How to Read a Pump Curve: Head, Power & Worked Example

Start before the first line

Before Reading the Curve, Confirm Its Basis

A pump curve is only useful when you know which pump configuration it represents. Before comparing a duty point or two replacement curves, confirm the pump size, speed, impeller diameter, units and liquid/test basis.

Pump size / curve IDMake sure the curve belongs to the exact hydraulic size being reviewed.

Speed / RPMA curve at 1750 RPM cannot be read as though it were the same curve at 3550 RPM.

Impeller diameterRead the correct trim line or trim family before estimating head and power.

UnitsConfirm GPM/ft/HP versus m³/h/m/kW before plotting the duty.

Liquid / test basisPublished centrifugal-pump curves are commonly water-based; specific gravity and especially viscosity may change the engineering review.

Curve revisionDo not use an old screenshot when a current approved curve or PDF is available.

Curve-reading rule

Do not compare two pump curves until speed, impeller diameter, units and liquid/test basis are compatible.

Curve anatomy

Read the H-Q Curve First—Then Add Efficiency, Power and NPSHr

The horizontal axis is capacity or flow. The vertical axis is pump total head. A family of impeller-diameter curves shows how the same pump size changes as the impeller trim changes. Efficiency, shaft-power and NPSHr information are then read at the same operating flow.

Pump Curve Anatomy

A normalized teaching diagram showing the direction of the main quantities. It is not pump-selection data.

Use a real manufacturer or approved replacement curve for actual selection values.

Head is not the same thing as pressure

For the same pump geometry and speed under comparable low-viscosity conditions, the pump head curve is not simply multiplied by specific gravity. The corresponding pressure and required power do change with liquid density.

Pump + system

The Pump Curve and System Curve Determine the Operating Point

The actual operating point is where the pump can produce exactly the head demanded by the system at the same flow. A valve position, pipe loss, static elevation or process change can move the system curve and therefore move the operating point even when the pump itself has not changed.

Pump Curve + System Curve

Increasing system resistance moves the intersection toward lower flow and higher pump head.

Centrifugal pump curve and system curve showing the operating point and a shift caused by higher system resistance

Active control systems can change system resistance dynamically; field valve/control position belongs in the operating-point review.

Operating region

BEP Is One Point; POR and AOR Are Operating Regions

Term	Engineering meaning	How to use it
BEP	Best Efficiency Point at the stated pump configuration.	Use it as a hydraulic reference point, not as the only point where the pump may operate.
POR	Preferred Operating Region around BEP where hydraulic and reliability behavior is generally preferred.	Use the applicable manufacturer / ANSI-HI guidance for the actual pump; do not assume one percentage band fits every pump.
AOR	Allowable Operating Region, normally wider than POR.	Check hydraulic loading, vibration, temperature rise, recirculation, NPSH and manufacturer limits before relying on the outer region.

No universal 70–120% BEP rule

POR/AOR depend on pump type, specific speed, design and application. A generic colored “safe zone” should not replace the actual curve and current operating-region guidance.

For the dedicated operating-region and suction-side discussion, see the NPSHa, NPSHr, POR, AOR & BEP selection guide.

Efficiency and absorbed power

Read Efficiency and Power at the Same Duty Point

After locating the operating point, read the efficiency contour and power information at that same flow/head condition. Specific gravity affects required pump input power even when the hydraulic head remains the same.

\[\mathrm{BHP}=\frac{Q\,(\mathrm{gpm})H\,(\mathrm{ft})SG}{3960\,\eta_p}\]

BHP = pump shaft/input horsepower; η_p = pump efficiency as a decimal.

\[P\,(\mathrm{kW})\approx\frac{Q\,(\mathrm{m^3/h})H\,(\mathrm{m})SG}{367\,\eta_p}\]

This is pump input/shaft power, not the motor electrical input at the supply.

Use the Pump Power & Motor Sizing Calculator for a transparent power check, then confirm motor selection against the actual pump curve, maximum absorbed power, starting/control method and project requirements.

Suction-side check

Read NPSHr From the Pump Curve; Calculate NPSHa From the System

NPSHrPump-side requirement read from the applicable pump curve at the actual flow and speed.

