP (according to the tag) that pushes 70 GPM (again according to tag) and a DE Fliter system that handles 32GPM. This is what was recommended to me by the pool company.

So my issue is this: I set everything up the hoses are all snugly on and I even changed the hoses because I figured this was the problem, When the pump gets going the pressure gauge slows 15PSI and the flow of water is very strong: the pump is flowing and all is normal: but within 10/30 mins (more often average of 15 mins) The gauge climbs to 20PSI and the flow of water pumping decreases GREATLY (from strong flow to a whispering push) And soon enough the water flow stops all together or is moving so slow that I think it stopped. The motor makes a whining grind sound (sometimes) when this happens.

So I clean it out, traps and all and turn it back on: repeats starts strong and within that time span the same results happen: No water flow or very slow water flow. I checked if there was air trapped and didn't appear to be any air leaking, or flow, hoses changed, and tightly on, Everything appears normal and connected correctly but I still lose the pressure.

I assume and, this is where I can't find an answer that the Pump is to strong for the fliteR? Again the 1.5 pushes 70 GPM and the filter says it can do 32GPMm, do I need a weaker motor? Or a bigger filter? or is my issue something completely unrelated?

Again check list:
Water is filled well above water line.
Hoses where changed and tightened
Filter was cleaned
No air present (that i see)
Filter traps cleaned, checked for clogs.

Any help on this subject would

User: Inyopools

frank - For your size pool you might have gone to a 2 HP pump but your 1 1/2" piping restricts you to a max GPM flow rate of around 60. So you are probably good with you 1 1/2 HP pump.

User: frank

My pool is rectangular, 20x40 about 33,000 gallons, with about 45 feet of head avg. 1 1/2 in pipe, 22 in hayward sand filter, I just ordered the Hayward super pump 1 1/2 hp sp2610x15 Is this a OK set up? I cant tell what my old pump is because the label is too faded(old century).

User: cgesq

I would like to replace a 25-year-old 1 HP Hayward pump. I have 15x30 inground pool with 2 inch pvc plumbing. Which Hayward model would you recommend.

User: Inyopools

nmax - You would want to match the new pump size to the size of your current filter. I cannot tell from your filter id number [340G34] which size filter you have. It doesn't come up in our system or the internet. Without the filter size, I would have to recommend that your replace the old pump with a motor that is the same size assuming they were matched correctly originally.

User: Inyopools

jb - I assume when you say "on the ground pool", you mean above ground and not in-ground. It makes a difference. For a 1HP above ground pump I would recommend a Hayward Matrix Pool Pump 1 HP.

User: nmax

I have a 17,000 gallon pool, the skimmer and drain lines are a total of around 60 ft (30 feet head?). I am estimating here since I wasn't around when the pool was built. It's 1.5 diameter pipe. I currently have a Hayward Perflex (340G34) that was installed a few years ago to replace the older one. The motor is shutting off every 30 mins or so to cool down. I think the bearings are probably shot. What size Super Pump should I get to replace the old one?

User: jb

I have a 16 x 32 on the ground pool, I need a new pump with filter basket, my pump was a 1 hp class cx 230/115 volt 6/12 amp 3450 rpm, what would you recommend, I have an established account with you,

User: Inyopools

Len - I would recommend Hayward Star Clear Cartridge Filter 50 Sq Ft for a cartridge filter. It would be compatible with either a new SP1750 pump or a converted SP2605x7 pump with a 1/2 HP impeller. See our guide on "How To Downsize a Pool Pump Impeller". If you stay with the existing 3/4 HP pump, I would recommend a larger filter like Hayward Star Clear Plus 75 Sq Ft. Filter 1 1/2" Ports.

User: Len

(follow up)

After some internet research, I might be able to downsize my impeller to 1/2 HP. Is this correct? will this alleviate my problem? I'm afraid it still might overpower my small sand filter. If I switch to cartridge, what Cartridge filter would you recommend with the 1/2 hp impeller?

