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

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. 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 298)

Add Comment

User: Inyopools

les gideons - Here's a link to our best priced 2 HP in ground pool pump - PureLine Prime Pool Pump 2 HP. Shipping is free. Total cost is $399.

User: les gideons

price of a 2 horse in ground pool shipping price included. thanks

User: Inyopools

lovesbiddy - I would replace your current pump with one that has the same HP and SF. See our guide on "How To Understand True Pump Horsepower - Up Rated vs Full Rated". Pool heaters will shut off at 20 to 40 GPM depending on the make and model of heater.

User: lovesbiddy

We have a pretty old pool but it is new to us as we just purchased this home. It does not have a bottom drain, only has 2 jets and 1 skimmer. We would like to replace the pump on it and by the calculations you have shown we need one with 51 GPM and just one skimmer at approx. 45 ft of head. Does the fact that it doesn't have a bottom drain create any extra issues? What about the pool heater, does it need to have the same flow requirements like the filter?

User: Inyopools

LuvMahPool - It sounds like most of your piping is 1 1/2" so I wouldn't go with a much larger pump. Variable speed is good. You can recoup most of your initial cost with an up to 80% reduction in operating cost. For a great price on great quality, I would recommend the newer "PureLine Prime Variable Speed Pool Pump 1.65 HP".

User: LuvMahPool

I recently bought a house with a pool. I'm a first time pool owner. The pool is 36,000 gal. I have two skimmers connected in series through one 1 1/2"(65' long) pipe to valve where it connects to the drain 1 1/2" (38' long) line. These are joined to one 2" (2' long) line to the pump. All the rest of the plumbing is 2" line until it get to the ground to return to the pool via the three jets. There, I split the 2" line to use the existing 1 1/2" (38' long) pipe to the three jets and also use the 1 1/2" (30' long) line to the Polaris (no longer used). I am now using this line as a fourth jet. My pump is a 1 1/2hp single speed pump and my flowmeter shows 70gpm, filter pressure is 20psi. Would I benefit from a larger pump on a variable drive or would the savings be negligible?

User: Inyopools

scpoolguy - I would recommend a 1 HP pump with 2" piping.

User: scpoolguy

I am looking to build a 14x28 rectangle, around 12,500 gallons, 1 skimmer, main drain,3 returns, 2 step jets. Based on wind direction and where I would like to locate my pump and filter I am looking at about 70 feet of pipe from the pump to the skimmer and about 65 feet of pipe to the main drain.

My questions are=
1. What size pump should I use?
2. What size piping should I use?

User: Inyopools

Wai Kane - Glad to help. If you have a rectangular pool, you multiply the pool's length times it's width times its average depth. That will give you your pool's cubic feet. There are 7.5 gallons of water in a cubic foot so multiply your pool's cubic feet by 7.5 to determine its total gallons. See step 1 of this guide for an example of this calculation. GPM is Gallons Per Minute. You get this number by dividing Gallons Per Hour (GPH) by 60. See Steps 5 and 6. Also, if you would like to see what steps a builder goes through to build a pool, see our guide on "How To Build an In-Ground Pool".

User: Inyopools

wahoo - You may well have a leak around you skimmer. As an older pool shifts in the ground over time, it may cause a crack in your skimmer around the piping. Installing a new motor and pump will not fix this problem if that is what you are suggesting, but it sounds like you are due for a new pump. The energy usage for your pool is the same with 110V or 220V.

User: Wai Kane

Thank you for posting how to calculate the amounts of gallons for a pool.
I'm trying to understand how to build a pool, not that I will build one, but just so I have some education on building one when I hire someone.
My question is how do you get the last calculating factor to figure out the amount of gallons?
LxWxDx? = GPH
Also, what does GFM mean?
Thank you once again for your educating me.

User: wahoo

20 year old motor,50 year old brass pump.

110 volt,1 1/2 pipe,hayward DE filter.

Might have leak in skimmer. When I install kreepy K hose and plate, It creates a air bubble under plate,even though it is under water.

Can i install just a new motor and pump?
Does it make sense to go to 22o volts?

User: Inyopools

ekh443 - A 1 1/2 HP VS pump should be adequate for your size pool if you don't have a lot of extra features.

User: ekh443

Hello, I just bought a home with a 18,500 gallon pool. It currently runs a Hayward Super II, 2HP single speed pump. I want to switch it out to a VS pump. Do I really need a 2HP?

User: Inyopools

faceache - Your pump is the right size for your system. Your suction cleaner should work at around 2400 RPM and the Spa should run well at full RPM, 3450. It sounds like you have a low water flow problem. See our guide on "How to Correct Low Water Pressure in Your Pool System". A clogged impeller is a common problem.

