Round Duct Sizing Chart & CFM Calculator

How to use this chart

Enter the CFM a run needs to carry and pick a friction rate target, then the calculator above returns the smallest nominal round duct size that clears it, along with the resulting velocity. To browse the full picture instead, the table lists every nominal size against three standard friction rates, 0.08, 0.10, and 0.15 in.wc per 100 ft, with the paired velocity for each. Read down a friction-rate column to find the largest CFM a given diameter can carry before crossing that rate; read across a row to see how capacity and velocity both climb with duct size.

Equal-friction sizing, in plain terms

"Equal friction" sizing means picking one friction rate and holding every run in the system to it, so that the fan has to overcome roughly the same resistance per 100 feet no matter which branch air is moving through. 0.10 in.wc per 100 ft is the default here because it's the most common residential design point when there's no load-calculation guidance (e.g. Manual D) pointing elsewhere: it strikes a workable balance between duct size (lower friction rates need larger, more expensive duct) and velocity (higher friction rates push air faster through a smaller duct, which gets loud). 0.08 trades a slightly larger duct for a quieter run, useful on branches feeding rooms sensitive to register noise. Whatever rate is chosen, applying it consistently across a trunk and its branches is what keeps a system balanced; mixing friction-rate assumptions between runs is a common cause of some rooms being starved while others run overpressured.

Why round beats rectangular for the same airflow

For a given cross-sectional area, a circle has the smallest possible perimeter of any shape. Since the Darcy-Weisbach friction loss in this calculator scales with how much duct wall the air drags against relative to how much air is actually moving, less perimeter per unit of area means less friction loss per foot for the same CFM. A rectangular duct sized to the same area as a round duct will always lose more pressure over the same run, sometimes considerably more if the aspect ratio is far from square. Rectangular duct still gets used constantly, mainly because it fits into flat spaces (above a dropped ceiling, inside a wall cavity) that a round duct of equivalent capacity physically can't. Where clearance allows either, round is the more efficient shape to size, install, and seal.

Velocity limits, and where they can bind first

Friction rate and velocity are two different constraints, and either one can be the tighter limit depending on the CFM and duct size involved. A common design ceiling is about 900 feet per minute for supply duct and 700 feet per minute for return duct, mainly to keep duct and register noise out of a quiet room. At small diameters and high CFM, velocity often exceeds a comfortable ceiling before the friction rate does, which is why this calculator reports both figures and lets an optional velocity cap override the friction-rate-only answer. If a run needs a larger diameter than the friction rate alone would suggest, a velocity limit is almost always the reason.

How these numbers are derived

Air velocity comes from CFM divided by the duct's cross-sectional area. Reynolds number comes from that velocity, the diameter, and standard air's density and viscosity. The Darcy friction factor comes from the Swamee-Jain equation, an explicit, non-iterative approximation of the Colebrook-White equation accurate to within about 1% in the turbulent flow range this calculator operates in. Pressure loss per foot comes from the Darcy-Weisbach equation using that friction factor. The one input specific to this page is roughness: this chart uses ε = 0.15 mm, the SMACNA/ASHRAE-calibrated figure for installed galvanized duct with joints (e.g. longitudinal-seam duct on roughly 2.5 foot centers), not the smoother 0.09 mm figure for bare, jointless sheet metal. Published friction diagrams are built against duct with joints, not an idealized seamless pipe, which is why the installed figure is the one used here. Every step assumes standard air at 20°C, sea level (ρ = 1.204 kg/m³, μ = 1.825×10⁻⁵ Pa·s); a different altitude or air temperature shifts the numbers slightly. Nothing here is transcribed from a scanned chart, it's computed live from the same public equations every duct designer already relies on, so the method is visible rather than just the output. Flexible duct behaves differently, its liner is far rougher, so it needs its own roughness figure and its own chart; see the flexible duct CFM chart for that.