Air Ducts  Major Friction Head Loss, Online Calculator
Head loss or major loss in air ducts  equations and online calculator for rectangular and circular ducts  Imperial and SI units.
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.
Air Ducts Friction Loss Calculator  Imperial Units
The head loss calculator below is based on formula (1) :
Head loss and air flow velocity for some common duct sizes and air flow volumes can be taken from the table below:
Air Volume (cfm)  Head Loss (inches water gauge per 100 feet duct)  

Air velocity (ft/min)  
Duct Size (inches)  
4  5  6  8  10  12  16  
100  0.65  0.21  0.09  0.02  0.01  
1146  733  509  286  183  
200  0.8  0.32  0.08  0.02  0.01  
1467  1019  573  367  255  
400  1.19  0.28  0.09  0.04  0.01  
2037  1146  733  509  286  
800  0.34  0.14  0.03  
1467  1019  573  
1600  0.12  
1146 
Air velocity should not exceed certain limits to avoid unacceptable noise generation.
Air Ducts Friction Loss Calculator  SI Units
This pressure loss calculator is based on formula (1) with input and output values modified for SI units.
Air Ducts  Velocity and Friction Loss  Excel Template
This Excel template can be used to calculate velocity and friction loss in air ducts.
Air Ducts  Friction Loss Diagram
The chart below can be used to estimate friction loss in air ducts.
The default values are for air flow 400 cfm (680 m^{3} /h) , duct size 8 in (200 mm) and friction loss 0.28 inH2O/100ft (2.3 Pa/m) .
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