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 tendency to purchase a bigger pump than is necessary thinking bigger is best. 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 8 hour period. This page will show you how to select a pump that filters all the water in your pool in 8 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 you pool. The formulas for calculating gallons depend on the shape of your pool. For a RECTANGULAR POOL, measure the length (ft), the width (ft) and the average depth. Average depth is determined by adding the depth at the shallow end to the depth at the deep end and dividing by 2. The formula for calculating total gallons in a rectangular pool is: Gallons = Length x Width x Average Depth x 7.5 Example (see picture): your pool is 30 ft long and 15 feet 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 = 6 ft. 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

ROUND SWIMMING POOL - To determine the number of gallons of water in your round pool, measure the diameter of the pool and its average depth. Average depth is determined by adding the depth at the shallowest part to the depth at the deepest part and dividing by 2. The formula for calculating total gallons in an oval pool is: Gallons = Diameter x Diameter x Average Depth x 5.9. 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 = 5 ft. 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

OVAL SWIMMING POOL - To determine the number of gallons of water in your oval pool, measure the long diameter, the short diameter and the average depth. Average depth is determined by adding the depth at the shallow end to the depth at the deep end and dividing by 2. The formula for calculating total gallons in an oval pool is: Gallons = Long diameter x Short diameter x Average depth x 6.7. Example (see picture): Your pool's long diameter is 25 ft, Short diameter is 15 ft and the pool's average depth is (3 + 7) / 2 = 5 ft. Pool's capacity is 25 x 15 x 5 x 6.7 = 12,563 gallons. Go to Step 5

Step 4
kidney swimming pool

KIDNEY SWIMMING POOL - To determine the number of gallons of water in your kidney pool, measure the largest width, the smallest width and the average depth. Average depth is determined by adding the depth at the shallow end to the depth at the deep end and dividing by 2. The formula for calculating total gallons in an kidney pool is: Gallons = (Longest width + Shortest width) x Length x Average depth x 3.38. 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. 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 8 hours. To come up with this flow rate simply divide your calculated gallons by 8. 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 amount 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 3 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 lenghts and divide by three. 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 requirement. 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 42 GPM 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 to 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, Harward 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 meets resistance at the filter. This will eventually burn out the pump motor and your filtration will also be compromised. We recommend over-sizing the filter 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: fountains, spas, waterfalls, solar heating, and infloor cleaning systems. The features generally require higher GFM rates which equates to a higher HP pumps. 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 GPM. 2" pipes can handle up to 100 GPM.

Comments (1 to 40 of 213)

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User: Inyopools

Duffiera - We use just the average distance from pool inlets to the pump, but this size calculation is a very basic estimate for a typical pool. A more thorough analysis involves factors like total length of piping, size of piping, number of fittings [turns in the piping], vertical heights [up to the solar panels] etc. For a more accurate analysis, you should have a pool designer look at it.

User: Duffiera

When calculating Feet of Head is it only the distance from the skimmer to the pump? Or, is the return distance included? And, what about a solar heating system on a second story? That distance too?

User: Inyopools

phantom - Sounds like your spa is part of your main pool [return water to pool is a waterfall] so I question why it need a separate pump for the spa. Can't the spa be supplied water though the pool system like most pool systems. I don't understand your pool configuration enough to comment on a separate pumping system for the spa.

User: Inyopools

tmo - I would also recommend a 1 HP pump to match your smaller DE filter. You will just have to run it longer to filter all the water in your large pool each day. And a Pentair WhisperFlo is a good choice.

User: Inyopools

spmorin - You have a large pool, but since it is old, its piping is probably all 1 1/2" which restricts flow. I would suggest that your new pump be 1 1/2 HP or less to keep flow under 60 GPM. With a 1 1/12 HP pump you should have at least a 22" diameter Hayward sand filter. If your current filter is smaller than 22"and you don't want to replace it with a largter one, I would suggest a 1 HP pump. You will just have to run the system longer to filter all the water in your pool. Both will support your solar system.

User: phantom

I have an 8 ft diameter in ground spa
3 ft deep. It has 1 bottom drain and return water to pool is a waterfall.
I have 30 ft to pump. Pump is 4 ft above
Spa water level or 7 ft above bottom drain. Little usage. DE filter.
Which pump do you suggest?

