Air Ducts are passages for air to travel within your home. The loss in pressure of the flow that occurs due to the air viscosity in the surface of the duct is termed as friction loss. You can use this online calculator to find the friction loss in ducts using flow volume, air passage length and diameter.
A 4 feet air duct with diameter 2 inch and air flow volume 156 cfm has a friction loss of
= (0.109136 * 1561.9 ) / (25.02)
= 1.97603 inches.
= (0.109136 * 1561.9 ) / (25.02)*(4/100)
= 49.40069 inches / 100 feet.