Pump Head vs Pressure: Why Centrifugal Pump Performance Curves Use Feet of Head Instead of PSI
Pressure gauges read PSI, but centrifugal pump curves are usually shown in feet of head. That is because pressure changes with fluid density, while head gives the pump industry one common way to describe performance across different liquids. A pump handling water, condensate, or a light hydrocarbon may show very different pressure readings even when the pump itself is producing the same head.
Head is Not the Same Thing as Pressure
A simple way to picture pump head is to imagine a straight vertical pipe connected to the discharge of a centrifugal pump. If the pump could push the liquid 115.5 feet up that pipe, the pump would be producing 115.5 feet of discharge head.
That is different from pressure. Pressure is what a gauge reads at a point in the system. Head is a way of expressing the energy the pump adds to the liquid. The two are related, but they are not interchangeable because pressure depends on the density of the fluid being pumped.
That distinction is why pump performance is usually expressed in feet of head instead of PSI.
Why the Pump Industry Uses Head Instead of Pressure
A centrifugal pump may handle water in one service and a hydrocarbon blend in another, but that does not mean the pump needs a different performance curve each time fluid density changes. The impeller and casing have not changed. What changes is how that hydraulic energy shows up as pressure once specific gravity is factored in.
If pump performance were shown only in PSI, the pressure curve would have to shift whenever the density of the liquid changed. A heavier liquid would produce more pressure for the same head. A lighter liquid would produce less. Using feet of head avoids that problem. It allows one pump curve to describe pump performance across liquids with different specific gravities.
That is why centrifugal pump curves are normally plotted as head versus flow, not pressure versus flow.
Same Head, Different Pressure
The relationship becomes easier to see with a simple example. Assume the pump is producing 115.5 feet of head. The only thing that changes is the specific gravity of the liquid.
| Specific Gravity | Head | Pressure |
| 1.2 | 115.5 ft | 60 psi |
| 1.0 | 115.5 ft | 50 psi |
| 0.70 | 115.5 ft | 35 psi |
Nothing changed about the pump. The head stayed the same in every case, the pressure changed because the liquid density changed.
That is the main reason the pump industry uses head instead of pressure when describing centrifugal pump performance.
How to Convert Differential Pressure to Feet of Head
When suction pressure, discharge pressure, and liquid specific gravity are known, pump head can be estimated from pump differential pressure using:
Liquid Head (ft) = (Differential Pressure × 2.31) / SG
Where:
- Differential Pressure = discharge pressure minus suction pressure, in PSI
- SG = specific gravity of the liquid relative to water
A liquid water column exactly 2.31 feet in height, will produce a pressure gauge reading at the bottom of the column of exactly 1 psi gauge reading. The 2.31 factor converts PSI to feet of head for water. Dividing by specific gravity adjusts that conversion for liquids heavier or lighter than water.
Pump Head Calculation Example
Assume a centrifugal pump has:
- discharge pressure = 85 psi
- suction pressure = 15 psi
- liquid specific gravity = 1.1
First calculate pump differential pressure:
85 psi − 15 psi = 70 psi
Then convert differential pressure to head:
Head = (70 × 2.31) / 1.1 = 147 ft of head
In other words, the pump is producing about 147 feet of head at those conditions.
What Head Does and Does Not Tell You
Head is the standard way to describe centrifugal pump performance, but it does not tell the whole story. Specific gravity changes the pressure associated with a given head, and viscosity or unusual fluid conditions can affect how closely a pump follows a published curve.
Still, if the question is why the pump industry talks in feet of head instead of PSI, the answer is straightforward: head gives a more consistent way to describe pump performance across liquids with different densities.
If you need help relating pressure readings, specific gravity, and operating conditions back to actual pump performance, PumpWorks can help evaluate the application and size the pump as part of a broader rotating equipment solution. Contact a specialist today.