The rate at which liquid flows through a pipe – actually the volume flow rate may depend on many factor. The most obvious are:

The viscosityof the liquid.

The cross sectionand lengthof the tube. We can simplify the analysis by assuming the pipe has a circular cross section of radiusas shown below.

The pressure differencebetween the ends of the tube.
An expression for the volume flow rate may be deduced by dimensional analysis. We assume thatwhereis called the pressure gradient and k is a dimensionless constant.
The dimensions of each quantity are:
hence
Equating powers for each of
Solving these three equations simultaneously givesandhence
can be shown to behence