Flexible Duct Sizing Calculator

Why Flex Duct Needs Its Own Size

Flexible duct isn't just a bendier version of rigid metal duct — its corrugated inner wall creates real friction that smooth sheet metal doesn't have, so air moving through it loses pressure faster for the same diameter. Size a flex run using a rigid-duct chart and you'll end up with a duct that's too small, which shows up later as weak airflow at the register, a noisy system working harder than it should, and a duct that runs hotter or colder than the room around it because it can't deliver the air the equipment is pushing.

How to Use This Calculator

Enter the CFM (cubic feet per minute) that run needs to carry — this comes from your Manual J load calculation or your equipment's rated airflow divided across its registers, not a guess. The friction rate defaults to 0.08 in.wg per 100 feet, the standard ACCA Manual D starting point for most residential systems; leave it as-is unless you already know your system calls for something else within the accepted 0.06–0.18 range.

How We Calculate It

We use the ASHRAE round-duct friction equation — d = (0.109136 × CFM^1.9 / friction rate)^1/5.02 — to get the exact rigid metal diameter that airflow requires, then round up to the nearest stocked duct size. ACCA Manual D calls for flexible duct to be sized two inches larger than the equivalent rigid metal size to offset its extra internal friction, so that's added next, again rounded up to the nearest size actually sold. For 150 CFM at the default 0.08 friction rate, that comes out to an 8" rigid equivalent and a 10" flex duct.

A Few Notes

This gives you a sizing target, not a full duct design — a real system has multiple runs, fittings, and a total equivalent length that a proper Manual D workup accounts for. It also assumes the duct is installed correctly: flex duct that's compressed, kinked, or left sagging between supports can lose 50–80% of its rated capacity regardless of what diameter you bought, so pull every run fully taut and support it at least every four feet.