Duct Friction Loss Calculator

Inputs

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Design airflow through this duct run.
cfm
Inside duct diameter for round or spiral duct.
in
Measured straight duct length for this run.
ft
Equivalent straight length for elbows, takeoffs, transitions, dampers and other fittings.
ft
Equivalent absolute roughness. Galvanized sheet-metal duct is often screened near 0.09 mm.
in
Air density at the duct condition. Room-temperature sea-level air is about 1.2 kg/m3.
lb/ft³
Static-pressure allowance for this duct run or for the total system budget you want to compare against.
in. w.g.

Results

Default result
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Duct pressure drop(dp)
0.1268in. w.g.
Pass

25.3% of available static

Also computed

Friction rate(dp/L)Caution0.1544in. w.g./100 ft

moderate friction; check blower static pressure

Duct velocity(v)Pass1,003ft/min

typical comfort-duct screening range

Static budget margin(margin)Pass0.3751in. w.g.

remaining static allowance

Static budget used25.3%

Total equivalent length(TEL)82.02ft

Velocity pressure(q)0.06248in. w.g.

Pressure drop vs Total equivalent lengthPressure drop scales linearly with total equivalent length once duct size, airflow and roughness are fixed.Pressure drop vs Total equivalent length00.10.20.3050100150selectedTotal equivalent length (ft)Pressure drop (in. w.g.)
Pressure drop scales linearly with total equivalent length once duct size, airflow and roughness are fixed.
Method notes 3 notes
  • Loss is calculated for a round or spiral duct using the entered straight length plus fitting equivalent length.
  • Pressure loss uses Darcy-Weisbach with hydraulic diameter, room-temperature air viscosity and a Swamee-Jain turbulent friction factor.
  • Final HVAC duct design should still include actual fitting loss coefficients, leakage, balancing, acoustics, terminal data, fan curves and local code requirements.

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