Duct Equivalent Length Calculator

Turn every fitting on a run into feet of straight duct so you can compare runs on one number: total effective length and its pressure loss.

Run inputs

Airflow (CFM)

Round diameter (in)

Straight length (ft)

Air temperature (°F)

Duct material

Fittings (quantity)

90° elbow, smooth radius, r/D 1.5 C 0.15

90° elbow, smooth radius, r/D 1.0 C 0.22

90° elbow, 5-piece segmented C 0.24

90° elbow, 3-piece segmented C 0.34

45° elbow, smooth radius C 0.1

90° mitred elbow, no vanes C 1.2

Tee or cross, flow into the branch C 1

Tee or cross, straight through C 0.1

45° conical take-off C 0.5

Gradual reducer, 30° included C 0.05

Gradual expander, 20° included C 0.25

Sharp-edged duct entry C 0.5

Abrupt duct exit C 1

Volume control damper, fully open C 0.2

Fire damper, curtain type C 0.5

Flexible connector at the plant C 0.3

Results

Enter a positive airflow and diameter, then select Calculate.

Method and assumptions

Equivalent length L = C × D / f, where C is the fitting loss coefficient, D the diameter (ft) and f the Darcy friction factor at this airflow — the same C-values used by the Fitting Loss calculator.
Because f changes with velocity and roughness, a fitting's equivalent length changes with airflow; fixed 'feet per elbow' tables are only approximations.
Friction via Darcy–Weisbach with the Altshul–Tsal friction factor; sea-level air at the entered temperature.
Round duct only; for rectangular duct enter its equivalent round diameter. Lengths shown to 1 ft, pressure to 0.001 in. w.g.

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Important Disclaimer

This calculator is for preliminary design guidance only. Final duct sizing, construction, and installation must be performed by qualified professionals and comply with applicable codes and standards.