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 q1.9 / de5.02 (1)
Δh = friction or head loss (inches water gauge/100 ft of duct)
de = 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):
Air Volume Flow - q - (cfm)
Equivalent duct diameter - de - (inches)
Duct Length - (feet)
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 |
|Duct Size (inches)|
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 Volume Flow - q - (m3/s)
Equivalent duct diameter - de - (m)
Duct Length - (m)
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 m3/h), duct size 8 in (200 mm) and friction loss 0.28 inH2O/100ft (2.3 Pa/m).