The duct sizing guides

• CFM to Duct Size Chart: What Size Duct Do I Need for 400 CFM? - 400 CFM needs a 9.7 in. round duct (10 in. stock) or 14x6 at 0.10 in. wg per 100 ft. Full CFM to duct size chart, 50 to 2,000 CFM, with velocity.
• Rectangular to Round Duct Conversion Chart (Equivalent Diameter Explained) - Full Huebscher circular-equivalent chart, 4x4 to 36x20 in. A 14x6 duct equals 9.8 in. round at equal friction and capacity - not equal area.
• Friction Rate in HVAC: What It Is and How to Calculate It - Friction rate = (Available Static Pressure x 100) / Total Effective Length, in in. wg per 100 ft. Full ASP deduction worksheet and the 0.06 to 0.18 design band.
• Duct Velocity Chart: Recommended FPM for Supply, Return, and Branch Ducts - Recommended duct velocity in FPM: 700-900 for residential supply trunks, 500-700 for branches, 500 or less at return grilles. Plus the V = CFM / area math.
• Duct Fitting Equivalent Length Chart and How to Find Total Effective Length - Equivalent length chart for duct fittings: square 90s run 75 to 80 ft, radius 90s 15 to 30 ft. Add them to straight duct to get total effective length.
• Flex Duct vs Rigid Duct: CFM Capacity, Sizing Up, and the Cost of Sag - Fully extended flex carries about 16% less CFM than galvanized at the same friction rate. Full capacity chart, compression penalties, and how much to upsize.