How to Select a Swimming Pool Pump

You might think bigger is better in terms of pump size. However, a pump that is too big can cause problems.

To get the right size pump, you need to know the volume of water in your pool. The volume is the pool’s total capacity, which is measured in gallons.

Start by figuring out the average depth of your pool. You will need to know how deep the shallow end is and the depth of the deep end. Measure the two ends if necessary.

Add the two depth measurements together and then divide by two to get the average depth.

Average Depth Sample Calculation

Shallow End Depth = 4 feet

Deep End Depth = 12 feet

4 + 12 = 16 feet

16 feet ÷ 2 = 8

The average depth of the pool in this example is 8 feet.

To determine the volume of water, use the formula for the shape of the pool you have: 

• Circular Pools: Diameter (ft.) x Diameter (ft.) x Average Depth (ft.) x 5.9 = Total pool capacity in gallons
• Oval Pools: Length (ft.) x Width (ft.) x Average Depth (ft.) x 6.7 = Total pool capacity in gallons
• Rectangular Pools: Length (ft.) x Width (ft.) x Average Depth (ft.) x 7.5 = Total pool capacity in gallons

Once you know the volume of water in your pool, you’re ready for the next step of calculations.

Calculate Flow Rate and Turnover

Next, you’ll need to figure out the flow rate and turnover required to circulate the water for the time you want.

Flow rate is the number of gallons of water the pump moves per minute. Turnover is the minimum amount of time needed to circulate all the water through the pool filter. 

Pool water gets dirty and cloudy if it isn’t all pushed through the filter at least once a day. 

Use this formula to figure out the turnover rate:

Pool Volume in Gallons ÷ Turnover Rate in Minutes = Flow Rate

Example: If you have a 25,000-gallon pool, and you want the water to turn over once every 8 hours: 

There are 60 minutes in an hour, so 8 hours would be 480 minutes:

8 hours x 60 minutes = 480 minutes

Then you would divide the pool volume in gallons by the number of minutes:

25,000 ÷ 480 = 52 gallons per minute

Your 25,000-gallon pool needs a pump with an output of 52 gallons per minute to circulate the water once every eight hours.

Calculate Maximum Flow Rate

The size of your pool’s pipes determines its maximum flow rate. 

Count the number of intake lines for your pool and refer to the common pipe sizes below:

• For each 1.5-inch intake line, the maximum flow rate is 42 gallons per minute.
• For each 2-inch intake line, the maximum flow rate is 73 gallons per minute.

Example: 

Two 1.5-inch intake lines = 2 lines x 42 gallons per minute = 84

The maximum flow rate for a pool with two 1.5-inch intake lines is 84 gallons per minute. 

Every pool filter has a maximum flow rate, measured in gallons per minute. The maximum flow rate for your pool pump should not be more than the maximum flow rate of your pool’s filter. If your pool pump pushes more water through the filter than it can handle, the filter won’t work correctly. 

If your pool pump has a higher maximum flow rate than your pool filter, you should either get a pool pump with a lower flow rate or a filter than can handle more water. When the numbers for the pool pump and filter don’t match, operating the pool pump can damage the filter.

Calculate Resistance