Pool Pump Flow Rate vs Pressure: Key Differences

Two different numbers on the same pump

A pool pump listing can state the volume of water the pump moves and the force it applies to move it. Those are two separate figures, even though one pump produces both at the same time. This article explains what each number means and how both relate to circulation and filtration.

Flow rate is a volume per unit of time. On pool pump listings it appears as gallons per minute (GPM) or gallons per hour (GPH). Some listings use liters per minute or cubic meters per hour for the same idea. The flow figure tells you how much water the pump can move under the maker's stated operating condition.

Pressure is the force the impeller applies to the water. Listings may state it as pounds per square inch (PSI) or as feet of head, which is the equivalent height of a water column the pump could push. The pressure figure tells you how much force the pump can apply to overcome resistance in the plumbing and filter.

A pump that moves a high flow rate in one system may move less water in a system with more resistance. The maker's published performance curve shows the full set of operating points.

This trade-off is why a listing that states only a flow figure does not tell you the pump's behavior in your system, and a listing that states only a pressure figure does not either. The two figures together, with a performance curve, describe the pump more completely.

• Flow rate units commonly seen on listings: GPM, GPH, liters per minute, cubic meters per hour
• Pressure units commonly seen on listings: PSI, feet of head, bar

How flow rate and pressure trade off

The two figures are not independent. A pump has a performance curve that describes the combinations of flow and pressure it can produce. When the pump faces more resistance, pressure rises and flow falls. When resistance drops, flow rises and pressure falls. This trade-off is the shape of every pump's performance curve.

Resistance in a pool system comes from the plumbing and equipment the water passes through. Pipe length and diameter, fittings, the filter, the heater, check valves, and any other component add resistance. A longer pipe run or a filter with fine media creates more resistance than a short run or an open return.

The same pump can operate at different points on its curve depending on the system it is installed in. The maker's published performance curve shows those operating points. The related article on feet of head explains the pressure side of that curve in more detail.

What each number does for the pool

Both flow and pressure are needed for a pool to circulate water through the filter. Flow rate is the volume of water that reaches the filter in a given time. Pressure is what pushes that water through the filter media and the rest of the system.

The ENERGY STAR Pool Pumps page describes the pool pump as the component that recirculates water through a filter to maintain water clarity and hygiene. The pump produces both the movement and the force for that job. A filter needs a certain flow to receive water and a certain pressure to push the water through the media.

On a pump listing, a stated flow figure tells you how much water the pump can move under stated conditions. A stated pressure figure tells you the force the pump can apply. Each figure is part of the maker's specification, and neither one alone describes the whole pump.

The related article on flow rate in GPM covers the flow side, and the article on PSI reading covers the pressure side in more depth.

Reading flow and pressure on a listing

A pool pump listing may state flow rate, pressure, both, or neither. Some titles give one figure and omit the other. When a listing states GPM, that is the maker's stated flow rate, usually under a defined operating condition. When a listing states feet of head, that is the maker's stated pressure capability.

A PSI figure on a listing is the stated discharge pressure, which is the pressure at the pump outlet under stated conditions. The related article on discharge pressure explains that figure in more detail.

When a figure is absent, the listing may say the value is not stated by the seller. That is useful information: the seller chose not to publish the number, and the maker's specification sheet or manual may state it.

The Pool & Hot Tub Alliance (PHTA) is accredited by the American National Standards Institute (ANSI) to develop national consensus standards for pool equipment, which is one reason specification terms like GPM and feet of head appear consistently across the industry.

What this site cannot say about flow and pressure

This site does not size a pump for a pool. The matching of a flow rate or pressure figure to a pool volume depends on the full system, including the plumbing, filter, and any other equipment on the pad. That calculation belongs to the maker's sizing page and an installer.

The related articles on this site explain horsepower, head, and flow rate as terms, and point readers to the maker's sizing page and an installer. The above ground pool pump guide and the inground pool pump guide list pumps from both pool types, and each guide compares the stated figures of the pumps it covers.

A filter gauge reading is a number on the filter, not on the pump listing. This article explains the terms flow rate and pressure; it does not tell you what reading your filter should show.

Questions to ask before buying

When comparing pumps, read the stated flow and pressure figures as the maker's specifications. Ask the maker for the performance curve so you can see how the two figures trade off across operating points. Hayward's site, for example, offers a total dynamic head calculator as a maker resource, and Pentair describes its IntelliFlo pumps as bringing variable speed and flow technology to the industry. For a pool owner comparing pumps, the variable speed pool pump guide covers pumps whose flow settings adjust across a range.

Ask an installer how the existing plumbing, filter, and any other equipment on the pad will affect the pump's operating point. The same pump can produce different flow and pressure figures in different systems.

• What flow rate does the maker state for this pump?
• What pressure or head figure does the maker state?
• Does the maker publish a performance curve?
• What operating point does the installer expect for the existing plumbing and filter?

What to pick for your situation

Questions

What is flow rate on a pool pump?

Flow rate is the volume of water the pump moves per unit of time, stated in gallons per minute (GPM) or gallons per hour (GPH). The related article on flow rate covers the term in more detail.

What is pressure on a pool pump?

Pressure is the force the pump applies to push water through the plumbing and filter. It may be stated in pounds per square inch (PSI) or feet of head. The related article on PSI reading explains the term.

Which is more important, flow rate or pressure?

Both are needed. A pump must move water to the filter and apply force to push it through. The maker's performance curve shows how the two trade off, and an installer can relate that curve to the existing plumbing.

Why do some listings state GPM and others state PSI?

Sellers choose what to include in a listing. Some state the flow figure, some state the pressure figure, some state both, and some state neither. When a figure is missing, the listing may say it is not stated by the seller.

Can you tell you the right flow rate for your pool?

This site does not match a flow rate to a pool size. That calculation depends on the full system, including plumbing and filter. Ask the maker's sizing page and an installer.

What is a performance curve?

A performance curve is a chart published by the maker that shows the combinations of flow and pressure a pump can produce. It describes how the two figures trade off as resistance changes.