User: tmo

28,000 gal, 1.5" lines, 2 skimmers, 2 drains, Pentair DE 36
Currently have a Hayward 1.5HP Superflo that is about ready to go. Local pool store recommended 1HP Pentair Whisperflo mainly due to filter size. I'd like to go as low as possible on the pump HP and run longer during the day.

User: spmorin

Our pump just died and we need to replace. Pool and most of the equipment is old.
Pool size is 18.5' x 36' average depth is 6'
One intake (skimmer) approximately 30' from pump and filter.
We also have solar panels on roof of house that water flows through after filter. Approximately 16' of 1 1/2 inch pipe up to roof, approximately 240 sq. ft' of solar panels on roof, back to pool is approximately 12' into what would normally be the outlet pipe form the filter.
Dead pump is Sta Rite Dyna Glas MPRA6E-147L
Filter is Hayward sand filter, unsure of size.


User: Inyopools

Clemsongirl - For your size pool customers typical buy a 1 1/2 HP pump and a 120 sq ft or 175 sq ft cartridge filter. I would recommend something like a Hayward Super II EE 1 1/2 HP Pump and a Hayward Star Clear Plus Cartridge filter: C12002 or C17502. The larger filter will last longer and require less frequent cleaning. For greater operational savings I would recommend a two speed pump (25% savings) or a smaller variable speed pump (50% to 75% savings).

User: Clemsongirl

20'x40' Rectangle 2' Radius

3'4" Shallow Depth, 8' Deep Depth
What size pump/model do you recommend?
Perimeter 116'-7""

Gallons 27900

One skimmer, Pentair salt system, 4-5 feet from pool to pump

User: Inyopools

kreiner - The 3/4 HP Pentair SuperFlo pump is a good choice for you size pool. Pentair is one of the top pump manufacturers and its SuperFlo pump is very efficient and quiet as well as economical. It is Pentair's motor of choice for small to medimun pools. These pumps are comparable to Hayward's popular Super Pumps.

User: kreiner

I have a 13,000 gal pool and 2 intakes with an average head of 35 ft. I also have a Pentair Clean & Clear catridge filter system rated for 320 gpm and a 3/4 HP booster pump for the Polaris 280 robot. The motor on my 3/4 HP StaRite pump went out and I would like to replace with single speed pump. I only run about 5 hrs per day and we are moving soon so don't want to spend the money on a high efficiency model. I was looking at the 3/4 HP Pentair SuperFlo 340037 or could you make a recommendation. Thanks.

User: Inyopools

4lil'swimmers - I would recommend a smaller 1.5 HP VS pump for your size pool. And a 24" Hayward Sand Filter is larger than you need but we generally recommend going bigger on filters. They will last longer and you won't have to backwash them as often.

User: 4lil'swimmers

Hello Just moved into a home last summer with 34X17 rectangular grecian shaped pool. 24000gal, 1 skimmer, 3 returns, 1.5in. plumbing, no features, and use a kreepy to clean. I estimate the skimmer is about 40-45ft from the pump. The pump and filter are ancient pacfab, nautilus stuff. According to calc. I need a 1HP pump. Would like to get a full size VS or if I can use a smaller VS? and would a 24in Hayward Sand filter work with this size? - Thanks

User: Inyopools

meemers - If your pool is 7,500 gallons, that is a relatively small pool and 2.5 HP is considerable overkill. You do not need anything over 1 HP even with your cleaner.

User: meemers

I have a 7,500 galloon 20 year old pool with a spa with spillover into the pool (4 jets). My current pump is 2.5 HP Hayward. I run a 2 wheel The Pool Cleaner vacuum from the skimmer intake. What pump should I be considering?

User: Inyopools

labergebnt - We usually recommend a 1 1/2 HP pump for your size pool. If you can afford the initial cost, we also recommend buying a 2-speed or variable speed pump for significant operating cost savings.

User: Inyopools

Chris - I would recommend buying a 3/4 HP or even 1 HP full rated dual speed pump for your pool configuration. As you point out, this would give you a low cost option for filtering your pool at low speed while insuring that you have enough flow for a pool cleaner at the higher speed.

