DuctSizeCalc Tip

Duct friction loss calculator

Friction loss rises steeply with airflow and falls very steeply with diameter: doubling the CFM almost quadruples the loss, and one size up can halve it. DuctSizeCalc uses 0.109136 × CFM^1.9 ÷ D^5.02 inches of water per 100 ft for round metal duct and the Huebscher equivalent diameter for rectangular duct. Multiply by the run length ÷ 100 for the loss of a straight run, then add fittings.

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    For 1,000 CFM a 14 in duct loses about 0.096 in. w.g. per 100 ft and a 16 in duct about 0.049.

    How the duct size calculator works

    DuctSizeCalc uses the equal-friction method. For round galvanized duct carrying standard air, friction loss per 100 ft = 0.109136 × CFM1.9 ÷ D5.02 (D in inches), a fit to the ASHRAE friction chart that ductulators are based on. Solving for D gives the diameter at your friction rate. A second diameter keeps the velocity at or below your limit: D = 12 × √(4 × CFM ÷ (π × fpm)). The larger of the two is rounded up to a standard round size. Rectangular sizes use the Huebscher equivalent diameter, De = 1.30 × (a·b)0.625 ÷ (a + b)0.25, at heights of 6, 8, 10 and 12 inches.

    Round duct size by airflow (0.08 in. w.g. per 100 ft, 900 fpm max)

    AirflowRound sizeVelocityFriction /100 ft
    50 CFM5 in367 fpm0.057 in. w.g.
    75 CFM6 in382 fpm0.049 in. w.g.
    100 CFM7 in374 fpm0.039 in. w.g.
    150 CFM8 in430 fpm0.044 in. w.g.
    200 CFM8 in573 fpm0.075 in. w.g.
    300 CFM10 in550 fpm0.053 in. w.g.
    400 CFM12 in509 fpm0.037 in. w.g.
    600 CFM12 in764 fpm0.079 in. w.g.
    800 CFM14 in748 fpm0.063 in. w.g.
    1000 CFM16 in716 fpm0.049 in. w.g.
    1200 CFM16 in859 fpm0.07 in. w.g.

    Flexible duct has more friction than metal duct, especially when it is not pulled tight, and every elbow, boot and register adds pressure drop, so the friction rate you design to should come from the blower's available static pressure divided by the total effective length (ACCA Manual D). Results are estimates for planning, not a substitute for a proper duct design.

    Frequently asked questions

    How do I calculate duct friction loss?

    Use the friction chart or its equation for the duct size and CFM to get the loss per 100 ft, then scale by the total effective length including fittings.

    What is the friction loss of 400 CFM in a 10 inch duct?

    About 0.092 in. w.g. per 100 ft at 733 fpm.

    What is total effective length?

    The straight duct length plus the equivalent length of every fitting (elbows, boots, takeoffs), from tables such as ACCA Manual D.

    Is DuctSizeCalc free?

    Yes, with no sign-up or ads.

    Are my numbers saved?

    Yes, in this browser only.

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