Things That Affect A Swimming Pool and Spa Hydraulics

Hi Pool Pros!  We have more on the subject of healthy Hydraulics.  Let’s first talk about some hydraulic terms, before we get into the meat of the discussion.  Knowing what these mean will help us understand future discussion points.

Head loss

Head loss is a factor applied to the resistance to flow in a swimming pool system.   Head loss can also be converted to Pounds per Square Inch (PSI).  1 PSI equals 2.31 ft of head.  Example:  A pump pumping water up 30 ft will generate 30 ft of head or 13 PSI.  The return side of the pump is measured in PSI, while the suction side in vacuum or negative pressure (there are gauges that will read positive and negative pressure).  For head loss on the suction side, 1 inch of mercury equals 1.13 ft of head.   Example:  6 inches of mercury equals 6.8 ft of head.    Remember, everything that water flows through has a head loss factor.

System Head Loss

When you add up all the head loss factors (filter, heater, pipe, etc.) for a system you have the system head loss which is also called total dynamic head or (TDH).

Gallons per Minute

When helping someone determine the head loss or pipe sizing for a pool, you need to ask what the turnover required for the pool and spa is.  This of course is measured in GPM.


We should consider velocity the most important in healthy hydraulics – it’s the speed at which water flows through a system.   Velocity is measured as feet per second (FPS). This is very important because as the velocity increases, so does the head loss.   Example:   At 50 GPM, through 1.5” pipe, the velocity is 7.09 and the head loss is 11.1.   At 70 GPM, the velocity is 11.3 and the head loss is 25.1.  As you can see from these examples, to determine head loss through any piece of equipment or piping, the starting point is flow or GPM.

We will bring this all together as we go through Healthy Hydraulic.  The information above will be used to determine the right pump for a given system.  We view a swimming pool or spa’s hydraulic footprint like we do fingerprints – every system is unique.   We also like to use the phrase “resistance to flow” in place of head loss since it’s easy to understand, particularly by pool owners.  So as we continue on the topic of healthy hydraulics, we will use both of these terms interchangeably.

Future discussion will cover the integral parts of Healthy Hydraulics – pumps, filters, heaters and all other essential components that are part of the filtration or water feature path.

We look forward to you jumping into the conversation.  Get involved, stay connected!