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

Written by:  Danny Rhodehamel
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 (3.81 OUT OF 5 STARS ON 26 RATINGS)

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 Length x Average depth x 5.9. 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 5.9 = 11,063 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 150)

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User: Inyopools

Sarah – First, the calculation for an accurate head of water is more complicated than what we present here. This guide provides a ballpark estimate for a typical pool setup which yours is not. I would doubt that you need 4-5 HP. Probably 3 HP would be adequate. Your issues are operational costs and equipment location. For costs, have you considered replacing your pumps or pump motors with variable speed pumps/motors. You can control the speed for initial startup [priming] and the back off to what you need for circulation and cleaning. This not only gives you HP [speed] flexibility but can save you considerable money on operational costs. See our guide on "How to Save Money Using a Variable Speed Motor".I would recommend the larger 3 HP pumps like the Pentair Intelliflo or the Hayward Ecostar. Can't suggest much on the equipment location. You might try the VS pumps first. Even if you decide to move the equipment, the VS pumps will decrease your operation costs considerably. Also you might look at putting in check valves between the pump(s) and the pool to keep the lines from emptying when the pump is off. Should help priming.

User: Sarah

Hi- I have a 44,000 gallon kidney shaped pool & in-ground spa, built in the early 1990's. We have a higher than typical bather load (4-8 kids daily for several hours). Our pool equipment is located about 7 feet higher than the pool water level. We have a Nautilus DE filter. Our pumps do not consistently prime so we manually run the equipment daily to keep the water clean. Several pool contractors scratch their heads as to how to fix our pool. A few have recommended relocating the pool equipment so it is level with the pool water (or up to 3 feet lower). Recently an engineer commented we likely don't have to much head for our 1.5 HP pump. Doing the math from your chart, we'd need a 4-5 HP pump. Would it be wiser in the long run to have the equipment at water level, or upgrade our HP for the pump? Our energy costs are very high (about $.30/kilowatt) but we're being quoted about $5k to relocate the equipment. Help!

User: Inyopools

Damon - If you have 2" piping, you could go with the 2 HP pump with the option of scaling back to 1-1/2 HP by changing the impeller (and maybe diffuser). If your pipes are 1-1/2", I would not recommend going to 2 HP.

User: Inyopools

nancy - Check you sand level. You state that your filter is 22" with 250 lbs of sand. The spec I see on that filter state 150 lbs of sand. If you don't have enough space between the top of the sand and the top of the filter tank, you won't provide enough room for filtering and backwashing properly. And you may be overloading your pump.

User: Damon

Thanks for your reply from 7/24. One last thing, with such a long run to the solar heater on my roof, would it make sense to go with a 2 hp and then downsize the impeller if the filter/equipment is getting too stressed. My understanding with impellers is that you can downsize them but not the other way around. Thanks again

User: nancy

I have a 16x32 inground pool with approximately 19,000 gallons
Replaced my old hayward filter 3 years ago with a waterway of same
size. 22" 250 lb sand filter. Has been noisy since day one. This year
replaced my hayward super pump 1 hp with sf of 1.10 with a hayward maxflo xl 1 hp with 1.25 sf. Also changed sand and inspected laterels, all fine. Filter still noisy. I have 40 feet of head with 1 1/2 in plumbing. Not filtering properly either. Any suggestions? Also there are no air leaks that I can find either.

User: Inyopools

Damon - For your size pool and connection to a solar panels on the roof, I would recommend a 1 1/2 HP full rated motor. Full rated motors have Service Factors {SF}of greater than 1.25.

User: Damon

Very informative. Thanks. I am trying to size a single speed motor replacement for my filter pump. The catch is that the pump is connected to a solar heating system on the roof. The distance from the pump to the pool is only 19ft but from the pump to the solar on the roof is 150ft with 10ft of that being vertical up to the roof. I'm figuring a 2hp motor will work. Any thoughts greatly appreciated. Thanks.

Pool is 13,000 gal
2" plumbing

User: Inyopools

CCWP - Your pool has about 2,250 gallons so I would recommend the smallest in ground pump you can get. The Sta-Rite Super Max 1/2 HP is a good, quiet and economical choice for a main brand (reliable) pump.

User: CCWP

I am looking for a pump for an inground baby pool measuring 10'x 20' x 18"deep. The pump will be located within 5' of the pool and level with the pool surface. Any suggestions?

User: Inyopools

DASMan- Yes, a 2-speed motor can save you about 25% in operation costs; a variable speed pump 75 to 80%. See our guide "How to Save Money Using a Variable Speed Motor" for more information.

User: DASMan

Thanks for the recommendation. Would there be any benefit to using a two speed or variable speed pump keeping any eye to power efficiency?

