DuctSizeCalc Tip

Duct size calculator

Enter the airflow in CFM (or the AC size in tons) and DuctSizeCalc sizes the duct by the equal-friction method used on a ductulator: the diameter that gives your friction rate, checked against a maximum velocity so the duct is not noisy. It rounds up to a standard round size, shows one size down for comparison, and lists rectangular ducts with the same capacity at 6, 8, 10 and 12 inch heights. The example is 400 CFM at 0.08 inches of water per 100 ft.

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    Switch to "Check a duct" to see the velocity and friction of a duct you already have.

    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

    What size duct for 400 CFM?

    At 0.08 in. w.g. per 100 ft you need about 10.3 in, so a 12 in round duct (509 fpm). At 0.10 in. w.g. a 10 in duct is enough (733 fpm, 0.092 in. w.g.).

    What friction rate should I use?

    Residential duct systems are often designed between 0.05 and 0.10 in. w.g. per 100 ft; 0.08 is a common starting point. The right figure comes from the blower static pressure and the length of the longest run.

    What is a good duct velocity?

    About 700–900 fpm for residential supply trunks and 500–700 fpm for branches; lower is quieter.

    Is DuctSizeCalc free?

    Yes, with no sign-up or ads.

    Are my numbers saved?

    Yes, in this browser only.

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