NPSHaSystem-side available suction head based on pressure, liquid level, temperature/vapor pressure and suction losses.

MarginApplication-specific; do not replace the review with one universal +3 ft or fixed percentage rule.

Use the NPSHa Calculator to audit the system-side terms. For field symptoms, see the centrifugal-pump cavitation field checklist.

Impeller diameter

Use Affinity Relationships as an Estimate—Not as the Final Trim Curve

\[\begin{aligned}Q_2/Q_1&\approx D_2/D_1\\H_2/H_1&\approx(D_2/D_1)^2\\P_2/P_1&\approx(D_2/D_1)^3\end{aligned}\]

D = impeller diameter. Diameter-based affinity relationships are screening estimates and have application limits.

Final trim rule: use affinity relationships to estimate where a trim may land, then confirm against the actual manufacturer/replacement trim curve or validated hydraulic data before machining or ordering.

Speed change / VFD

Changing Speed Reshapes the Pump Curve

\[\begin{aligned}Q_2/Q_1&\approx N_2/N_1\\H_2/H_1&\approx(N_2/N_1)^2\\P_2/P_1&\approx(N_2/N_1)^3\end{aligned}\]

N = rotational speed. These relationships assume the same pump geometry and an appropriate affinity-law application.

Speed ratio	Approx. reference flow	Approx. reference head	Approx. reference power
100%	100%	100%	100%
90%	90%	81%	73%
80%	80%	64%	51%

Normalized Speed Curves

The curve moves left/down as speed is reduced. The actual duty still depends on the system curve.

Normalized centrifugal pump curves at 100 percent 90 percent and 80 percent speed for VFD affinity-law explanation

Final operating point = changed pump curve × actual system behavior.

For quick screening, use the Pump Affinity Laws & VFD Calculator.

Liquid properties

Specific Gravity Changes Power; Viscosity Can Change the Curve Itself

Property	What it changes	Engineering consequence
Specific gravity / density	Pressure corresponding to a given head and required power	Do not size power from water horsepower without applying the actual SG/density.
Viscosity	Can change flow, head, efficiency and required power relative to water performance	A viscous-liquid review is not the same as multiplying a water curve by SG.

Viscosity boundary: do not use “water curve × SG” as a complete viscous-liquid correction. Use the applicable viscosity-correction method and then re-check the operating point.

PDF-backed walkthrough

Worked Example: Read an Actual 2×3-8 Curve at 1750 RPM

The approved performance-curve PDF contains a 2×3-8 curve at 1750 RPM with multiple impeller trims, efficiency contours, horsepower contours and NPSHr contours. The figure below is extracted from that approved PDF at high resolution rather than recreated by AI.

Reference Duty Walkthrough

Illustrative duty: 160 GPM at 55 ft. The point falls between adjacent impeller-trim curves, which is why the full trim family must be read before a trim decision.

Actual 2 by 3 by 8 centrifugal pump curve at 1750 RPM annotated with an illustrative 160 GPM 55 ft duty point

Source-backed visual. Use the full-resolution approved performance-curve PDF for final numerical reading.

Identity

Confirm size & RPM

2×3-8 at 1750 RPM in the approved curve set.

Duty

Plot Q and H

Use the required flow and total head on the same axes.

Trim

Read adjacent trims

If the point lies between trim curves, treat interpolation as a screening step—not a machining instruction.

Efficiency

Read η at duty

Use the efficiency contour crossing or nearest the operating point.

Power

Check HP/kW

Read the power contour and cross-check the formula using actual SG.

Suction

Read NPSHr

Read NPSHr at the same flow/speed; compare it with system NPSHa and the required margin.

For additional sizes and curves, use the pump performance curves library. The curve—not an approximate affinity calculation—should remain the primary hydraulic reference for the selected configuration.

Installed-pump reality check

Validate the Curve With Field Measurements

When an installed pump no longer behaves as expected, compare measured operating data with the correct curve before ordering parts or increasing motor size.