User: Inyopools

DASMan - For your size pool, 28' round and your preference for a IG pump/filter set, I would recommend a Hayward Super Pump 1.5 HP SINGLE Speed W/ DE4820 48 Sq. Ft. DE Filter. This pump has a 1.0 SF so the true HP is 1.5 HP. The pump has 1.5" ports but the filter has 2" ports. You can reduce the filter ports to 1.5" with a reduction fitting available at any HW store.

User: DASMan

Hey there I have a question that I hope you can shed some light on. I have a 28' round AG pool and the pump seized. The manufacturer doesn't make that pump any longer and because of where the inlet of the filter is, the new pumps will no longer line up. Instead of going with traditional AG filter kits, I would like to use an IG pump and filter. I have one skimmer and two returns all hard plumbed with 1 1/2" pipe; the skimmer is about 15" away from where the pump will be located and I do have a heater. What size pump do you recommend and what size filter. I prefer to use D.E. As I have had great experience with that in the past.

Thanks in advance for your help.

User: Inyopools

Ccbcr2000 - A 1 HP should be fine for your size pool. And I would recommend a 2-speed pump for operational cost savings.

User: Inyopools

poolenvy - The 1.5 HP pump is a good size for your size pool. Did you use pool sand? Try backwashing and rinsing the sand 2 or three times to clear whatever is in the sand.

User: Ccbcr2000

I have an above ground pool that was put in with 1 1/2 pipe. It has a bottom drain as well as 2 skimmer baskets. I had a 3/4 hp pump and when it went bad we replaced it with a 1 hp. This pump now needs to be replaced. I have read about the dual speed pumps and I am uncertain what to buy now. Do you recommend dual speed or single? Someone told me I needed a 1 1/2 hp dual motor. I think this is too much since my pool only has 8640 stallions of water. It is rectangular 12 x 24 and 4' deep.

User: poolenvy

I have a Hayward s244t pool filter and a 1.5 pump, for a 23000 gallon pool, replaced the sand and laterals but still seems like their is sand coming into the pool. cloudy water but chemicals are all normal levels, pump was off for at least 1-2 days and pool was clear could see drain, turned on pump and was cloudy again, CAN'T SEE drain. we use to have a 3/4 hp pump, do you think the pump is too big?

User: Inyopools

justin - I assume you are talking about a 1 HP Pentair Supermax since you seem to be going with the example above. That pump will work well with either the c900 or c1200 filter. I would go with the larger filter, if possible. It will last longer and will require less cleanings.

User: justin

I pretty much have the same situation you have listed above. 15x30 pool and the piping(2in) is around the same distance. Right now I have a Pentair Challenger pump. I am going to replace my pool filter to a Hayward c900 or c1200(opinions please) and I have a brand new Pentair Supermax. My current filter is broken and not fixing and the pump so far has no problem, but old. Will the Supermax and the c900-c1200 work fine together? Dont want to re-pipe but once.
Thanks

User: Inyopools

Kate - With 1 1/2" piping you shouldn't go over a 1 1/2 HP size pump. If you can change out the piping to 2", you could go to 2 HP. Depending on the rebate guidelines; you may be able to use an Energy Efficient [EE] pump, a 2 speed pump or a variable speed pump.

User: Kate

Hello, my local utility is offering $400 rebates for high efficiency pool pumps and ours is at end of life. I need some help ordering the right pump. Can you help?

Here are the facts:
I have a 33,000 gallon pool
One intake/skimmer about 10 feet from pump
Hayward 24" sand filter
1.5" pipes

What else do you need to know? Which pumps do you carry that would fit the bill?

Thanks, Kate

User: Inyopools

bucky - Yes, you can use an in-ground pump and filter system on an above ground pool. You just cannot use an above ground pump and filter system on an in-ground pool.

User: Inyopools

Katmathis - If your old system has 1 1/2" piping, you should not go above 1 1/2 HP. The 2 HP pump would generate more GPM than the piping could handle. If you have 2" or 2 1/2" piping, you could go to a 2 HP pump.

User: bucky

can i use an inground filter and pump for a 27foot above ground pool

User: Inyopools

Andrew - Can't say why the grids broke. Sometimes the pump is too large for the filter, but the DE 4800 has sufficient GPM for a 1 1/2 HP pump. Check to make sure the manifold and base that hold the grids are sound.

User: Katmathis

Hello! We have an OLD (early 60's) rectangular inground pool that is 20 x 40 and 10 feet at the deepest point with the skimmers and jets being about 60 feet from the pump. We currently have a 1.5 hp pump that we are replacing. Should we consider a 2 hp?

User: Andrew

My grids seem to be broken but the mesh is still intact. I just replaced the grids last year. I have a De 4800 filter and a 1 1/2 hp pump. I went up a 1/2 hp from the old pump. Now it seems that de is blowing back in the pool. Why will the grids brake like this? The last grids I had lasted 9 years....will this happen again?

