Pump Flow Rate and Head: Meaning and Units

Understand pump flow rate Q and head H, their units, and why delivery head differs from pressure and the elevation of a pumping system.

Pump flow rate describes the volume of liquid delivered per unit time. Pump head describes the energy transferred to the liquid per unit weight. Flow rate answers how much liquid is delivered; head describes the energy increase associated with that delivery. They are different quantities. KSB volume-flow definition · Grundfos delivery-head definition

Flow rate: volume per unit time

The usual symbol is Q. For a centrifugal pump, KSB defines useful delivery at the pump outlet and distinguishes it from inlet flow, leakage and flows taken off for other purposes. The location and meaning of the reported flow therefore matter. KSB pump flow rate

Common units include cubic metres per second (m³/s), cubic metres per hour (m³/h) and litres per second (L/s). These express volume divided by time; they do not express mass per time or the speed of the liquid. KSB flow-rate units

Head: energy per unit weight

The usual symbol is H, commonly expressed in metres (m). The length unit represents energy per unit weight; it does not mean the pump itself has that height. KSB distinguishes the head transferred by the pump from the head required by the complete system. KSB head definition and units

Quantity	Symbol	Example unit
Volume flow rate	Q	m³/h
Pump head	H	m

Head, pressure and elevation

Head and pressure are related, but their reported values are not interchangeable. A source may explain head through an equivalent liquid-column height or simplify it to a pressure difference. That explanation does not make a head value in metres a pressure value in bar. This page does not provide a pressure-conversion rule. Grundfos explanation · KSB energy basis

The geodetic head is the elevation difference between the system’s outlet and inlet sections. It is only one system quantity. Flow through pipes, bends and valves also involves friction losses, so the required system head cannot generally be reduced to vertical height alone. KSB geodetic-head distinction · Grundfos friction losses

Reading the two quantities together

Pump literature often presents head and flow together as a QH curve. A quoted head therefore needs its corresponding flow context. KSB also distinguishes nominal flow from flow at the best-efficiency point and permissible operating limits; the labels do not all mean the same thing. Grundfos QH curve · KSB named flow rates

For the components that transfer and contain this energy, see pump casing and impeller. These definitions do not establish a duty point, a safe operating range or a suitable pump for a particular installation.