Most pumps are listed with GPH (gallons per hour) flow rates to give you an idea of how much water they will move, however this number is with zero head pressure. In reality, most aquarium return pumps have a good amount of head pressure against them, so the flow rate at zero doesn’t necessarily tell us a whole lot.
What is pump head pressure?
Generally speaking, it is the total resistance pushing against the water coming out of the pump. Yeah, it’s that simple. However, accurately calculating your head pressure is more difficult. And without an accurate estimate of your head pressure, it is much more difficult to determine what size return pump you are going to need.
Factors that Impact Head Pressure
So what creates head pressure on your return? In a typical aquarium set-up, these are the biggest factors:
- Pump Lift Distance: It is the change in height between the return pump and the location where water returns back to your aquarium.
- Diameter of Return Tubing: Generally speaking, smaller diameter tubing will add more head pressure than larger diameter tubing.
- Total Length of Return Tubing: The total distance water travels through the tubing.
- Pipe Fittings: Any fitting, like a 90 elbow, a tee or a valve, that the water passes through.
- In Line Equipment: Any equipment, like filters, chillers, and UV sterilizers, that the water passes through.
Impact of Pump Lift Distance
This is usually the biggest factor in determining head pressure and it is fairly easy to determine.
- For Submersible Return Pumps: Measure the height in feet between the water line in the sump and the location where water returns back into your aquarium.
- For External Return Pump: Measure the height in feet between the top of the return pump and the location where water returns back into your aquarium.
Impact of Diameter and Length of Return Tubing
Easily estimating the impact, the diameter and the total length of tubing has on head pressure is difficult. This is because it is impacted by the flow rate of the pump; ie the faster the pump, the greater the impact. Below is a chart you can use to estimate the impact based on the size of the return tubing and the target flow rate you are trying to achieve.
| 500 gph | 750 gph | 1000 gph | 1500 gph | |
| 0.5” Tubing | Add 0.60/Foot | Add 1.40/Foot | Add 2.50/Foot | Add 5.20/Foot |
| 0.75” Tubing | Add 0.15/Foot | Add 0.40/Foot | Add 0.65/Foot | Add 1.40/Foot |
| 1.0” Tubing | Add 0.05/Foot | Add 0.10/Foot | Add 0.20/Foot | Add 0.40/Foot |
| 1.5” Tubing | N/A | Add 0.02/Foot | Add 0.03/Foot | Add 0.06/Foot |
| 2.0”+ Tubing | N/A | N/A | N/A | N/A |
So if you are using 0.75” tubing at a length of 5 feet and are trying to achieve a flow rate of 1000 gph, you would have an additional 3.25 feet of head pressure (0.65 x 5 feet) on your return pump.
In general, it is best to use as large of a tube or pipe that is reasonable possible as this will minimize the amount of head pressure on your pump. While larger tubing and fittings will be more expensive and take up more room, you will generally make that back up by being able to purchase a smaller, less expensive return pump. Smaller pumps also have the added benefit of using less energy, so you’ll save a bit on your energy bill too. If you want to read more about flow through pipes, research the Hazen-Williams Equation.
Impact of Pipe Fittings
- For ¾” Tubing and Smaller: Add 1 foot of head pressure for every 90-degree turn.
- For 1” Tubing and Larger: Add 1/2 foot of head pressure for every 90-degree turn.
Impact of Inline Equipment
This is another tough one to estimate. As a general rule, we do not recommend running an equipment in-line with your return pump. Doing this will not only negatively impact your return flow rate, it will also generally be too much flow for your equipment to run properly. If it’s unavoidable, here are some approximations you can use:
- Filters: Approximately 2 to 4 feet of additional head pressure
- Chillers: Approximately 2 to 4 feet of additional head pressure
- UV Sterilizers: Approximately 1 to 2 feet of additional head pressure
Formula for Calculating Head Pressure
So where does that leave us? Add up all the impacts described above, and this should give you a good estimate of the head pressure your system has. That in combination with how much flow your drains can handle will help you determine what size pump you need.
Total Head Pressure = Pump Lift Distance + Diameter and Length of Return Tubing + Pipe Fittings + Inline Equipment
To learn how to calculate how much flow your drains can handle, please read our blog about how to choose the correct size return pump.
A Helpful Tool
Pentair Calculator: Looking for an easy way to figure this all out? Pentair has developed a very simple yet effective calculator to help determine how much head pressure is applied to your pump: https://pentairaes.com/pump-calculator