PsychrometricsDuct calculator
Size round and rectangular ducts from airflow, friction rate, and velocity, or check an existing round, rectangular, or flat-oval duct.
Inputs
Design friction rate is a target you choose, not a value you're given. 0.08 in/100 ft is a common starting point (0.05–0.10 is typical). Typical max velocity: supply mains up to 900 fpm residential and 1,500 fpm commercial. The duct is sized to meet every checked limit.
Don't know it? Estimate from static pressure
Available static pressure is the blower's rated external static pressure minus the coil, filter, grille, and damper losses. Total effective length is the longest supply run plus the longest return run, including fitting equivalent lengths. Friction rate = available static × 100 ÷ total effective length (ACCA Manual D). The result fills in the design friction rate above.
Include fitting equivalent lengths in the run to get total pressure loss. Compressed flex duct can run 2–4× the fully extended friction.