User: labergebnt

I have a 17' x 35' rectangular pool at 3' area and 8' deep area ( 5.5 )
I have 1 skimmer and 2 return. the pump is 30 feet away from the pool. as per calculation, it is a 51.13 gpm. Right now I have a 1hp and it seems so little. 2" pipe. Should I take a 2hp or 1.5 hp?

User: Chris

I recently bought a foreclosure property that has a pool. I’m going to be reopening it this spring, but while it was vacant, the power end sub-assembly of the motor and accessories was stolen and so I am replacing everything on the pump except the volute, skimmer basket, and clamp top. The pool itself is a gunite 18’x36’ oval with a 3’ shallow and 7.5’ deep end for a total of approximately 20,000 gallons. It has only one skimmer, in line between the main drain and the pump/filter. The skimmer is 28’ away from the pipe and the main drain is 9’ farther away horizontally for an average total head of approximately 37 feet. The current filter is a Pentair Tagelus TA 60D, 1.5’’ PVC Pipes, and Pump was a Pentair Whisperflo WF-23, ¾ HP up-rated pump. I planned on buying the same ½ HP, single-speed, full rate motor sub-assembly as before to replace it. I would like to purchase a kreppy krauly or a vacuum similar to clean the pool sometimes. Will this motor be sufficient to clean and for everyday use or should I get a more powerful or dual speed motor? Thanks for the advice!

User: Inyopools

kt - When you replace a pump, you want to look at its "real" or Total HP (THP). THP is determined by multiplying the stated HP with the stated Service Factor (SF). I would guess that the THP of your old Northstar is close to 1.5 THP while the replacement pump you are looking at is 1 THP [1.0 Hp x 1.0 SF]. If you want to replace you existing pump with an equivalent one, look for a pump with 1.5 THP.

User: Inyopools

notso - I would go with the Pentair Whisperflo Dual Speed full rated 2 HP Pump. But take one more look at VS pumps. See our guide on "How to Save Money Using a Variable Speed Motor". Don't know what your negative comments were, but you will save more in operating costs with a VS pump.

User: kt

I have a 26k gallon pool with a waterfall. the waterfall is independently running from a 1hp hayward northstar pump which just started making a lot of noise. Can i replace it with a 1 hp pump which has sf 1.0 hmax15m qmax 79gpm? i rarely run the waterfall in summers. the new pump looks much smaller than Hayward northstar for some reason.

User: notso

26k gal. inground, 30ft head, 2" inlet/outlet lines, 4 returns, new 60 sf cart/DE. not very good reviews of vari speed pumps, so looking at 2 speed, prewired for 230. replacing 22yr. whisperflo 2hp
thank you.

User: Inyopools

USCARCH - The smaller VS pumps including these two have 1 1/2" ports, but they are easily converted with adapters to fit 2" piping without affecting performance.

User: Inyopools

USCARCH - For a 2 degree per hour rise in temperature in a 15,000 gal pool, you will need a heater with around 300 BTU output and because you live in CA, you have to buy a LowNox version. Two models that fit these requirements are: Hayward's Pool Heater Universal H-Series Low Nox 300K BTU NG and Raypak's LoNox Heater 335000 BTU NG Elec Ign. You set the max temperature that you want to stop heating. Also be sure to buy a pool cover to save the heat that you are putting into this pool.

User: Inyopools

Justin - For your size pool and the short amount of time you will need to filter your pool water, you will not save that much scaling down a VS pump vs running the 3/4 HP pump. I would go with the 3/4 HP single speed pump.


One more question about the pump. All my lines are 2". Would one of the two pumps you suggest be appropriate for the 2, 2" suction and the 2, 2" return lines? Thanks


Thanks for the info on the pump and filter. Both pumps qualify for the rebate. As far as the heater goes, I am looking at around 2 degrees for hour, max. 25 degree temp rise. I live below 2000' in Los Angeles, CA. I have natural gas plumbed to the location. What model / models would your recommend. Thanks

User: Inyopools

USCARCH – For your size pool and setup I would recommend the smaller 1.5 HP VS pump like the SuperFlo VS 1-1/2 HP Variable Speed Pump or the Hayward Super Pump VS Variable Speed Pump. For a DE Filter - Hayward Pro Grid DE Filter 60 Sq Ft w/o Valve or Jandy DEV60 60 Sq. Ft. DE Filter w/o Valve. I’m not sure what you mean by “max 25 degrees temperature rise”. Need to know over what period of time. I would guess you would want a heater that generates 300K BTUs for your size pool. For more information on selecting a heater, see our guide on “How To Select a Gas Heater For Your Pool or Spa”.