FlowMeasured or reliably inferred operating flow

Suction pressureNear-pump pressure with pressure basis and gauge location documented

Discharge pressureMeasured at a known location and stable operating condition

SpeedActual RPM / VFD frequency

LiquidTemperature, SG/density and viscosity

DriverMotor current/input, control mode and nameplate data

ConditionVibration, bearing temperature, valve position and observed symptoms

\[H\,(\mathrm{ft})=\frac{\Delta P\,(\mathrm{psi})\,2.31}{SG}\]

Depending on pressure-tap locations, total pump head may also require elevation and velocity-head corrections.

\[H\,(\mathrm{m})\approx\frac{\Delta P\,(\mathrm{bar})\,10.2}{SG}\]

Use the full system/head relationship rather than treating differential pressure alone as universal TDH.

Use the Total Dynamic Head Calculator when elevation and line losses must be included in the duty reconstruction.

Calculated Example: Turn Pressure Readings Into a Curve Point

Assume a pump handling water at 20 °C at a fixed speed of 2,900 rpm. The readings and curve values below are assumed teaching inputs for a complete calculation sequence.

Input	Value and reference
Flow	100 m³/h (0.027778 m³/s)
Suction / discharge pressure	0.30 / 2.80 barg; both gauge readings use the same reference
Internal diameter at the taps	100 mm suction; 80 mm discharge
Tap elevation	Discharge tap is 0.40 m above the suction tap
Properties and gravity	ρ = 998 kg/m³; g = 9.80665 m/s²
NPSHa basis	Atmospheric pressure 101,325 Pa; vapor pressure 2,340 Pa; NPSH datum at the suction tap
Illustrative curve readings	Pump efficiency 72%; NPSHr 3.2 m at the evaluated duty

1. Convert the readings to total pump head

Total head between pressure taps

\[ \begin{aligned}H&=\frac{p_d-p_s}{\rho g}\\&\quad+(z_d-z_s)\\&\quad+\frac{v_d^2-v_s^2}{2g}\\v&=\frac{Q}{A}\\H&=25.54+0.40+0.92\\&=26.86\,\mathrm{m}\end{aligned} \]

p: Pa; ρ: kg/m³; g: m/s²; z and H: m; v: m/s; Q: m³/s; A: m². d: discharge; s: suction.

Steady incompressible liquid flow. Pressure taps correspond to the stated reference sections.

The stated internal diameters give v_s = 3.537 m/s and v_d = 5.526 m/s. Total head is 26.86 m. Pressure difference contributes 25.54 m; elevation and velocity supply the remaining terms. The taps are assumed to be at the pump reference sections.

2. Plot the duty and read power and suction data

Plot Q = 100 m³/h and H = 26.86 m on the curve for the documented impeller at 2,900 rpm. With the assumed 72% efficiency, hydraulic power is 7.30 kW and pump shaft power is 10.14 kW.

Hydraulic, shaft and electrical power

\[ \begin{aligned}P_{\rm hyd}&=\frac{\rho gQH}{1000}\\P_{\rm shaft}&=\frac{P_{\rm hyd}}{\eta_p}\\P_{\rm electric}&=\frac{P_{\rm shaft}}{\eta_m}\end{aligned} \]

P: kW; Q: m³/s; H: m; ρ: kg/m³. ηp: pump efficiency; ηm: motor efficiency, expressed as fractions.

The final relationship applies to this example without a VFD. Include drive losses when applicable.

At an assumed motor efficiency of 92%, electrical input would be 11.03 kW for this example without a drive. Include VFD losses when applicable.

NPSHa at the suction datum

\[ \begin{aligned}p_{s,\rm abs}&=p_{\rm atm}+p_{s,\rm gauge}\\\mathrm{NPSH}_A&=\frac{p_{s,\rm abs}-p_v}{\rho g}\\&\quad+\frac{v_s^2}{2g}\end{aligned} \]

Pressures: Pa; pv: vapor pressure; vs: mean velocity at the suction tap; NPSHa: m.

The example NPSH datum is at the suction-tap elevation. Include the elevation correction for another datum and review the required margin.

With the assumed NPSH datum at the suction tap, NPSHa is 13.82 m. Compare it with the 3.2 m curve requirement at the same flow and the manufacturer/project margin. Include the appropriate elevation term when the datum differs.

Calculated teaching diagram showing pressure readings, total pump head, the operating point and shaft power. — Original calculated teaching example with assumed readings and curves; not commercial model performance.

