There are a few primary factors that go into finding the right size pool pump. Below, we’ll run through how to quickly figure it out step-by-step and choose your pool pump confidently.
How to Figure Out What Pool Pump Size You Need
- Calculate the volume of your pool
- Calculate minimum flow rate
- Determine maximum flow rate
- Determine your flow rate range
- Add in Total Dynamic Head (TDH)
- Settle on your final specs
First, you need to figure out how many gallons of water are in your swimming pool. This calculation differs based on pool shape but is pretty straightforward. Use my pool volume calculator below, and then I’ll start calculating the minimum flow rate you need for your pump.
All good? Okay, let’s move on to the next step.
Minimum Flow Rate
Once you have the approximate number of gallons of water in your pool, I have to figure my minimum flow rate for the pump using a Gallons Per Minute (GPM) calculation. GPM considers your pool size and the number of “turnovers” you need per day (your turnover rate). For both traditional chlorine pools and saltwater pools, the industry standard is two turnovers per day.
Before I get into the calculation, you have a few options here. You can choose to run your pump 24 hours per day (decreasing your GPM needs) or twice a day in different segments. The typical two-a-day segments are 6-hour cycles, 8-hour cycles, and 10-hour cycles. So, let’s calculate the minimum flow rate we need for all four of these options.
The ultimate goal is efficiency – balancing electricity bill savings with pump horsepower and balanced water chemistry. Running at low horsepower for longer periods of time results in a more balanced water chemistry, but can be a sink on your electricity bill. The opposite is true for higher horsepower and shorter run times.
This is where variable speed pool pumps come in handy – they can speed up and slow down when you need them to, resulting in a much safer pool chemistry and energy efficient setup, which is a nice savings on that electric bill.
Okay, let’s move on to the next part of my pool pump sizing – figuring out your maximum flow rate.
Maximum Flow Rate
Fortunately, you don’t have to do a bunch of fancy math to figure your maximum flow rate. It is completely dependent on your plumbing system and other pool equipment, all of which should have documentation on maximum flow rates. We’ll start with filters.
There are three types of pool filters (sand, cartridge, and DE) and all of them have different maximum flow rates based on their surface area. Make sure you stay below the maximum flow rates for your filter system or you could end up damaging it. Here are the flow rates for common filter types and sizes.
As a general rule of thumb, sand filters typically fall between 19-22 Max GPM per square foot of surface area.
- 1.8 square feet surface area = 40 GPM
- 2.3 square feet surface area = 50 GPM
- 3.1 square feet surface area = 60 GPM
- 4.9 square feet surface area = 100 GPM
Cartridge filters are a lot bigger and a little less exact. Typically, you’re going to average 0.3 to 0.35 GPM per square foot of surface area.
- 100 square feet surface area = 32-38 GPM
- 200 square feet surface area = 55-75 GPM
- 300 square feet surface area = 80-112 GPM
- 400 square feet surface area = 100-150 GPM
DE filters are a little rangy as well. Plan for approximately 1.75 GPM per square foot of surface area, assuming the middle of the ranges below.
- 24 square feet surface area = 36-48 GPM
- 36 square feet surface area = 54-72 GPM
- 48 square feet surface area = 72-96 GPM
- 60 square feet surface area = 90-120 GPM
Now that we have filters settled, let’s move on to maximum flow rates for plumbing.
You never want to put more pressure in your pipes than they can handle. The maximum flow rate of your plumbing setup depends on the pipe size and should be clearly labeled on your system. If not, you can use these common values or ask your contractor.
- 1.5″ pipes = 43 GPM
- 2″ pipes = 73 GPM
- 2.5″ pipes = 120 GPM
Heads up: your plumbing setup may be different across your entire pool environment. For example, the pipes leading to your spa could differ from the main pool area. Always remember to take the lowest of the maximum flow rates for sizing up your pool pump. Otherwise, you could risk damaging the setup for part of your pool environment.
Min and Max Flow Rate Range
Almost there! Now that we have the minimum and maximum flow rates for your pool setup, put them together to get to your flow rate range: one of the two main specs for determining the pool pump size you need.
So, let’s say you have a 15,000 gallon inground pool. With two turnovers per day, your minimum flow rate comes to 20.8 GPM.
Let’s also say that you have a 2.3 square foot sand filter (50 GPM max) and 1.5″ plumbing (43 GPM). You have to take the lower of those two values to make sure you don’t overwork any components of your pool, so your maximum flow rate comes to 43 GPM.
In this example, our flow rate range comes out to 20.8-43 GPM.
Let’s figure out the last main spec you need for your pool pump: total dynamic head.
Total Dynamic Head (TDH)
Total dynamic head is a resistance metric – it tells you the total amount of resistance in the hydraulic system of your pool. The filter, pump, pipes, heater, backwash valve, return, and other accessories have an effect on how water flows throughout your system. Most inground pools will be somewhere around 50 TDH. Above ground pools typically fall around 30 TDH. We’ll go into more specifics on TDH in another post.
Okay, we now have everything we need! All pool pump models will have charts with horsepower estimates based on flow rate ranges and total dynamic head. In my inground pool example, we need a pool pump with a 20.8-43 GPM flow rate range and 50 TDH (also called Feet of Head).
Questions? Shoot me a message and I’ll be happy to help.