How to Size a Pool Pump

When it comes to your pool, it’s the pump that does all the hard work behind the scenes to push water through your filtration system. That’s why it’s so important to buy the right size pump. Buy the wrong one and it may not be able to keep up with the demand, resulting in cloudy—or worse—green water.

Properly sizing your pump isn’t difficult, but it will take some careful calculations to get it right. Let’s walk through the process step by step so you can determine the best pump size for your swimming pool.

How Much Water Does Your Pool Hold?

The first step in determining the proper size pump for your pool is to calculate how much water it holds. The reason for this is that your pool pump should be able to turn over the water in about 8 hours. (For a more exact calculation on how many hours to run your pump, check out How Long to Run a Pool Pump.) In other words, all the water in your pool should run through your pump in that time span. And that should happen once a day.

To determine how much water your pool holds, start by looking at the paperwork that came with your pool. If you can’t find the number of gallons it holds, don’t worry. With a little simple math, you can figure it out on your own.

Just follow these formulas, depending on your pool’s shape, or use our pool volume calculator:

For a rectangular pool: Length (ft.) X Width (ft.) X Average Depth (ft.) X 7.48 = Total Pool Gallons

For a round pool: 3.14 x radius squared (ft.) x average depth (ft.) x 7.48

For an oval pool: 3.14 x ½ Length (ft.) X ½ Width (ft.) X Average Depth (ft.) X 7.48 = Total Pool Gallons

To determine the average depth in your pool, which you will need for the above formulas, add the shallowest part of your pool and the deepest part of your pool and then divide the number by two.

Determine the Gallons of Water Per Minute

The next step in the process is to determine how many Gallons Per Minute (GPM) the pump needs to pump to turn over the water in your pool at least once a day. To find this number, you need to first figure the Gallons Per Hour (GPH). To do that, follow this formula:

Pool Volume in Gallons ÷ 8 = GPH

Since most pool pumps come with a Gallons Per Minute (GPM) rating, you now have to convert the GPH to GPM. Here’s the formula you need to do that:

GPH ÷ 60 = GPM

Find the Perfect Pump

You’ve done the hard part, all the calculations to determine exactly what size pool pump will filter your pool’s water in a day. But before you run off to the store, let’s talk about a couple of other considerations you should think about.

For example, you have three speed choices for your pump. They are:

• Single-speed pumps. When pool pumps were first invented, they were all single speed. Some states, like California and Arizona, don’t allow them anymore because they use so much energy.
• Dual-speed pumps. These pumps have two speeds: low and high. The high-speed setting is equivalent to a single-speed pump, while the low speed is much less. Depending on the size of your pool, the low speed may not be enough to turn over the water in your pool within a day.
• Variable speed pumps. These pumps are more energy-efficient because they use a magnet motor as opposed to the induction motors the other two types use. Magnet motors produce less friction, which increases efficiency. They’re also quieter than the other types. You will pay more for a variable speed pump, but save in the long run.

What Pump Size Is Right for Your Pool?

Now that you’ve done the math and understand the ideal size pump for your pool, it’s time to start shopping. After all, the dog days of summer are just around the corner, and an important step to having that perfectly blue pool is choosing the right size pool pump.