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 228)

Add Comment

User: Inyopools

Chaskinsnc - We generally recommend a 1 1/2 Hp pump for your size pool. I would recommend a "Hayward Super Pump 1 1/2 HP Dual Speed" for your pool.

User: Chaskinsnc

I have a 26,730 gallon 18x36 rectangle roman ends end ground vinyl liner with 1 skimmer, 1 drain and 3 jets. GPH=3341, Feet to Head avg 70' 1 hp jacuzzi 2 speed pump with 22.5 sand filter. (Both discontinued) 12 seasons with ionizer system. I run on low 24 hrs and high when in use or cleaning.
I need to replace thE pump. What do you recommend?

User: Inyopools

jlew - First understand that you will need an in-ground pool pump for your pool. Above ground pumps are gravity fed since they are installed at the base of the pool. Your above ground pool has to be considered an in-ground since it is mostly below ground and the pump is installed almost at the surface of the pool. For your size pool I would recommend a 1.5 HP Hayward Super pump with a Hayward C17502 175 Sq. Ft. Cartridge Filter or with a Hayward S310T2 30" Sand Filter & Valve.

User: jlew

Hello, I'm installing a 15k gallon above ground pool but am sinking the pool into ground with ~12" left above ground. It has one skimmer and one return. I want to run an auto vacuum from the skimmer and will be using a salt-water generator. The pump/filter will be hard plummed under ground for ~20ft and then back up out of the ground at the pump. I would like to know: (1) What size plumbing should I use? (2) Recommendation for pump and filter sizes for both sand and cartridge (I'm undecided on which). Thanks!

User: Inyopools

Patel - Assuming you have 1 1/2" piping, for your size pool, I would recommend a smaller VS pump like the "Hayward Super Pump VS Variable Speed Pump". For a cartridge filter I would recommend the "Star Clear Plus 120 Sq Ft. Filter 1.5" Ports"

User: Inyopools

helpplease - Hayward is one of our most popular brands for quality and price. You will probably have to do some minor cutting and gluing to fit the new pump into the system piping. See our guide on "How To Replace a Pool Pump With An Energy Efficient Pool Pump" for an example of what's involved in replacing a pump. Your other option is to just replace the motor on your pump. See our guide on "How To Replace the Motor on Your Pool Pump" for more information. And lastly, we have many guides related to pumps that you might want to review for general knowledge. Just click "How To Guides" at the top of the page and scroll down the MIDDLE list to Pool Pumps. Click on Pool Pumps to see list of pump guide topics. Start with "How a Pool Pump Works".

User: helpplease

Thanks for your help! Can you recommend a brand of pump? I'm really at a loss of knowledge here, husbands away. Just went to the pool store and I'm thinking Hayward again so we don't have to mess with different fittings?
Thanks again

User: Inyopools

helpplease - I would recommend a 1 HP full rated pump. Service Factor (SF) should be 1.3 or above. See our guide on "How To Understand True Pump Horsepower - Up Rated vs Full Rated" for further information on SF.

User: Patel

It's actually model number C48l2n134c2 a.o Smith 1 HP pump with 115/230 volt that i need to replace.
Want to replace with Hayward pool system (pump+filter)

User: Patel

I want to replace my A.O. Smith -ST1102 (1 HP; RPM 3450, 115/230 Volts,THP/SF: 1.5, AMP 18.6/9.3).

Can you suggest replacement for this pump (possibly dual speed or variable speed) and should i replace my sand filter with sand carthridge?? I am okay to spend little extra to save cost on operating.

I have rectangular pool - 10k gallon

User: helpplease

Hi. Pump just died after only 4 years. Pool is 12400 gallons. On skimmer, 2 jets probably 30 feet from pump, plus waterfall 15 feet from pump. What size do you think I need?

User: Inyopools

Terri For your size pool (~33,000 gallons) I would recommend one of the larger VS pump like: "Hayward EcoStar Variable Speed Pump W/ Lock Out" or "IntelliFlo Variable Speed".

User: Terri

I have an inground 20x40 3ft-9ft pool. I'm thinking about changing to a vs pump. What size and kind do you recommend?

User: Inyopools

MCHILT - Yes, you can use a reducer on the discharge end of the VS pump change the 2" port to 1 1/2".


I currently have Pentair Challenger 2HP. Is is plumbed with 2" inlet and 1-1/2 out. the 1-1/2 then goes to DE filter, heater and back to pool. The Pentair Intellipro VS has 2" ports for both intake and out. Can I use a reducer on the out flow to 1-1/2?

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.

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