The determination of viscosity is based on Poiseuille's law:
where t is the time it takes for a volume V to elute. The ratio  depends on R as the capillary radius, on the average applied pressure P, on its length L and on the dynamic viscosity η.
The average pressure head is given by:
with ρ the density of the liquid, g the Standard gravity and H the average head of the liquid.  In this way the viscosity of a fluid can be determined.
Usually the viscosity of a liquid is compared to a liquid with an analyte for example a polymer dissolved in it. The relative viscosity is given by:
where t and ρ are the elution time and density of the pure liquid. When the solution is very diluted 
the so-called specific viscosity becomes:
This specific viscosity is related to the concentration of the analyte through the Intrinsic viscosity [η] by the power series:
or 
where  is called the viscosity number.
The intrinsic viscosity can be determined experimentally by measuring the viscosity number as function of concentration as the Y-axis intercept.