User: Justin

Thanks Inyopools! So given the small pool size and short head runs, a variable speed would not save much on the energy bill over the 3/4 HP uprated motor? I usually only run the motor 8hr/day. Forgot to mention that the pool has a very small attached hot tub with heater. Thanks


I have a 15,000 gallon (~800 Sq. ft. surface area) in-ground pool with 2 return lines, 1 skimmer and 1 dedicated vacuum port. Max, 20' of head. My Vacuum is the Baracuda G3 Residential Advanced Suction Side Automatic Pool Cleaner. Need to replace pump with a Variable speed pump to get an SCE rebate. What would you recommend? Also need to replace by DE filter, Would like to stay with DE. What would you recommend?. Would also like to add a heater. Max 25 degree temperature rise Gas line is already in place. What would you recommend. Thanks

User: Inyopools

Justin - For your size pool, I would recommend a 3/4 HP pump with a Uprated motor (Service Factor (SF) of less than 1.15 - like this Hayward 3/4 HP Super Pump. Note: I am assuming this is a small in-ground pool. If it is above ground, you will need an above ground pump.

User: Justin

I'm looking to replace an overkill 2HP Pentair Whisperflo WFE-8 on my 8,000 gallon pool with 1 skimmer and 1 drain with an average 13 feet of head all returning through magnasweeps. What would you recommend?

User: Inyopools

Jc - For your size pool and its associated operational costs, I would recommend getting a large 3 HP Variable Speed (VS) pump. That way you could run it most of the time at a lower speed to circulated the water and then run it at full speed when you want to use the spa. It will save you up to 75% on operational costs over using a single speed 2 HP pump. See our guide on "How to Save Money Using a Variable Speed Motor" for more information. This VS pump would take care of all the features you listed. You would not need an additional pump.

User: Jc

I am building a very large pool 60x25 with spa 15x7, cascade and slide. I figured around 56,175 gallons of water with ur calculations. Do i need a 2hp motor? And if so seperate motors for spa and cascade?

User: Inyopools

S2BEIT - I would recommend a 1 HP above ground pump for your setup. Two factors to consider when replacing your pump: Total Horse Power (THP) and type of pump. The actual or THP of a pump is determined by multiplying its HP by its Service Factor (SF) - both stated on the label on the pump's motor. The Hayward PowerFlo has a SF of 1.0 so your old pump's motor had a THP of 1.0 x 1.0 or 1.0 THP. The Pentair SuperFlo pump motors have a SF of 1.65 so for the 3/4 HP motor, its actual THP is .75 HP x 1.65 or 1.15 THP. The acutal THP of 1.0 HP motor is 1.0 x 1.65 or 1.65. In this case the 3/4 HP motor would have been the better match. However, factor two, the Pentair SuperFlo pumps are designed for inground pool which work through suction (pulling water to the pump). Your Hayward PowerFlo pump was designed for above ground pools and work off of gravity - pump is lower than the pool water surface and water drops into the pump. I would go back to the 1.0 HP Hayward PowerFLo or similar above ground pool pump.

User: S2BEIT


I have an above ground pool 18x30oval around 46inch depth. What size pump do I need. My existing 1hp hayward Powerflo burned out. I was considering a 3/4 pentair supeflo or maybe 1 hp?

User: Inyopools

Capt Jack - I would go with a 1.5 VS pump for your size pool. 2 or 3 HP VS pumps sounds like overkill, especially with they have Service Factor (SF) of over 1.0. Total or real HP is the product of HP and SF. If a 2 HP pump has a SF of 1.5, your total HP (THP) is 3 HP. I would be looking for a VS pump of 1.5 to 2 THP.

User: Capt Jack

HI, I have a 16,000 gal in-ground pool with 1 drain and 1 skimmer. Currently have a 1.5HP Challenger single speed pump that I want to switch to VS. This pump serves the cartridge filter (PacFab Seahorse 300 sq ft), pool cleaner (Polaris 360) and my roof top solar heaters. I have been getting varying HP recommendations locally (anywhere from 2 to 3 HP) and I am wondering what your thoughts are.

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