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    HVAC / Ductwork

    Duct Fitting Losses

    The pressure loss through a duct fitting from its loss coefficient and the velocity pressure. Fittings often cost more pressure than the straight run, which is why a duct layout is decided at the elbows.

    Velocity pressure21.6 PaLoss per fitting6.5 PaTotal for the run52 Pa
    Velocity pressure
    21.6 Pa
    Dynamic pressure
    Loss per fitting
    6.5 Pa
    K 0.30
    Total for the run
    52 Pa
    8 fittings
    Fan power
    0.05 kW
    Per m³/s of airflow
    Loss against velocity
    Δp = K · 0.5 · ρ · V². K comes from the ASHRAE fitting tables for the geometry.
    Loss against velocity
    Velocity (m/s)Velocity pressureLoss per fitting
    35.4 Pa1.6 Pa
    515.0 Pa4.5 Pa
    729.4 Pa8.8 Pa
    948.6 Pa14.6 Pa
    1286.4 Pa25.9 Pa
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    Loss against velocity
    Pressure loss goes with the square of velocity — the reason oversized ducts save fan power.
    Loss against velocity0.0005.8111.6117.4223.2229.033 m/s3 m/s5 m/s5 m/s7 m/s7 m/s9 m/s9 m/s12 m/s12 m/sDuct Fitting Losses
    Canonical equations
    The exact relationships implemented by this engine.

    Pv = 0.5·ρ·V² · Dynamic pressure.

    Δp = K·Pv · K from the fitting tables.

    engineering methodology

    A clear path from inputs to results.

    The pressure loss through a duct fitting from its loss coefficient and the velocity pressure. Fittings often cost more pressure than the straight run, which is why a duct layout is decided at the elbows.

    Step-by-step methodWorked exampleAssumptions explained

    How to use

    1. Set air velocity (m/s).
    2. Set loss coefficient k.
    3. Set number of fittings.
    4. Set air density (kg/m³).
    5. Read the headline metrics, then check the table for the detail.

    The formula

    Pv=0.5⋅ρ⋅V2Pv = 0.5·ρ·V²

    Reference: Δp = K·0.5·ρ·V²; K from ASHRAE fitting tables.

    Worked example

    At the default inputs

    Inputs
    • Air velocity (m/s): 6; Loss coefficient K: 0.3; Number of fittings: 8; Air density (kg/m³): 1.2
    Calculation path
    1. 1

      Velocity pressure: 21.6 Pa · Loss per fitting: 6.5 Pa · Total for the run: 52 Pa · Fan power: 0.05 kW

    Result

    Velocity pressure: 21.6 Pa · Loss per fitting: 6.5 Pa · Total for the run: 52 Pa

    Interpreting the result

    At 6.0 m/s each fitting costs 6.5 Pa, so 8 of them cost 52 Pa.

    Assumptions & limitations

    • K is constant for the fitting geometry.
    • No system effect from the fan outlet.

    Frequently asked questions

    Why do fittings dominate?

    A single elbow can cost more than ten metres of straight duct.

    Does velocity or size matter more?

    Velocity, squared — which is why duct sizing drives fan power.

    Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.