The major loss, or friction loss, in a circular duct in galvanized steel with turbulent flow can for imperial units be expressed. Δh = 0.109136 q 1.9 / d e 5.02 (1). where. Δh = friction or head loss (inches water gauge/100 ft of duct). d e = equivalent duct diameter (inches). q = air volume flow - (cfm - cubic feet per minute). For rectangular ducts the equivalent diameter must be calculated.