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:
Head Loss (inches water gauge per 100 feet duct)  

Air velocity (ft/min)  
Air Volume (cfm)  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).
Related Topics

Ventilation
Systems for ventilation and air handling  air change rates, ducts and pressure drops, charts and diagrams and more.
Related Documents

Air Duct Velocities
Typical air duct velocities in applications like ventilation systems or compressed air systems. 
Air Ducts  Friction Loss Diagram
A major friction loss diagram for air ducts  Imperial units ranging 10  100 000 cfm. 
Air Ducts  Friction Loss Diagram
A major friction loss diagram for air ducts  SI units. 
Air Ducts  Velocity Diagram
Air flow volume, duct size, velocity and dynamic pressure. 
Duct Sizing  the Equal Friction Method
The equal friction method for sizing air ducts is easy and straightforward to use. 
Duct Systems  Pressure Classifications
Duct systems are commonly divided into three pressure classifications. 
Duct Velocity
Calculate velocities in circular and rectangular ducts  imperial and SIunits  online calculator. 
Ducts Sizing  the Velocity Reduction Method
The velocity reduction method can be used when sizing air ducts. 
Ductwork  Seal Classes
Ductwork air leakage classes. 
Equivalent Diameter  Rectangular vs. Circular HVAC Ducts
Rectangular equivalent diameter for air flows between 100  50000 cfm. 
Fluid Flow  Equivalent Diameter
Convert rectangular and oval duct geometry to a equivalent circular diameter  online calculator with imperial and SIunits. 
Fluid Flow  Hydraulic Diameter
Calculate hydraulic diameter for pipes and ducts. 
Sizing Circular Ducts
A rough guide to maximum air volume capacity of circular ducts in comfort, industrial and high speed ventilation systems. 
Spiral Ducts  Dimensions
Standard dimensions of spiral ducts  Imperial units. 
Velocity Classification of Ventilation Ducts
Recommended air velocities in ventilation ducts