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    HVAC Duct Sizing Calculator — Free Friction Rate Tool

    What size duct do I need for a 3-ton AC unit?

    A 3-ton system needs roughly 1,200 CFM total. The main trunk typically sizes to 14"×8" or 16" round at 0.08 IWC friction rate, with branch ducts in the 6"–8" round range depending on per-room CFM. Always size from a full Manual D calculation, not by tonnage alone.

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    Estimates and general guidance only. Verify against your local code and AHJ before any installation. Gas, electrical, and refrigerant work is hazardous to untrained users.

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