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Duct size chart

The chart below lists common airflows and the standard round duct DuctSizeCalc picks for each at 0.08 in. w.g. per 100 ft with a 900 fpm limit, along with the actual velocity and friction of that size. Use the calculator above for other airflows, or change the friction rate: at 0.10 in. w.g. most ducts carry about 12% more air than at 0.08.

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    Charts printed for flex duct give smaller airflows for the same diameter; this one is for smooth metal duct.

    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 much CFM can a 6 inch duct carry?

    About 95–100 CFM at 0.08 in. w.g. per 100 ft in metal duct; 100 CFM gives 0.085 in. w.g.

    How many CFM through a 14 inch duct?

    About 800–900 CFM at 0.08 in. w.g.; 800 CFM runs at 748 fpm and 0.063 in. w.g.

    Is a duct size chart the same as a ductulator?

    Yes, both come from the same friction chart; DuctSizeCalc uses its equation so you can enter any value.

    Is DuctSizeCalc free?

    Yes, with no sign-up or ads.

    Are my numbers saved?

    Yes, in this browser only.

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