Bought a house with a pool. mixmatch equipment Pac-Fab tagelus TA-30D sand filter and a Hayward sp2605x7 pump. The pool is a older pacific pools in ground 19 foot round. est about 8500 gal. I think the pump is overpowering the filter. Bigger filter or smaller pump. Thinking SP1750 pump and maybe switch to a cartridge filter. Recommendations? Equipment is less than 20ft from the pool single skimmer and single return.

User: Len

Bought a house with a pool. mixmatch equipment Pac-Fab tagelus TA-30D sand filter and a Hayward sp2605x7 pump. The pool is a older pacific pools in ground 19 foot round. est about 8500 gal. I think the pump is overpowering the filter. Bigger filter or smaller pump. Thinking SP1750 pump and maybe switch to a cartridge filter. Recommendations? Equipment is less than 20ft from the pool single skimmer and single return.

User: Inyopools

what size? - Sounds like the old shaft seal may have leaked and sprayed the motor bearings and damaged them. Generally when the motor starts making a load noise, owners have to replace the motor. We generally recommend a 1 1/2 HP motor for a 30" round AG pool but that's with a flat bottom. Depending on your pool's average depth, you may need something bigger and since your pool was originally fitted with a 2 HP motor, I would stay with that.

User: Inyopools

Allan - For your size pool and with 2" piping, the 1.5 HP Pentair Superflo VS would be a good selection for you.

User: what size?

I have a 30' round above ground pool with deep end. We recently changed out the shaft seal. When we put everything back together the motor/pump was making loud humming noises. Should I replace the motor or motor/pump? What size? It currently has a 2hp motor.
It appears to be running great just sounds much louder

User: Allan

Looking for a new 230v variable speed pump. I have an in ground with about 21,500 gal. All 2" PVC. 2 skimmers, 3 jets, main drain, and a 1/4" stream coming out of a small fish sculpture. Pump is 45' from near end of the pool, so I estimated an average head of 89' to the skimmers and drain. Failed pump was Pentair Challenger II, total HP 1.65. 24" Triton II sand filter rated at 60 gpm. I want to go with a VS pump. I've looked at Pentair Superflo VS (and a Hayward equivalent) and Pentair Intelliflo 3HP. My instinct is the Superflo VS 1.5 might be sufficient. I noticed your previous comment that stepping up from 1.5 suction and discharge ports to 2" PVC will not affect performance. What pump do you recommend? Thank you.

User: Inyopools

Chikkita - With a 20,000 gallon pool, you are on the edge of what we recommend for a 1.5 HP or 2 HP pump. I would go with the 1.5 HP pump for lower operating costs.

User: Inyopools

jenna13 - Yes. You can replace the motor of your super pump with a Century 2Green Motor using the same wires currently going to your super pump motor. The switching capability is incorporated in the motor. See our guide on "How to Replace a Single Speed Motor with a 2Green Motor" for more information. Other 2 speed options generally require 3 wires plus ground and another timer.

User: Anthony

I have a 20x40 inground, 33000 gal, with 1.5 inch plumbing. Skimmer and a bottom intake.
Filter is a Hayward 4820 48sqft max rate 96 gpm.
I calculate an average dynamic feet per head of somewhere around 30 fph this is a 30ft skimmer run.. level ground and the bottom - 8ft rise and 12 feet away from pump.
The issue is, the DE filter pressure is always 30 psi. If you use one intake it's 20. (Yes I clean my filter grids way more than the average person.)
There is a bunch of twists and turns on the pressure side because of a heater.
Is the 1.5 drawing in too much water for this system? Should I go to a 1hp Hayward Maxflo??
Thanks so much .. pulling my hair out :-)

User: Chikkita

I am about to construct a lapping pool with the following dimensions 25 meters x 2.5 meters , 4 feet depth. My contractor suggests a 1.5 Hp pump. Another suggests a 2 Hp. What can you suggest and what other information might you need to make a sound recommendation? Thanks

User: jenna13

My 1hp superpump is dead. Can I get a two-speed pump in it's place w/o changing all of the electrical requirements?

  1     2     3     4     5     6     7    

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.