DuctSizeCalc Tip

Duct velocity calculator

Velocity = airflow ÷ duct area. A 6 in round duct has 0.196 sq ft of area, so 100 CFM moves at 100 ÷ 0.196 ≈ 509 fpm. DuctSizeCalc works this out for round or rectangular ducts, adds the friction loss per 100 ft, and flags ducts that are likely to be noisy (over about 900 fpm in a home) or oversized. This page opens on the "Check a duct" tab with 100 CFM in a 6 in duct.

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    Register and grille noise depends on face velocity too, so check the register maker's chart as well.

    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 velocity?

    Divide CFM by the duct area in square feet. For a round duct, area = π × (diameter in inches ÷ 12)² ÷ 4.

    What is the velocity of 150 CFM in a 6 inch duct?

    About 764 fpm, with 0.18 in. w.g. friction per 100 ft, which is high for a branch; a 7 in duct is better.

    What is the maximum velocity for residential duct?

    Around 900 fpm for supply trunks and 600 fpm for branches and returns is common guidance for quiet systems.

    Is DuctSizeCalc free?

    Yes, with no sign-up or ads.

    Are my numbers saved?

    Yes, in this browser only.

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