3. Use any deviation to choose the next check

A measured point on the applicable pump curve directs the next review toward system resistance and process demand when flow is insufficient. A point below that curve at the same speed calls for checks of instruments, rotation, impeller configuration, liquid properties and internal condition. Match the configuration and process conditions before comparing points.

Keep the required system head calculation distinct from head developed between the pump taps. Cross-check shaft power and use the NPSHa calculator for the suction-side calculation.

Request a duty-point review with the curve, RPM, impeller diameter, flow and pressure-tap locations.

Technical basis: Hydraulic Institute: System Curves; KSB: Head; KSB: Power input, pump efficiency and power measurement; KSB: NPSH.

Replacement boundary

A Matching Hydraulic Curve Does Not Prove a Drop-In Replacement

Verification layer	What still has to be checked
Hydraulic	Q-H, efficiency, power, NPSHr and expected operating region
Dimensional	Nozzles, mounting, shaft interface, baseplate/foundation relationship
Mechanical	Impeller/shaft interfaces, seal chamber, coupling/rotation and configuration
Process	Materials, viscosity, temperature, solids and seal arrangement

For a replacement review, see the Goulds 3196 replacement and Durco Mark III replacement pages after the hydraulic duty has been confirmed.

Avoid false conclusions

Common Pump-Curve Reading Errors

Using shutoff head as duty headDuty head must be read at the actual flow.

Wrong RPMA curve at another speed can lead to a completely different duty conclusion.

Wrong impeller trimRead the installed/proposed trim, not the maximum diameter by default.

Ignoring the system curveThe pump curve alone does not define where the system will run.

Universal POR percentageDo not assume every pump has a 70–120% BEP preferred region.

Fixed NPSH marginDo not apply one +3 ft or one percentage margin to every service.

NPSHr at the wrong flowRead suction requirement at the actual operating condition.

Ignoring SG in powerDensity matters when checking absorbed power.

Ignoring viscosityViscous-liquid performance may not follow the water curve directly.

Overusing affinity lawsThey are engineering estimates, not a substitute for a real trim curve.

Curve match = interchangeabilityHydraulic matching does not prove mechanical or dimensional fit.

Curve review inputs

What to Send for a Pump-Curve Review

Existing pumpModel, size and frame/group if known

CurvePDF, screenshot or photo of the applicable performance curve

ImpellerInstalled diameter / trim if known

SpeedRPM / frequency / VFD setting

Flow rangeMinimum, normal and maximum

HeadTDH or the pressure/elevation/loss data needed to reconstruct it

Suction conditionTank level, suction pressure, temperature and suction-line losses

Liquid propertiesSG/density, viscosity, temperature and solids where applicable

MotorPower, speed and electrical/control data

SymptomsLow flow, vibration, cavitation signs, overheating or overload

Start with what you have.

A curve screenshot plus actual flow, pressure, speed and liquid properties is often enough to identify the next engineering check.

Frequently asked questions

Centrifugal Pump Curve FAQ

What should I check first on a centrifugal pump curve?

Confirm the pump size, speed, impeller diameter, units and liquid/test basis, then plot the required flow and head on the correct curve.

What is the difference between BEP, POR and AOR?

BEP is one best-efficiency point at a stated pump configuration. POR and AOR are operating regions. Their limits depend on the pump design, specific speed, manufacturer data and application rather than one universal percentage.

Does specific gravity change the pump head curve?

Not by simply multiplying the head curve by SG under comparable low-viscosity conditions. Specific gravity changes the corresponding pressure and the power required to deliver the same flow and head.

How much NPSH margin should I use?

There is no single universal margin for every pump and service. Compare system NPSHa with pump-specific NPSHr at the actual duty, then use the applicable manufacturer, project and ANSI/HI guidance for the required margin.

Does a matching pump curve prove a replacement pump is interchangeable?

No. Hydraulic matching is only one layer. Dimensional interfaces, mechanical configuration, materials, seal arrangement and the actual operating range still require verification.

How do I convert suction and discharge pressure readings to pump head?

Use the pressure difference divided by density and gravity, then add the elevation and velocity-head differences between the defined measurement sections. Record pressure reference, internal diameters and tap locations.

Technical sources

How to Read a Centrifugal Pump Curve: BEP, Power, NPSH & Operating Point