Size a new duct from airflow and friction rate, or check an existing duct's velocity and friction — on the job site, from your phone. No account, nothing uploaded, nothing stored.
Beta. This tool is live for real-world testing. Size or check a real duct and tell us what to fix — nothing is uploaded or stored.
Two directions, one page.Size a new duct: enter CFM and a friction rate, get the round size (and a rectangular option). Check an existing duct: enter CFM and the size you have, get its velocity and friction rate. Both modes flag noise risk, friction outside the residential band, and bad aspect ratios.
Size a new duct
Residential band 0.08–0.10. Manual D default 0.08.
Flex is rougher, so it sizes larger at the same friction.
Height is solved from the equivalent diameter, then rounded up to the next whole inch.
Check an existing duct
What this tool does — and doesn't. It sizes a duct run. It does not design the system: total external static pressure, fittings, and the equipment's rated ESP still govern. A 90° elbow adds roughly 25 ft of equivalent length — fittings, not straight runs, are what usually blow a friction budget. For a whole-house design, do a full ACCA Manual D. Rectangular ducts run roughly 15–25% higher friction than a round duct of the same area — the equivalent diameter below already adjusts for that; keep the aspect ratio under 4:1. Flex duct: pull it tight and support it — compressed or sagging flex is rougher than modeled, so the flex size here is a planning size, not a guarantee. And don't forget the return: sizing the supply and ignoring the return starves the blower even when the supply math looks fine.
