Reading a pump curve
The one chart every pump comes with: more flow means less head.
⏱ 5 min
- Everyday comparisonStep 1 of 5
Your thumb on the hose
Put your thumb over the end of a garden hose. Less water comes out, but it shoots much further. Take your thumb away: lots of water, but it just flops out.
A pump behaves the same way: it can give a lot of flow at low head, or a little flow at high head — never both at once.
- Try itStep 2 of 5
Open the valve
This chart is a . Flow is along the bottom, head up the side. The orange dot shows where the pump is working.
Open and close the outlet valve and watch the dot slide along the curve.
Challenge: Close the valve completely. What is the head now? Then open it fully: what happens to the head?
- ExplainedStep 3 of 5
The top of the curve: shut-off
With the valve closed, nothing flows and the pump makes its highest head: the . On this pump it is 40 m.
Don’t leave a pump running like this for long. All the motor’s energy turns into heat in the trapped water, which can boil and damage the pump.
- ExplainedStep 4 of 5
The right end: lots of flow, little push
Open the valve wide and the flow climbs while the head falls. Far to the right, the pump moves lots of water but can barely lift it.
Running too far right is also bad: the water rushes through, pressure at the eye drops, and cavitation (end of this module) can begin.
- The impeller connectionStep 5 of 5
Every pump has its own curve
Pump makers test each model and publish its curve. Engineers put the curves of different pumps side by side to choose the one that fits the job.
A bigger impeller or a faster motor moves the whole curve up and to the right.
📌 Remember this
A pump curve shows the head a pump gives at every flow rate. With the valve closed you get the most head and no flow; as flow increases, the head goes down.