Flow and head: how to size a pump

Every pump is chosen from two quantities: flow and head. Together they define the "duty point" — where the pump must operate on its curve.

Flow (Q)

The volume of liquid moved over time, in m³/h or l/min. It answers: how much water do I need?

Head (H)

The height the pump can lift the liquid to, in metres. It is not just the geometric lift: it also includes pipe friction losses and the pressure required at the outlet. It answers: with how much force?

The curve and the duty point

Every pump has a Q-H curve: as flow rises, head falls. The duty point is where the pump curve meets the system curve. A well-chosen pump works near its best efficiency point (BEP), not at the extremes.

Example

Need 5 m³/h at 30 m head? On our selector enter Q=5 and H=30: we show the Grundfos pumps that cover that point, with your duty point marked on each curve.