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    Air distribution / network

    Duct network calculator

    A duct run is a series of resistances, and the fittings usually cost more than the straight pipe. This calculator sums friction and fitting losses across three segments, converts the total into fan shaft power, and shows how quickly pressure rises with velocity.

    Flow1.00 m³/sSegments3Efficiency70%
    Total pressure loss
    80 Pa
    Friction plus fittings
    Friction share
    22%
    17 Pa
    Fitting share
    78%
    63 Pa
    Fan shaft power
    0.11 kW
    At 70% efficiency
    Segment losses
    Friction from the straight run, fittings from the local loss coefficients.
    80 Pa
    Segment losses
    SegmentFrictionFittingsTotal
    20 m at 5 m/s · K 1.512 Pa23 Pa34 Pa
    12 m at 4 m/s · K 2.54 Pa24 Pa28 Pa
    8 m at 3 m/s · K 3.01 Pa16 Pa18 Pa

    Fittings usually dominate: a single elbow can cost more than ten metres of straight duct.

    Inspect the model
    Choose a quick result view, a scaled comparison, or the complete diagnostic set.
    Total pressure against velocity
    Pressure rises with the square of velocity, so fan power rises with the cube.
    Total pressure against velocity0.00062.35124.70187.05249.40311.753.03.03.63.64.24.24.84.85.45.46.06.06.66.67.27.27.87.88.48.49.09.0Pa
    Friction against fittings
    Which part of the network dominates the pressure budget.
    Friction against fittings0.00014.8029.5944.3959.1973.9917.30FrictionFriction62.70FittingsFittingsPa
    Canonical equations
    The exact relationships implemented by this engine.

    Δp = f (L/D)(ρv²/2) · Darcy–Weisbach per segment.

    Δp = K (ρv²/2) · Sum of the local loss coefficients.

    P = Q·Δp / η · Shaft power; add motor and drive losses.

    engineering methodology

    A clear path from inputs to results.

    A duct run is a series of resistances, and the fittings usually cost more than the straight pipe. This calculator sums friction and fitting losses across three segments, converts the total into fan shaft power, and shows how quickly pressure rises with velocity.

    Step-by-step methodWorked exampleAssumptions explained

    How to use

    1. Enter the system airflow and fan efficiency.
    2. For each segment, enter length, velocity and the sum of fitting K factors.
    3. Read the total pressure and fan power.
    4. Check which share is fittings.

    The formula

    Δp=∑(fLD+∑K)ρv22\Delta p = \sum \left( f \frac{L}{D} + \sum K \right) \frac{\rho v^2}{2}

    Reference: Darcy–Weisbach friction plus local loss coefficients; standard duct design practice.

    Worked example

    Three segments, 1 m³/s, 70% fan efficiency

    Inputs
    • Segments: 20 m at 5 m/s (K 1.5), 12 m at 4 m/s (K 2.5), 8 m at 3 m/s (K 3).
    • Fan efficiency 70%.
    Calculation path
    1. 1

      Friction across the three segments: 17 Pa.

    2. 2

      Fittings: 63 Pa — nearly four times the friction.

    3. 3

      Total pressure: 80 Pa.

    4. 4

      Fan shaft power: 1 × 80 / 0.7 = 114 W.

    Result

    80 Pa total · 63 Pa from fittings · 0.11 kW fan.

    Interpreting the result

    Fittings dominate almost every real network. That is why layout decisions — fewer elbows, gentler transitions, straight approaches to terminals — save more fan energy than swapping the duct material. Pressure rises with the square of velocity, so the fan power rises with the cube.

    Assumptions & limitations

    • Constant friction factor of 0.02.
    • Standard air density 1.2 kg/m³.
    • No terminal units, filters or coils.
    • No leakage or balancing damper losses.

    Frequently asked questions

    Why are fittings so large?

    Each elbow or tee has a loss coefficient of 1–3, equivalent to many metres of straight duct.

    What efficiency should I use?

    Total fan efficiency of 60–75% is typical for a belt-driven centrifugal fan.

    Does higher velocity save money?

    It saves duct size but raises fan power with the cube of velocity, so the running cost usually wins.

    Methodology last reviewed 2026-10-21. Results are informational and not financial or engineering advice — see our disclaimer.