Why do centrifugal pump manufacturers publish performance curves in head (meters or feet) rather than pressure?

Centrifugal pumps are kinetic energy machines that impart a specific potential height (head) to a liquid regardless of its density. A pump running at a specific rotational speed and flow rate will generate the exact same head (e.g., 50 meters) whether pumping water, heavy oil, or dense brine. However, the resulting discharge pressure scales directly with the fluid's density.

How do you convert pump head to pressure in metric and US customary units?

In metric units, Pressure (Pa) = Density (kg/m³) × 9.81 (m/s²) × Head (m). Divide Pascals by 100,000 to get bar. In US Customary units, Pressure (psi) = Head (ft) × Specific Gravity / 2.31, or Pressure (psi) = Head (ft) × Density (lb/ft³) / 144.

Does pump discharge head change when liquid density changes?

No. For a fixed impeller diameter and operating speed, a centrifugal pump develops the same total dynamic head regardless of liquid density. However, discharge pressure and required shaft brake horsepower (BHP) increase linearly with higher fluid density.

Converting Pump Head to Pressure

How to convert a pump head to a pressure?

How to convert a pressure to a pump head?

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Introduction
Converting a Pump Head to a Pressure
Converting a Pressure to a Pump Head
Interactive Online Conversion Calculator
Plant Engineering Rules of Thumb & Safety Limits
Change of Head and Pressure with Liquid Density
Pump Manufacturers Reference
Free Excel Calculation Tool

1. Introduction

Pump manufacturers typically refer to pump head (expressed in meters or feet of liquid) to define the performance curve of their centrifugal pumps. However, Process Engineers very often need to reuse the pump head for downstream hydrodynamic calculations, to evaluate pressure drops in pipe networks, size control valves, or check relief valve setpoints. It is therefore necessary to convert the pump head to a pressure unit such as bar, kPa, or psi.

2. Converting a Pump Head to a Pressure

Having a pump characteristic curve giving total dynamic head in meters (\(H\)), it is possible to calculate the generated pressure (\(P\)) in Pascals (\(\text{Pa}\)) using the fundamental hydrostatic equation:

P = \rho \cdot g \cdot H

Where:

\(P\) = Differential pressure generated (\(\text{Pa}\))
\(\rho\) = Density of the pumped liquid (\(\text{kg/m}^3\))
\(g\) = Acceleration due to gravity = \(9.81 \text{ m/s}^2\)
\(H\) = Pump differential head (\(\text{m}\))

To convert the calculated pressure to bar, divide by \(100,000\):

P_{\text{bar}} = \frac{\rho \cdot g \cdot H}{100\,000}

For US Customary / Imperial units, the pressure in \(\text{psi}\) is calculated as:

P_{\text{psi}} = \frac{H_{\text{ft}} \cdot \rho_{\text{lb/ft}^3}}{144} = \frac{H_{\text{ft}} \cdot \text{SG}}{2.31}

Worked Example:

A centrifugal pump generates a differential head of \(20\text{ m}\). The process fluid is water at \(25^\circ\text{C}\) with a density of \(999\text{ kg/m}^3\):

P = 999 \times 9.81 \times 20 = 196\,003.8 \text{ Pa} \approx 1.96 \text{ bar} \quad (28.4 \text{ psi})

3. Converting a Pressure to a Pump Head

Converting pressure to pump head is the exact inverse calculation:

H = \frac{P}{\rho \cdot g}

If the pressure is expressed in bar, multiply by \(100,000\):

H = \frac{P_{\text{bar}} \times 100\,000}{\rho \cdot g}

In US Customary units, to convert \(\text{psi}\) to feet of liquid head (\(H_{\text{ft}}\)):

H_{\text{ft}} = \frac{P_{\text{psi}} \cdot 144}{\rho_{\text{lb/ft}^3}} = \frac{P_{\text{psi}} \cdot 2.31}{\text{SG}}

⚡ Interactive Pump Head & Pressure Calculator

⚠️ ENGINEERING NOTICE & EDUCATIONAL DISCLAIMER: This interactive calculator is provided exclusively for preliminary estimation and educational purposes. It is not intended for detailed hydraulic design or equipment procurement without certified vendor rating. No warranty, expressed or implied, is provided, and no liability is assumed.

Unit System: SI Metric (m, kg/m³, bar, kPa) US Customary (ft, lb/ft³, psi)

Calculation Mode:

Pump Head H (m):

Pressure P (bar):

Fluid Density ρ (kg/m³):

Calculation Results

💡 Plant Engineering Rules of Thumb & Design Margins

Density Independence of Head: A centrifugal pump impeller running at fixed speed creates the same head (meters or feet) for any clean fluid of similar viscosity, regardless of whether it is water (\(SG = 1.0\)), gasoline (\(SG = 0.72\)), or concentrated sulfuric acid (\(SG = 1.84\)).
Motor Shaft Power Scaling: Absorbed brake horsepower (\(\text{BHP}\)) scales directly with fluid Specific Gravity (\(SG\)). If a pump designed for water (\(SG = 1.0\)) is switched to heavy brine (\(SG = 1.25\)), the motor load increases by 25%. Always check motor nameplate power ratings!
Viscosity Correction Factors: Liquids with high kinematic viscosity (\(> 20 \text{ cSt}\)) degrade pump head, flow rate, and efficiency. Apply Hydraulic Institute (HI) correction charts for viscous fluids.
Shut-off Pressure Rating: Maximum suction pressure plus pump shut-off head determines pipe flange ratings (ANSI Class 150 vs. Class 300) and relief valve (PSV) thermal setpoints.

4. Change of Head and Pressure with Liquid Density

What is important to highlight is that a centrifugal pump, running with the same impeller diameter and at the same speed, will develop the exact same head regardless of the liquid pumped.

That is why pump manufacturers express the performance curves of their pumps in dynamic head (\(\text{m}\) or \(\text{ft}\)) rather than pressure units. They test the pump with water, measure the head produced, and the head curve remains valid for any other liquid of similar viscosity.

On the other hand, the Process Engineer must know the actual liquid density (\(\rho\)) to calculate the true discharge pressure developed in the piping system. For detailed conversion of liquid static head, see our guide on how to calculate fluid head from pressure.

5. Manufacturers

If you are looking for specific pump performance curves to estimate head and select the right pump size for your process application, you can visit Pompes GrosClaude: https://www.pompes-grosclaude.com/en/home/

(Note: MyEngineeringTools has no commercial link with this company.)

6. Free Excel Calculation Tool to Convert Head to Pressure

You can access our companion Excel Calculation Tool to convert head to pressure, and pressure to head, matching all the formulas on this page.

Warning: This calculator is provided for educational and preliminary estimation purposes only. It is not intended for detail design. Please consult a qualified supplier or process expert for certified pump performance verification.

Screenshot Pump head to pressure calculator Excel tool