Air Duct Friction Loss: Pressure Drop Calculator & Chart
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)
where
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) :
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 m3/h) , duct size 8 in (200 mm) and friction loss 0.28 inH2O/100ft (2.3 Pa/m) .
