Friction Loss in Air Ducts

Friction 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

Δp = (0.109136 q1.9) / de5.02         (1)


Δp = friction (head or pressure 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 friction loss calculator below is based on formula (1):

Air Volume Flow - q - (cfm)

Equivalent duct diameter - de - (inches)

Duct Length - (feet)

Pressure loss and air flow velocity for some common duct sizes and air flow volumes can be taken from the table below:

Pressure Loss (inches water gauge per 100 feet duct)
Air velocity (ft/min)
Air Volume
Duct Size (inches)
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

Air velocity should not exceed certain limits to avoid unacceptable noise generation.

Air Ducts Friction Loss Calculator - SI Units

The friction 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)

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