In this article you will learn how to calculate the correct size of pump you need for your swimming pool.
When designing a pool system, it is important to select the appropriate pump and filter combination. This involves ensuring that the pump and filter are sized appropriately for each other in terms of flow rate.
The pump’s flow rate determines how quickly water is circulated through the pool system, while the filter’s flow rate determines how much water can be filtered at a given time. If the pump’s flow rate is too high compared to the filter’s flow rate, it can put strain on both the pump and filter, reducing their efficiency and potentially shortening their lifespan.
Table of Contents
Calculating swimming Pool Pump Size
What is the correct rating of pool pumps and why?
Pool pumps should really be rated by the gallons per minute (GPM)Â of water flow instead of the motor’s HP.
While horsepower (HP) is often used as a general indicator of a pump’s power, it’s not always the best measure of a pool pump’s performance. This is because HP only describes the amount of power that the motor can generate, not the actual amount of water flow that the pump can produce.
Instead, the best measure of a pool pump’s performance is its gallons per minute (GPM) flow rate. The GPM rating takes into account the actual amount of water that the pump can move through the plumbing system, and it’s directly tied to the pool’s volume and the amount of turnover you need to achieve.
Generally, from experience
The general rule of thumb is that the entire volume of water in the pool should be pumped through the filter system in 8-10 hours.
For smaller pools, a pump with a flow rate of around 40 GPM may be sufficient to accomplish this task within the desired time frame. For larger pools, a higher flow rate of around 80 GPM may be necessary to achieve the same goal.
The range of size of pools can vary, but generally, a small pool is considered to be one that holds less than 10,000 gallons of water and a large pool can generally have a volume greater than 30,000 gallons. This could include above-ground pools, inflatable pools, and some in-ground pools.
Remember the above estimation of the pump is general if you need a specific sizing of a ool continue reading.
The actual size of a pool can depend on various factors, such as the shape, depth, and intended use of the pool.
Note. The pump size needed for a pool can also depend on other factors, such as the length and type of plumbing used, the type of filter, and the desired turnover rate.
Factors to consider when choosing size of pool pump
There are several factors to consider when choosing the size of a pool pump, including:
Pool size: The larger the pool, the larger the pump needed to circulate the water properly.
Plumbing size: The size of the plumbing pipes in the pool’s circulation system will also impact the pump size. A pump with a higher flow rate may require larger plumbing to handle the increased water flow.
Type of pool: Different types of pools, such as above ground or in-ground, may require different pump sizes to properly circulate the water.
Desired flow rate: The desired flow rate of the water through the pool’s circulation system will impact the pump size needed. A higher flow rate will require a larger pump to handle the increased water flow.
Type of filter: The type and size of the pool filter will also impact the pump size needed. A pump with a higher flow rate than the filter’s recommended flow rate may put strain on both the pump and filter, reducing their efficiency and lifespan.
Elevation and distance: The elevation and distance between the pool and the pump may also impact the pump size needed. Pumps may need to be larger to overcome these factors and properly circulate the water.
Climate: The climate in which the pool is located can also impact the pump size needed. Pumps may need to be larger in warmer climates to properly circulate the water and maintain the desired water quality.
Below are the detailed steps for determining a pool pump size with examples
1. Determine how many gallons of water your pool holds
Here are the formulas for calculating the gallons of water in different pool shapes
Here are the formulas for calculating the gallons of water in different pool shapes:
Square or Rectangular Pools:
Gallons = Length (ft) x Width (ft) x Average Depth (ft) x 7.5
7.5 is a conversion factor that are used to convert the rectangular volume of the pool from cubic feet to gallons.
Oval Pools:
Gallons = (Length (ft) + Width (ft)) / 2 x Width (ft) x Average Depth (ft) x 6.7
6.7 is a conversion factor that are used to convert the Oval pool volume from cubic feet to gallons.
Circular Pools:
Gallons = Diameter (ft) x Diameter (ft) x Average Depth (ft) x 5.9 5.9 is a conversion factor that are used to convert the circular pool volume from cubic feet to gallons.
To use these formulas, you’ll need to measure the length, width, and average depth of your pool. Average depth can be calculated by measuring the depth at the shallow end and the depth at the deep end, adding them together, and dividing by 2.
For example, let’s say you have a rectangular pool that is 25 feet long, 10 feet wide, and has an average depth of 6 feet. To calculate the gallons of water in the pool, you would use the formula:
Gallons = Length (ft) x Width (ft) x Average Depth (ft) x 7.5
Gallons = 25 x 10 x 6 x 7.5
Gallons = 15,000
So, the pool holds 15,000 gallons
2. Determine the number of gallons to be pumped per hour
To calculate the number of gallons that should be pumped per hour to clean the entire pool water, you should decide the turnover time. Basing on the standards the turn over should be between 8-10 hours. In our example we are gong to use 8 hours.
To do this, divide the total number of gallons of water in your pool by 8.
For example, using our previous example of a pool of 15,000 gallons of water, divide that number by eight:
15,000 gallons ÷ 8 hours = 1,875 gallons per hour
So, you would need to pump 1,875 gallons per hour to clean all the water in your 15,000-gallon pool in eight hours.
3. Convert gallons per hour to gallons per minute (GPM)
Since pumps are usually rated in gallons per minute (GPM), divide the gallons per hour obtained above by 60.
