Flow rate (L/min) vs pressure (kPa): how to read the performance curve?
When you look at a pump’s datasheet, you will see a diagram with a curved line – this is the performance curve (or Q‑H curve). It is the single most important tool for selecting the right pump. Ignore it, and you risk buying a pump that delivers far less flow than you need, or one that wastes energy.
This guide explains what the curve shows, how to read it, and how to use it to choose a pump that works in your real system.
1. What does a performance curve tell you?
A performance curve shows the relationship between flow rate (usually on the horizontal axis, in L/min) and pressure (on the vertical axis, in kPa, bar, or metres of head).
The curve slopes downward: as pressure increases, flow decreases. At zero pressure (outlet fully open), flow is at its maximum – this is often called “open flow” or “free flow”. At zero flow (outlet completely blocked), pressure is at its maximum – this is the “shut‑off pressure”.
Point on curve
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Flow
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Pressure
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Real‑world meaning
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Far left
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0 L/min
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Maximum pressure
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Pump dead‑headed against a closed valve
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Somewhere in the middle
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Moderate flow
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Moderate pressure
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Normal operating rang
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Far right
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Maximum flow
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0 kPa
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Pump outlet open, no resistance
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No pump can deliver both its maximum flow and its maximum pressure at the same time.
2. How to read a typical curve step by step
Let us use an example: a small diaphragm pump with the following points on its curve:
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At 0 kPa: flow = 5.0 L/min
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At 100 kPa: flow = 3.8 L/min
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At 200 kPa: flow = 2.5 L/min
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At 300 kPa: flow = 1.0 L/min
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At 400 kPa: flow = 0 L/min (shut‑off)
Step 1 – Find your system’s required pressure
Measure or calculate the total resistance your pump must overcome: pipe friction, vertical lift, filter back pressure, nozzle restriction, etc. Suppose your system needs 200 kPa.
Step 2 – Locate that pressure on the vertical axis
Go up to 200 kPa.
Step 3 – Read horizontally to the curve
From 200 kPa, move right until you hit the curve line.
Step 4 – Drop down to the horizontal axis
From that point on the curve, go straight down to read the flow. In our example, at 200 kPa, the flow is 2.5 L/min.
Step 5 – Compare with your required flow
If you need at least 2.5 L/min at 200 kPa, this pump works. If you need 3.0 L/min at 200 kPa, this pump is too weak – choose a pump with a higher curve.
3. Why you cannot rely only on “maximum flow”
Many buyers see “max flow 5 L/min” and assume the pump will deliver 5 L/min in their system. That is almost never true, because real systems always have some resistance.
Example: You need 2 L/min at 300 kPa. A pump with an open flow of 5 L/min might only deliver 1 L/min at 300 kPa – far below your need. Without the curve, you would never know.
Always ask for the performance curve and check the flow at your actual operating pressure.
4. How system resistance affects the operating point
Your system is not just an open pipe. Every component adds resistance:
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Long or narrow hoses
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Filters (clean or dirty)
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Valves (solenoid, check, manual)
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Nozzles or spray tips
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Vertical lift (pumping upward)
The total resistance at your desired flow is called the system curve. Where the system curve crosses the pump’s performance curve is the actual operating point – the flow and pressure the pump will deliver in your system.
In practice, most users do not draw a system curve. Instead, they estimate the required pressure and check the pump’s flow at that pressure. That is usually sufficient for selecting a micro pump.
5. Common mistakes when reading performance curves
Mistake
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Why it is wrong
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Looking only at the far‑right point (max flow)
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That flow is only at zero pressure; real systems have pressure.
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Looking only at the far‑left point (max pressure)
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That pressure is only at zero flow; you need flow, not just pressure.
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Assuming the curve is the same for all voltages
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A pump running at lower voltage has a lower, flatter curve.
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Ignoring viscosity
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The curve is for water (1 cP). For thicker liquids, both flow and pressure drop.
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Thinking a higher curve is always better
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A higher curve consumes more power. Choose what you actually need.
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6. Pressure units conversion quick reference
Pump curves may use different units. Here is how they relate:
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1 kPa = 0.01 bar
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100 kPa = 1 bar
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10 m of head ≈ 100 kPa ≈ 1 bar
For example:
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200 kPa = 2 bar ≈ 20 m head
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500 kPa = 5 bar ≈ 50 m head
Always check which unit the curve uses before reading.
7. Example: selecting a pump for a coffee machine
Requirement : 1 L/min at 900 kPa (9 bar).
Candidates :
Pump A: open flow 3 L/min, but at 900 kPa the curve shows 0.5 L/min → too weak.
Pump B: open flow 4 L/min, curve shows 1.2 L/min at 900 kPa → suitable.
Pump C: open flow 6 L/min, curve shows 1.0 L/min at 900 kPa → also suitable but consumes more power.
Choose Pump B – it meets the need with less energy waste.
8. How to get the performance curve from a supplier
A reputable pump supplier will always provide a performance curve. If they do not, treat it as a red flag.
When you request a curve, ask for:
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Flow units (L/min, GPM, etc.)
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Pressure units (kPa, bar, psi, m head)
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Test conditions (fluid temperature, viscosity – normally water at 20°C)
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Voltage at which the curve was measured (DC pumps may have curves for different voltages)
With this information, you can confidently match a pump to your application.
9. Conclusion
The performance curve is your map to selecting the right pump. It tells you the real flow you will get at the real pressure your system demands.
Read the curve at your required pressure.
Compare that flow to your need.
Choose a pump whose curve is above your operating point.
Ignore the curve, and you risk buying a pump that looks powerful on paper but fails in practice. Learn to read it, and you will never be disappointed.
Sammy