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Swimming Pool Circulation - Physics

Updated on September 17, 2013

Understanding Velocity and Turnover

In the pool business there are many reasons why pool builders should have paid attention in high school. Alas, many did not so doing this sort of math can hurt their brains. Trust me, this water stuff can be a tad on the dry side. Even as a state licensed swimming pool contractor (custom pool builder in the Central Florida/Orlando area) this is not the prettiest part of our business (but it still is important). Mark (my partner & husband) has helped me explain it on a very basic level---one that I myself can be at peace with!


Velocity is important in moving the water yet can be a part of a potentially dangerous situation. If the pool velocity is too high there may be an increased risk of suction entrapment (I am a complete believer in suction-less - drainless - entrapment-free - whatever you want to call it - pools that use only surface skimming to remove the water from the pool. We definitely want to stop suction entrapment. See our Pool and Spa News article Also, velocity that is too high can put pressure on your piping system that might result in damage to the piping ---the stuff that is underneath your deck --- really hard to fix --- and nearly impossible to fix without making a huge mess. You also can cavitate a pump (think -- bottom of the milkshake) and that can ruin a pump too. I explain velocity as analogous to your blood pressure. Pipes that move the water without excess pressure is best for the heart of the system (the pump), the filtration media, the piping and your power bill. Because of this, the size pump you use is quite important (see


For proper circulation you need to sanitize the water ---all the water--- so you need to turnover 100% of the volume of water. To do that you need to know the flow rate. In simple terms, if the flow rate was 100 gallons per minute in one hour you would circulate 6,000 gallons (100 gallons times 60 minutes). Oh if it were that easy. You see there are other factors to consider. First, what about the pump? What are the gallons per minute (GPM) that it is able to move? You can read the literature on the pump and see what the flow rate is BUT you have to adjust to the total resistance to flow (Feet of Head), which, in general, looks at the loss of velocity based on several factors:

  • the distance of the piping run - you know this concept from drinking from either a long straw or a short straw
  • the friction of the numerous piping turns (less is better) - Each turn slows you down - for instance, just one 90 degree bend creates as much reduction in flow as 11 feet of straight pipe (friction loss is what we call it -- it is similar to VD.....which if you are not an electrician you might think of something other than Voltage Drop.)
  • the change in elevation from the pool to the equipment - a ‘flooded' downhill location for the pump requires essentially no effort to get the water to it while pulling it uphill 10 feet or so can put quite a bit of extra strain on the pump system.

Keep in mind too that the cleanliness of your filter will impact the flow rate. In fact, seasoned pool owners know the ‘feel' from the return inlets and can tell when the flow rate is being reduced. This reduction in flow can seriously impact the effectiveness of sanitization so keep that filter clean.

Ever hear of a ‘bottleneck"? Your submerged suction outlet (drain) and/or your skimmer can only allow so many gallons per minute of flow. Generally, the piping size and pump GPM cannot overcome this limitation with additional effort.

Together this flow rate will tell you how long you will have to run your pump system in order to get the job done.

Some Formulas to use to impress your friends are shown below -- hopefully your pool builder will understand them. If not, you might want to re-think that. Just saying.

In summary, look for a system with low pressure levels at the filtration system while still enough velocity to accomplish the turnover rate required by your local building code. Doing this will make your pool clean and nice and allow you to worry more about the furniture placement and less about physics. Enjoy your swim!


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      Amy Appleton 

      9 years ago

      "Keep in mind too that the cleanliness of your filter will impact the flow rate." I'm glad you brought this up, it applies to hot tubs too. People don't seem to understand that they have to maintain their spa for the jets to work at full blast.

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      9 years ago

      You're very welcome!

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      Karen Reader 

      9 years ago

      Thanks for breaking it down like this. I appreciate the step by step that you have provided for us to go off of.

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      9 years ago

      That is some very interesting, technical information. Thanks for sharing with good explanation.


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