Last Updated: August 23, 2021

Pool Pump Size Calculator (Flow Rate & Volume)

Written By: Pool Care Guide

What Size Pool Pump Do I Need?

The ideally-sized pool pump should be capable of filtering all of your pool’s water three to four times per day. This recommendation is largely based upon the results of computations and experiments that were presented as Gage and Bidwell’s Law of Dilution

Before we get into that, let’s go over a few quick definitions.

  • Flow Rate: The flow of water the pool pump is able to create. We calculate that flow in Gallons Per Minute (GPM) or Gallons Per Hour (GPH).
  • Pool Volume: Volume can be measured in a few ways. In this article, pool volume refers to how many gallons fit into your pool.
  • Turnover Rate: How many minutes or hours it takes to filter your pool volume a single time.

To figure out what pool pump you need, we first need to know your pool’s volume in gallons and the turnover rate that you want.

The first thing we need to do is figure out how many gallons your pool holds.

Step 1: How Much Water Does Your Pool Hold?

To figure out the size of the pool pump that we require, we first need to know how many gallons of water your pool can hold. Provided that you already know this, you can skip to the next section. If you don’t know how many gallons your pool can hold, we recommend using our pool volume calculator to figure that out.

Just in case, if you would rather calculate the pool’s volume yourself, you can use the volumetric formulas below for the most common pool shapes. If you have a different shape or multiple shapes, you might want to click over to our previously mentioned pool volume guide.

Circle

  • Area: 3.14 x radius²
  • Volume:  Area x Depth
  • Total Gallons: Volume x 7.48 Gallons

Squares & Rectangles

  • Area: Length x Width
  • Volume: Length x Depth
  • Total Gallon Capacity: Volume x 7.48 Gallons

Kidney

  • Area Formula: .45 x (Diameter Circle 1 + Diameter Circle 2) x Pool Length
  • Volume Formula: Area x Depth
  • Total Gallon Capacity: Volume x 7.48 Gallons

Variable Depths

If your pool has a shallow end and a deep end, you’ll need to account for that. Pool depths transition in two ways: gradual and sudden.

Gradual transitions are simple. Add the shallow depth to the deep depth and divide the sum by 2. Sudden transitions should be measured and calculated separately. Treat them like separate pools. After you have calculated the total capacity of these “separate pools,” add them up together.

Step 2: How Many Turnovers Per Day Do You Want?

Gage-Bidwell’s Law of Dilution

Unfortunately, the specifics of the previously mentioned Gage-Bidwell’s Law of Dilution aren’t available. They were either lost or we just haven’t found the original source. Since we can’t use this “law,” we will just derive some generalized guidelines.

Using a standard but unknown pool pump and filter, this duo found that 7 turnovers are needed each day to cleanse 99.9% of the water. Below that, we find the following:

  • 1st Turnover: 63%
  • 2nd Turnover: 86%
  • 3rd Turnover: 95%
  • 4th Turnover: 98%
  • 5th Turnover: 99.3%
  • 6th Turnover: 99.7%

As you can see, each additional turnover provides diminishing returns. Also, your results will vary, so it’s best not to take this as gospel.

Goldilocks Principle - Bigger Is Not Always Better

Some might assume bigger is better, but that isn’t necessarily the case. The perfect pool pump will not be determined by horsepower, but by its gallons per minute (GPM). The GPM is the measurement of how many gallons the pool pump can push through the filtration system each minute.

It doesn’t take a physics degree to realize increasing the GPM will also increase the pressure within the filtration system. Filters are designed to work within a certain range of pressure. When the pool pump’s GPM is outside of that range, the filtration suffers.

If the GPM is too low, water will not flow through the filters fast enough to keep up with the constant contamination. The water will not be cleaned fast enough. The ending result is that algae and other contaminants will slowly overtake the pool.

If the GPM is too high, the overwhelming pressure will force the contaminants right through the filters. The water will not be effectively cleaned, and the algae and other contaminants will slowly overtake the pool. We can find the ideal pool pump size by calculating what flow rate we need. To calculate that, we first need to know how many gallons the pool holds.

The Pool Pump Should Be Capable of Multiple Turnovers Each Day

An ideally sized pool pump’s turnover rate will be between 6 and 8 hours, but 10 hours is also considered acceptable. For normal day-to-day maintenance, you only need the water to turn over a minimum of once per day to achieve an acceptable level of filtration.

That said, there will be times pool maintenance will require more turnovers in a single day. That’s why your pool pump’s calculated turnover rate should be capable of 3 to 4 turnovers per day.

Step 3: Finding the Flow Rate

The flow rate is the measurement of how many gallons a pool pump can process every minute or every hour. We can use the pool volume and desired turnover rate to calculate what flow rate we need.

Flow Rate (GPH) = Total Gallons / Turnover Rate / 60

Flow Rate (GPM) = Flow Rate (GPH) / 24

When window shopping through potential pool pumps, you should be able to find the gallons per minute (GPM) listed within the product specifications. 

If you already know the turnover rate that you want and your pool’s volume, you can use the chart below to find the GPM that you'll need.

Required Flow Rate to achieve desired Turnover Rate

Gallons6 Hour Turnover8 Hour Turnover10 Hour Turnover
10,00028 GPM21 GPM17 GPM
15,00042 GPM31 GPM25 GPM
20,00056 GPM42 GPM33 GPM
25,00069 GPM52 GPM42 GPM
30,00083 GPM63 GPM50 GPM
40,000111 GPM83 GPM67 GPM
50,000139 GPM104 GPM83 GPM

Final Thoughts - Climate Considerations

Your local weather, temperature, flora and fauna, and swimmer sanitation habits will influence how much filtration you need. In general, locations closer to the equator have warmer temperatures. Algae grow faster in warm climates and slow to a crawl in temperatures below 61°F (16°C). With that in mind, those of you that live in warmer climates should consider purchasing pool pumps with a higher GPM. On the other side of the coin, colder climates can get away with slower turnover rates.

If you want to know how long and often you should run your pool pump, you might want to click over to our guide, “How Long Should You Run a Pool Pump?” That guide gives narrowed-down recommendations based on temperature ranges, pool sizes, and your personal priorities (frugality vs. filtration). However, whether you are located in the central plains of Ireland or a Floridian coast, your pool should turn over at least once per day.

Questions and Answers

Can a Pool Pump Be Too Big?

A pool pump can be too big if you do not have sufficient control of the flow rate. Filters only operate effectively within specified ranges of pressure. If your pool pump overpowers the filters, your filtration will suffer. The increased PSI will push those contaminants straight through the filter. For this reason, you should never buy a single-speed pool pump that is rated higher than your filter’s manufacturer’s recommendations. Instead, if you choose to buy an overpowered pool pump, look into variable-speed pool pumps. Dual-speed pumps can work if the lower setting fits within the filter’s effective range.

Is it Okay to Run a Pool Pump 24 Hours a Day?

Definitely! A pool pump running 24/7 will use up a lot more electricity, so your utility costs will bump up each month. However, from a filtration perspective, an endlessly running pool pump will keep your pool crystal clear.

Some believe that one turnover per day is enough when paired with regular pool maintenance. The pump that you need will ultimately be determined by whether you agree or disagree. Increased filtration will undoubtedly decrease pool issues and make pool maintenance a bit easier. On the downside, it will also increase the cost. 

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