Pool pumps circulate water through the pool filtering system, mixing chemicals and sanitizing the water to help keep it clean. The pump must be the right size for your pool so that the filtering system operates efficiently.
This guide will show you how to calculate the pool capacity, flow rate and turnover, maximum flow rate and resistance, which will give you the numbers needed to determine the proper size pump for your pool. Need supplies fast? Order before noon for convenient scheduled same-day delivery.
Sizing the Pump for Your Swimming Pool
You might think bigger is better in terms of pump size, but a pump that is too big can cause serious problems.
The first step in sizing your pool is to know the volume of water in your pool, which is the average depth:
Calculation - Example
Measure the shallow end depth. - 4 feet
Measure the deep end depth. - 12 feet
Add them together. - 16 feet
Divide by two. - 16 ÷ 2 = 8 feet
In the example above, the pool's average depth is 8 feet.
Now use the formulas below to determine the volume of water in your pool.
- 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 to calculate the flow rate and turnover required to circulate the water in the number of hours you want.
Calculate Flow Rate and Turnover
Flow rate is the number of gallons the pump moves per minute, and turnover is the minimum about of time to circulate all water through the filter.
Use this formula to determine your 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 eight hours:
25,000 ÷ 480 (60 minutes x 8 hours) = 52 GPM
Your 25,000-gallon pool needs 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 the 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 GPM.
- For each 2-inch intake line, the maximum flow rate is 73 GPM.
Example: Two 1.5-inch intake lines = 84. The maximum flow rate is 84 GPM.
This number is important because your pool filter has a maximum flow rate, which is measured in GPM. The pool pump's GPM rating should be below the pool filter's maximum flow GPM rating.
If the pool's turnover rate is higher than the filter's maximum GPM, the filter is undersized and will not work properly. If the filter is undersized, it should be replaced, or the pump should be undersized to prevent damage to the filter.
Every piece of equipment connected to your pool’s circulation and filtration creates resistance to water flow.
The length and size of your pool’s pipes, type of filter, and features like heaters and pool cleaners increase the resistance.
Use a pressure meter and this formula:
- Check the pressure of water flowing into the filter tank and multiply that number by 2.31.
- Get a vacuum reading on the pump suction line and multiply that by 1.13.
- Add the two numbers together and the result is the total dynamic head.
Example: If the water flowing into the filter tank is 10 PSI, and the vacuum reading on the pump suction line is 5 PSI:
Water flow into filter tank: 10 PSI x 2.31 = 23.10
Vacuum reading on pump suction line: 5 PSI x 1.13 = 5.65
Total = 28.75 ft. of resistance
Now that you have the numbers for your pool, use the pump manufacturer performance charts that identify the model and horsepower that is appropriate for your size pool.