User: faceache

We have a 10,000 gal. pool with a spa. Recently, we switched from a single 1 1/2 THP speed pool pump to a variable 1 1/2 THP speed pool pump. The water seems to filter fine at 2000 rpms. However, our vacuum cleaner does not have enough suction at 3000 rpms to work efficiently as it did with the single speed pump. Also, our spa feature is not working. Do we we need a larger pump?

User: Inyopools

mvcain - If you go with the larger 3 HP VS pump, I would recommend upgrading your Tagelus 60 to a Tagelus 100. We always recommend sizing your filter to the next level to have a buffer in filter capability. And you won't have to backwash as frequently.

User: Inyopools

mvcain - A 3 HP VS pump would not hurt your system. Because it is a VS pump, you have the flexibility of running just the speed (HP) you need to meet you pools system needs. The larger pump is probably more than you need currently, but a 1.5 pump is on the edge of meeting your current pool requirements and the larger pump would give you the option to add other features onto your system later. If you choose the larger VS pump, the Intelliflo pump is a popular choice.

User: mvcain

The current sand filter I have is a Tagelus 60. Not sure if that would need to be upgraded to accommodate the larger pump. thanks!

User: mvcain

Hello, I am looking to replace my 1.5hp Pentair single speed pump with a Pentair variable speed pump. My pool is 19,300 gallons, with 65 feet of pump head. I have a slide and 3 deck jets. Would a 3hp model be harmful? I have 2 inch pipes and a relatively large sand filter. I was looking at the Intelliflo Variable speed pump (011018). Any suggestions? Thanks!

User: Inyopools

Diana - Hayward just came out with the Hayward Super Pump VS Variable Speed Pump (115V). It is the only VS pump I know of that uses 115V. It has .85 Total HP.

User: Inyopools

CTRICH - For your size pool, I would recommend a 1.5HP pump with a larger sand filter - like the Hayward Super Pump 1.5 HP SINGLE Speed W/ S310T2 30" Sand Filter & Valve. If you are going to the effort of digging around the pool, I would definitely replace the 1.5" pipes with 2" pipes. The 1.5 pipes are borderline for the flow you need. And I'm not sure why you are elimination the deep end drain. You will have to run a suction or robotic cleaner longer to clear the bottom debris and circulate the bottom water.


My pool is 18x36 inground with 8ft deep end. (26,730gal). would like to turn over in 8hrs. (56-60gpm). Looking for new Hayward sand filter and pump. Digging up around pool in spring and eliminating deep end drain. Only suction will be skimmer. Should I convert from 1.5" pvc on suction line to 2"? I also have a heater with min flow of 57gpm and max 70gpm. What size filter and pump should I get? currently have old S220T and 1 HP Superpump. Thinking of S244T with 1hp?

User: Diana

We need to replace our pool pump. We only have 110 volt at the pump house. I'd like to go with a variable speed our gpm 42.2 and I think we need a 1.5 horse power pump. What do you rec2.
Thank you

User: inyopools

Mujahid - Your pool will hold approximately 16,000 gallons of water. For that size we would recommend a 1 1/2 HP pump with a 30" sand filter like the Hayward Super Pump 1.5 HP SINGLE Speed W/ S310T2 30" Sand Filter & Valve.

User: Mujahid

I am going to construct a new indoor skimmer swimming pool in rectangular shape,
Pool size: 9m * 4.5m
Pool depth: 0.95m to 1.75m
Total volume: 60.75m3 (Approximately)

Can you please help me out that which size of sand filter I can install and how many HP it can be?

Thank you

User: Inyopools

david - I would recommend a set like the "Hayward Super Pump 1 HP Single Speed W/ S244T 24" Sand Filter & Valve" or comparable.

User: david

i have apool 13500 gallons, its head is about 55ft,the pumpbeing at the opp end from skimmer box and pump is about 6 ft below top of pool

what size pump and sand filter should i get?


User: Inyopools

radus - If I have the numbers right, you have a 32,400 gallons pool which is comparable to the one we discuss in a guide on "How To Set Up In Ground Pool Equipment - Part 0 - Overview. It goes into detail on what equipment to consider for installing a large 30,000 gallon pool including discussions on circulation, water features, heaters, automatic chemical feeders, lighting, and control systems. In general terms this pool system has one pump and filter for circulation and a second pump and filter for the several water features. If your pool system does not have extensive water features, you could probably get by with one pump and filter. See our guide for details.

User: radus

I have a new in-ground pool 12'x72'x5'(depth). In a long side there are 4 skimmers, in the other 8 returns and 2 drains; everything built in 2" pipes.
Now is time to build the plumbing; some pool professionals give me 2 options:
- 1 pump, 1 filter and 1 electric heater
- 2 pumps, 2 filters, 2 heaters
Other professionals give me combinations between those two.
What's your opinion? And how many HP for the pump(s)?

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.

  1     2     3     4     5     6     7     8    

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.