Size round and rectangular ductwork using the equal friction method at 0.1" WG per 100 ft. Results include equivalent rectangular sizes at 8", 10", and 12" heights with velocity checks.
SMACNA/Manual D standard: 0.08–0.10 residential, 0.10 commercial
Used to evaluate velocity against SMACNA limits
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Enter airflow and friction rate, then click Calculate.
Standards & Method
Equal Friction Method — duct sized so friction loss is constant per unit length throughout the system. Design friction rate 0.1" WG per 100 ft per SMACNA and ASHRAE Fundamentals.
Round diameter — calculated using ASHRAE Fundamentals formula: Hf = 0.109 × Q¹·⁹ / D⁵·⁰², solved for D. Result rounded up to nearest standard size.
Rectangular equivalents — calculated using ASHRAE hydraulic diameter equivalence: De = 1.30 × (a×b)⁰·⁶²⁵ / (a+b)⁰·²⁵. Width rounded up to nearest even inch.
Standard sizes — round duct in 1" increments 4"–24", then 26", 28", 30". Rectangular widths rounded to nearest even inch.
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