What friction rate should I use for residential duct?
Most residential systems are designed around 0.08–0.10 in. wg per 100 ft. But the right number is the one you compute for your job: take the blower's rated external static pressure, subtract the coil, filter, and register/grille losses to get available static pressure, then divide by the total effective length and multiply by 100. The friction-rate helper on this page does exactly that. Using a made-up 0.10 on a system with a high-static coil is how ducts end up undersized.
How many CFM will a round duct carry?
At 0.10 in. wg per 100 ft in galvanized round, this engine gives roughly: 6" ≈ 110 CFM, 7" ≈ 165, 8" ≈ 240, 9" ≈ 325, 10" ≈ 435, 12" ≈ 710, 14" ≈ 1070, 16" ≈ 1525. Velocity climbs with each size, so a 16" at 0.10 is already near 1100 fpm. Switch to capacity mode to get the exact number for any diameter and friction rate.
How much larger does flex duct need to be than metal?
Usually one to two sizes. Flex has a corrugated liner, and any compression makes it far worse: pulled tight it runs about 1.3× the friction of metal, at a typical 4% compression about 1.9×, and sagging at 15% roughly 3×. So a 10" metal round is closer to a 12" flex once you account for real-world compression. Pull flex tight and support it — compression is the single biggest airflow killer on a flex system.
How do I convert a round duct to a rectangular one?
Use the equivalent-diameter formula De = 1.30 × (a·b)^0.625 ÷ (a+b)^0.25, where a and b are the rectangular sides. Size the rectangle so its De is at least the required round diameter, and keep the aspect ratio under about 4:1 — a squished 3×14 moves the same air as a 7" round but costs more metal and more pressure than a near-square shape. This tool lists the practical rectangular options for you and honors a max-height limit.
What duct velocity is too high?
Velocity drives noise. Common residential guidelines are about 900 fpm for supply trunks and 700 fpm for branches and returns; commercial systems tolerate 1200–1500 fpm. When the pick exceeds the guideline for your application, the tool flags it and suggests going up a size. High velocity at registers and grilles is what people hear as a whistling or rushing system.
Is this the same as a Ductulator?
It uses the same equal-friction method as a slide-chart duct calculator (Ductulator is a Trane trademark), but the numbers here are computed live from the Altshul-Tsal friction factor and the standard velocity-pressure equation in ASHRAE Fundamentals rather than read off a printed wheel — so you also get the shown math, velocity checks, and the inverse (capacity) direction.