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Rectangular duct size calculator

A rectangular duct carries less air than a round duct of the same area because the corners add friction. The Huebscher equation gives the round duct with the same friction and airflow: De = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25. DuctSizeCalc finds the narrowest whole-inch width at each common height that matches the round size you need. This page starts with 800 CFM: about 13.4 in round, or 20 × 8, 16 × 10 or 13 × 12 in rectangular.

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    Keep the aspect ratio (width ÷ height) under about 4:1; very flat ducts need more metal and have more friction.

    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 is 12 × 8 duct equal to in round?

    About 10.7 in round (Huebscher equivalent diameter), not the 11.1 in you would get from equal area.

    What rectangular duct for 800 CFM?

    At 0.08 in. w.g. per 100 ft: 20 × 8 in (720 fpm), 16 × 10 in or 13 × 12 in.

    How do I convert round duct to rectangular?

    Pick the height that fits the joist space and find the width whose equivalent diameter is at least the round size; DuctSizeCalc does this for 6–12 in heights.

    Is DuctSizeCalc free?

    Yes, with no sign-up or ads.

    Are my numbers saved?

    Yes, in this browser only.

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