Compare pump head at the flow you actually need
Choosing a replacement from maximum flow and maximum head alone can leave a home or small shop without enough pressure when several outlets run together.
This calculator organizes manufacturer coordinates and site requirements into a common set of units for a discussion with your installer.
It compares the head available at a particular demand flow with the head required at that same flow.
The scope is one pump supplying clean water from an atmospheric open source.
Enter the source-to-outlet elevation, terminal pressure and verified pipe losses yourself.
Missing evidence is not replaced by an average, and performance outside the entered curve is not estimated.
The result is a purchase-review worksheet, not final design approval or a product recommendation.
What the comparison provides
Requirements by scenario
Add up to three concurrent-use scenarios, each with its own flow and verified loss head.
Elevation and terminal pressure are shared across scenarios.
Give scenarios names that distinguish normal use from simultaneous demand.
Head by pump candidate
Enter manufacturer coordinates for up to three pumps.
The calculator interpolates available head and reports the difference in metres and kilopascals.
Missing evidence and flow outside the supplied range have separate pending reasons.
Colour and line pattern identify pump candidates; stars identify demand points.
The chart always uses L/min on its horizontal axis, regardless of the original input units.
The table provides required head, available head, differences and pending reasons without relying on the graphic.
Candidates stay in input order; the largest head surplus is not ranked as the best purchase.
Define elevation, pressure and loss boundaries
Measure elevation from the source surface
Measuring only from the pump floor to the outlet can omit the suction-side lift.
Use the lowest expected source water level and the outlet elevation.
A higher outlet is positive; a lower outlet is negative.
For changing tank or well levels, agree on the adverse level with the installer and save separate reports when changing the shared elevation.
Use terminal pressure under flow
Required gauge pressure belongs at the fixture or device, not at the pump discharge nozzle.
A closed-valve static reading is different from pressure during simultaneous use.
The source is assumed to be open to the atmosphere, so existing mains pressure is not added or subtracted.
If your requirement is already expressed as pressure head, select the m option.
Verify losses at each flow
Include relevant pipes, elbows, valves, check valves, filters and devices.
Pipe length alone is not used to invent losses, diameter, roughness or friction factors.
The velocity-head difference must be negligible or included in verified additional head.
Do not enter a loss again if it is already included in the pressure requirement; align the boundaries of the source calculation first.
Conversions and required head
Simplified clean-water energy balance
Hp = p / (ρ × g)
Hreq = Δz + Hp + Hloss
ρ = 1000 kg/m³ · g = 9.80665 m/s²
Density is a declared approximation for this tool, not the measured density of water at every temperature.
Do not apply it to hot, viscous or wastewater fluids.
For jets or other installations with material outlet velocity head, first reconcile the required head from a professional calculation with this input model.
Pressure and flow equivalents under the clean-water assumptions| Quantity | Equivalent input | Converted value |
|---|
| Pressure | 1 bar = 100 kPa = 0.1 MPa | About 10.197 m |
| Flow | 2.4 m³/h | 40 L/min |
| Flow | 1 L/s | 60 L/min |
Changing a unit converts valid existing values to preserve the physical quantity.
Blank inputs stay blank; a converted value outside the supported input range triggers a range error.
Do not paste pressure-axis values directly into the curve’s head fields, which accept metres only.
Obtain head coordinates or verify the units and fluid assumptions before converting the source graph.
Transcribe curves and interpolate between coordinates
Hpump(Q) = H1 + (H2 − H1) × (Q − Q1) / (Q2 − Q1)
The two coordinates bracketing demand flow are joined with a straight line.
Unordered input is sorted by flow, but duplicate flows put that candidate on hold because there is no unique coordinate sequence.
Add points near pronounced bends to preserve the original shape.
Each candidate accepts 2–12 coordinates.
Maximum head may belong near zero flow, so never combine maximum head and maximum flow into one point.
Do not mix different frequencies, speeds, impeller diameters or control settings on one curve.
Record the model, document revision date or version, and operating conditions together.
A rising segment may be real, so the tool warns and still interpolates it.
Check for transcription mistakes and ask the manufacturer about operating stability.
Exact range endpoints are included, but the line is never extended to estimate an out-of-range flow.
Graph-reading uncertainty and manufacturing tolerances are not automatically corrected.
Step-by-step workflow
- List concurrent fixtures and demand flow, then confirm source level and terminal pressure for the actual combination of uses.
- Obtain a loss calculation or measurement basis at each flow; leave unknown losses blank and save a worksheet to request the missing evidence.
- Transcribe coordinates around demand flow from each manufacturer curve; different frequencies or impellers should be separate candidates.
- Record the source, revision and settings; check only items you have verified, and review converted values when switching units.
- Review shortfalls, surplus and pending reasons; save TXT or print output and send it with original curves and site evidence.
The fictional example fills values and hypothetical confirmation boxes.
Its warning remains after edits so example assumptions are not mistaken for certified data.
