The  figures and Poiseuille's Law material below, courtesy of Dr. Rod Nave, Georgia State University

Poiseuille's law can be used to calculate volume flowrate only in the case of laminar flow. Any of the parameters below can be changed. When you have finished entering data, click on the quantity you wish to calculate in the formula above. The different parameters will not be forced to be consistent until you click on the quantity to calculate. For pressure difference: x 10^ Pa = kPa = atmos = mmHg = lb/in^2 = cm water = inches water applied to a tube of radius = cm = inches (corresponding to area =cm^2 = in^2) and length = cm = inches, for a fluid with viscosity = poise = x the viscosity of water, the volume flowrate will be = cm^3/s =in^3/s=liters/min=ft^3/min= U.S. gal/min. Default values will be assigned to parameters to which you have not given values. You can change any of those default values as part of your exploration.