The 60 represents the number of minutes in an hour.
We obtained 1,875 gallons per hour
1875 ÷ 60 = 31.25 GPM (gallons per minute), which is the flow rate you need for your pump.
So for a pool of 15000 gallons, you will need a 31 GPM water pump for a turn over of 8 hours
Note. In the above example we considered a turn over of 8 hours, if a turn over of 10 hours s considered a pump size will differ as shown below.
If you choose a turnover of 10 hours
Divide the gallons in the pool by 10 from our example of 15000 gallons
15000 ÷ 10 = 1,500 Gallons per hour. Convert the gallons per hour to gallons per minute (GPM)
Divide gallons per hour obtained above by 60. This is because 1 hour has 60 minutes
1500 ÷ 60 = 25 Gallons Per Minute (GPM)
So, a pool of 15000 gallons requires a pump of 25 GPM if a turnover of 10 hours is considered.
In conclusion.
For a rectangular pool of 15000 gallons, you need a pump of 25 – 31 GPM (gallons per minute)
Once you have determined the flow rate of your pool in gallons per minute (GPM) select a suitable pool pump model based on the manufacturer and horsepower (HP).
 It’s important to note that pool pumps from different manufacturers come in varying flow rates (GPM) and horsepower (HP), so it’s important to choose a pump that is compatible with your specific pool requirements.
Tips swimming pool pump sizing
When a pump is located farther away from the pool, it has to work harder to push the water through the pipes and filters, which can decrease its GPM. Additionally, valves and PVC turns in the plumbing system create friction that can further restrict the flow of water and lower the pump’s GPM. This is important to consider when selecting a pump for your pool and designing the plumbing system, as it can affect the overall performance and efficiency of the system.
Â
When you add water features like spas, fountains, waterfalls, or any other water-related accessories to your pool, you are increasing the amount of water that needs to be circulated through the system. This means that you need a pump with a higher GPM (gallons per minute) rate to keep the water flowing and to properly maintain the water quality.
Â
A higher GPM rate requires a more powerful pump with a higher horsepower (HP) rating to maintain the necessary flow rate. The higher the flow rate required, the more powerful the pump needs to be. So, when adding extra water features to your pool, it’s important to choose a pump with the appropriate HP rating to ensure that your pool and water features are properly maintained.
When it comes to choosing a pool pump, it’s important to consider both the pump’s GPM and the filter’s GPM. Ideally, the filter’s GPM should be higher than the pump’s GPM. This is because if the pump is pushing more water than the filter can handle, it will create additional strain on the pump motor and potentially cause it to burn out prematurely.
Â
However, it’s also important to keep in mind that the size of your pool’s pipes will play a role in determining the appropriate GPM and HP for your pump. If the pump’s GPM is too high for the pipe size, it can cause damage to the plumbing system and reduce the pump’s efficiency.
Maximum flow rate in pool pipes
A commonly used guideline in the pool industry limits the flow rate in the pipes to a certain GPM
Each pool has a maximum flow rate which is determined by the size of its pipes.
Intake Line Size
Maximum Flow Rate
Recommended Maximum Flow Rate*
1.5 inches
42 GPM
34 GPM
2 inches
73 GPM
58 GPM
2.5 inches
120 GPM
96 GPM
3 inches
160 GPM
128 GPM
Recommended maximum flow rates are 80% of the maximum flow rates to ensure proper water flow and to reduce strain on the pool equipment.
It’s important to keep in mind that exceeding the recommended maximum flow rate can cause damage to your pool equipment and also lead to inefficient water circulation, which can result in dirty or cloudy pool water.
Pools with pipes of different sizes
In a pool’s plumbing system, different piping sizes are often used to accommodate various equipment and features, such as the main pool, spa, and water features. Each piping size has a specific maximum flow rate that should not be exceeded, as exceeding it can cause damage to the system and potentially create safety hazards.
To ensure that the pool’s maximum flow rate is not exceeded, it’s best to use the smallest piping size in the system as the determining factor. This means that even if most of the pool’s plumbing system is made up of larger pipes, the smaller pipes should be used as the limiting factor.
For example, if a pool has primarily 2-inch pipes but also has a spa with 1.5-inch pipes, the maximum flow rate should be calculated based on the 1.5-inch pipes’ limit, which is 42 GPM. This ensures that the water flow is regulated and distributed evenly throughout the system without causing any strain or damage to the pipes, pump, or other equipment.
Overall, it’s important to understand the different piping sizes and their respective maximum flow rates to ensure proper pool operation, efficiency, and safety.
1 thought on “How do I know what size pool pump I need?”
Shammy Peterson
It sure was helpful when you pointed out the need to choose a pool pump with a higher GPM if you are planning to add accessories. This reminds me of my husband’s plan to shop for a new energy-efficient pool pump next Friday. He has to consider that we have plans to install waterfalls on the pool to make it look more enticing, so your tips must make sense.
What are the Chemicals Needed for a Pool and their uses?
It sure was helpful when you pointed out the need to choose a pool pump with a higher GPM if you are planning to add accessories. This reminds me of my husband’s plan to shop for a new energy-efficient pool pump next Friday. He has to consider that we have plans to install waterfalls on the pool to make it look more enticing, so your tips must make sense.