Use Reset to begin a fresh real-data entry.
Remove unused empty scenarios or candidates; complete candidates can still be compared independently.
Worked example: two pumps against 30 m
Fictional coordinates, not product advice
Assume 40 L/min concurrent demand, 10 m elevation, 15 m terminal pressure head and 5 m verified loss at that flow.
Required head is 10 + 15 + 5 = 30 m.
Candidate A uses (20, 31) and (60, 23); candidate B uses (20, 38) and (60, 30), with flow in L/min and head in m.
Candidate A: 3 m shortfall
Interpolated head is 27 m at the middle flow.
27 − 30 = −3 m, equivalent to −29.420 kPa.
Comparing maximum head alone could miss this shortfall.
Candidate B: 4 m surplus
Available head at the same flow is 34 m.
34 − 30 = 4 m, equivalent to 39.227 kPa.
This is head surplus at the specified point, not proof of efficiency or control suitability.
For the second scenario, use 60 L/min and a separately verified loss of 8 m: required head becomes 33 m.
A supplies 23 m and is 10 m short; B supplies 30 m and is 3 m short.
The third scenario at 80 L/min lies outside both entered ranges, so available head and margin remain unknown.
Do not generalize a positive result from one demand scenario to every simultaneous-use condition.
Interpret shortfalls, surplus and pending results
- Head shortfall: interpolated head is below the requirement at demand flow; review piping changes or another candidate.
- Equal head: no additional margin remains; equality does not cover reading error, fouling or changing source levels.
- Head surplus: available head exceeds the requirement; oversizing, control and efficiency still need separate review.
- Pending: evidence is incomplete or flow is outside the coordinate range; blanks do not become zero or a positive verdict.
A nonpositive required head is routed to separate gravity-supply and pressure-control review.
It is not a final declaration that no pump is needed.
Only machine-representation noise is normalized at equality; a small real shortfall is not turned into equality through display rounding.
Very small nonzero differences use exponential notation to retain their sign and status.
Use the worksheet in replacement and irrigation planning
Homes and small shops
Save separate normal and peak-use conditions with the correct elevation difference.
For filter changes, pipe resizing or lower source levels, obtain revised losses before recalculating.
This makes changed requirements visible instead of simply reordering the old model.
Irrigation and storage tanks
Confirm emitter pressure and zone flow before comparing pumps that could supply it.
The garden irrigation calculator divides known supply capacity into zones, which is a different task.
If fewer zones require a larger flow, verify the new losses instead of copying the previous value.
Tell the installer where elevation was measured, when flowing pressure was checked and what filter condition was assumed.
If pressure and loss values came from different documents, ask whether their boundaries overlap.
These records support a purchase comparison but do not replace performance guarantees in an installation contract.
Frequently asked questions
Can I buy the pump if there is a head surplus?
Head comparison alone is insufficient.
Review the actual duty point, minimum continuous flow, efficiency, motor, controls and suction conditions with the manufacturer and installer.
Are maximum flow and maximum head enough?
You need the corresponding flow and head coordinates.
Do not combine the two maxima into a single point; at least two actual curve coordinates are required.
Why is an out-of-range flow not calculated?
There is no evidence that the entered slope continues outside the supplied range.
Request original manufacturer data covering that flow instead of extrapolating.
Can unknown pipe loss be entered as zero?
Unknown and verified zero are different.
Use zero only when justified; otherwise keep the field blank and retain the pending status.
Is a rising curve segment always an error?
It can be a real characteristic and is interpolated with a warning.
Check transcription and ask the manufacturer about unstable operation.
Can I include existing mains pressure at the source?
The model assumes an atmospheric open source with zero gauge pressure.
Pressurized sources and closed loops need different pressure boundaries and separate review.
Is the star on the chart the actual duty point?
A star is the requested flow and required head.
The actual duty point is the pump/system curve intersection, which this tool does not determine.
How can I share the input evidence?
TXT includes coordinates, units, sources, revisions, confirmation states, pending reasons and example provenance.
Print can save a PDF; attach original curves separately.
Technical sources and limits
Technical sources were checked on 2026-10-01.
References cover Grundfos curve interpretation, KSB’s head concepts and NIST conversions.
The density approximation, linear interpolation and input limits are tool assumptions, not manufacturer certification criteria.
No statutory design-year rules or pump inspection pass criteria are applied.
NPSH, cavitation, water hammer, efficiency, power consumption and series/parallel operation are excluded.
Do not use this worksheet to approve fire systems, wastewater or hot-fluid installations.
Recheck each candidate’s current curve revision and operating settings, and consult its manufacturer about the permitted operating range.
Prepare the worksheet and original curves together
Resolve pending items by obtaining the loss calculation, current performance curves and installation conditions.
Compare again with complete evidence, record which demands produce a shortfall, and request actual duty-point and selection review.
Saving a report only downloads a file; it does not contact a supplier or create a contract.
Return to pump curve inputs