User: Inyopools

Charter Oak - It sounds like your pump is too large for your size pool. You can reduce the HP on your current new pump by 1/2 HP by replacing the impeller with one that is paired with a 1 HP motor of the same model. The pump's motor would then be driving water with a smaller impeller generating less GPM. A general note: a motor can always drive a smaller impeller; it cannot drive a larger impeller (one step up) without overheating. See our guide on "How To Replace A Pool Pump Impeller".

User: Charter Oak

I have a small in ground pool 15X27 12,000 gallons. I just installed a Hayward 2610 1.5hp motor. Thinking more is better, and the price wasn't much different. As the pump builds preasure it starts to click like its straining. I am wondering if this is back preasure and is there a fix. I was thinking about by passing the heater just to see if this improves the flow. Your thoughts would be greatly appreciated.

User: Inyopools

Stelly2 - I would stay with the 1 1/2 HP pump. You should have 2" piping to handle a 2 HP pump. Look for an energy efficient replacement pump. And I would try to get the main drain working to improve flow and keep your pool cleaner.

User: Stelly2

Help! I have a 45x18 oval pool about 26,000 gallons. The main drain return has been disabled years ago, the only return is from the surface skimmer about 80' from the pump. It has a Hayward 1.5hp pump that's aging. It's harder to keep the water clear and after a lot of research I'm suspecting the pump (at least ten years old) needs to be replaced. Would a 2Hp pump be better and work in this configuration? All piping is 1.5". Thanks

User: Inyopools

JT - My apologies. I misread the size of your pool. And your analysis correct. A 3/4 HP motor would be fine for your size pool. As for the piping, if you could replace your 1 1/2" piping with 2" piping you would be able to decrease your feet of head. Much of the resistance to water flow is in the size of the piping. As you open up the piping, you decrease the resistance to flow. Also if you use a pair of 45 degree elbows to make a more gradual 90 degree turn, you will also decrease resistance to flow.

User: JT

I apologize if this shows up twice but it appears the first one didn't post.
I was looking at a 3/4 HP pump rated at 47gpm@40ft of head, 38@50, 27@60, 12@70. Since I only need about 22 gpm for 8 hour turnover shouldn't this be sufficient? Why would I need 1 HP. The pump and the pool both have 1 1/2" fittings. I believe a 1 1/2" pipe will only have a maximum flow of 43gpm. Wouldn't any thing above that be a waste of energy. Another question. If I put a 2" adapter on the fittings and run 2" pipe would my flow increase or due the smaller inlet and outlet not allow a higher flow regardless of the pipe size?

User: Inyopools

18,000K pool - When you replaced your pump, did your replace it with an equivalent size pump. To match a pump, you need to look at the pump's Total HP which is HP x SF [service factor]. A pump with HP=1 and SF=1.0 has a THP of 1.0. A pump with HP=1 and SF=1.5 has a THP of 1.5, half again as powerful as the first pump. If you did buy an equivalent pump, look at our guide on "How To Get Rid Of Algae In Your Swimming Pool" for further information.

User: Inyopools

JT - We generally recommend a 1 HP pump for that size pool. Also, if it is placed at or slightly below the pool surface level, you will need to buy an in-ground suction pump rather than an above ground pump. The AG pumps rely on gravity to pull water from the pool. On the question of head, variable speed pump work below 20 ft of head because as you decrease speed and water flow [GPM], you significantly decrease resistance to flow [head] in the pipes. Your 20 ft of head is only 20' at full speed.

User: 

I have an in ground liner pool, 32 X 15, 18,000 gallons. I have a problem with the pool turning green all the time. Had the pump changed but I am not sure it is the correct size. In the past it took 24hrs or so for the color to change to blue. Now it is hard to keep clear. It is green or cloudy blue. The chems are fine... I have changed the pump, the sand in the filter, changed the water. I seems like the filter is not working right or the pump is the wrong size. What do you suggest?

User: JT

I am installing a 11,000 gal 48" deep pool. The pump will be located about 40ft from the pool. The plumbing will run under ground and the pump would sit at approximately the same height as the pool or slightly lower. To turnover in 8hrs I figure I need about a 20 GPM flow at 40 feet of head. Do you agree? Also can a variable speed pump at low speed circulate any water. The stats for most pumps show no flow rating after about 20ft of head.

User: Inyopools

Birch - The Pentair Whisperflo is a very popular pump. For your size pool, I would stay with a 1 1/2 HP full rated pump. And to save energy I would go with an Energy Efficient like the Pentair Whisperflo Energy Efficient 1.5 HP Full Rate Pump. It will save you 20% in operating costs.

User: Birch

I have an 18x32 rectangle pool. The skimmers are are about 50' and 60' feet from pump and drain is about 70'. I have a jacuzzi magnum pump that I am looking to replace. I was looking at a 2hp pentair whisperflo. Would the 2hp be too much for my 1 1/2 piping? Any feelings about this pump?
Cheers!

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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.