Enter the airflow and get the right round duct size, the air velocity, and an equivalent rectangular duct — using the standard equal-friction method HVAC pros use. Free, instant, no sign-up.
Typical friction rate for residential/light-commercial: 0.08–0.10 in.wg per 100 ft. Lower = bigger, quieter duct.
Sizing a duct means finding a cross-section that carries the required airflow (CFM) without moving the air too fast (which is noisy and wastes energy) or too slow (which wastes metal and space). The most common professional method is equal friction: you pick a friction rate — how much static pressure the duct loses per 100 feet, typically 0.08–0.10 in.wg/100 ft for homes and light commercial — and size every run to that same rate. This calculator solves the standard round-duct friction equation for galvanized sheet metal and gives you the diameter, then converts it to an equivalent rectangular duct.
Round duct is the most efficient shape (least friction and material per CFM), so the calculator sizes the round duct first, then offers rectangular sizes that carry the same airflow at the same friction using the ASHRAE equivalent-diameter relationship. Keep the rectangular aspect ratio at or below 4:1 — long, flat ducts get inefficient and noisy.
Air velocity (feet per minute) is the noise and comfort check. Rough guides: < 700 fpm is quiet enough for branch runs near living spaces, 700–1200 fpm is typical for supply branches and main trunks, and much above 1500 fpm starts to whistle. The calculator flags the velocity so you can go up a size if it's too fast.
Informational tool for HVAC estimating — verify final designs against ACCA Manual D and local code.