Sizing versus checking
The other calculators on this site answer "what size duct do I need for this CFM and a target friction rate?", that's sizing: picking a duct for a target you haven't built yet. This page answers the opposite question: "I have a duct this size, carrying this CFM, what is it actually doing?" That's checking, useful when a run already exists (or its size is fixed by the space available) and the question is whether it performs acceptably, not what size to build. Both directions run on the exact same physics, this page just runs it forward, from a known size and CFM to velocity and friction rate, instead of solving backward from a target.
Velocity and friction loss are different problems
Velocity, in feet per minute, is how fast air moves through the duct; it mostly matters for noise; too fast and registers and duct runs start to hiss, rush, or whistle. Friction loss is the pressure the duct consumes to move that air, measured in inches of water column; it mostly matters for the fan or blower, every inch of friction loss across a system's ductwork is pressure the equipment has to supply, and undersized ductwork with excessive total friction loss is a common cause of a furnace or air handler that runs constantly but never quite delivers rated airflow.
Two ceilings, and a run can fail either one
A common design ceiling is about 900 feet per minute for supply duct and 700 feet per minute for return duct, driven by noise, not by the equipment's ability to move air. Friction rate has its own separate budget: the total friction loss across a duct system has to fit inside what the blower can supply after subtracting the losses from the equipment itself, filters, coils, and fittings. It's entirely possible for a run to sit comfortably under the velocity ceiling while still contributing more friction loss than the system's fan budget allows, or the reverse, low friction rate but high velocity in an undersized duct on a strong blower. Checking both numbers, not just one, is the point of this page.
Why material changes the answer
Velocity depends only on CFM and cross-sectional area, so it is identical for the same size and airflow no matter what the duct is made of, that's why the velocity table above doesn't change when the material dropdown does. Friction rate is different: it depends on surface roughness, and roughness varies a lot by material. The same 10 inch duct carrying 400 CFM loses roughly 0.086 in.wc per 100 ft in smooth, bare galvanized duct, about 0.089 in a typical installed galvanized run with joints, and roughly 0.119 in flexible duct, whose fibrous, corrugated liner is far rougher than any metal surface. That's not a small difference; a flex run at those conditions loses about a third more pressure per foot than the same size in smooth metal, which is exactly why the flex sizing chart on this site uses its own, higher roughness figure instead of reusing the galvanized number.
Total loss adds up over length
Friction rate is normalized per 100 feet, which makes different runs comparable but isn't itself the number a fan has to overcome, that's total friction loss: friction rate multiplied by the actual run length divided by 100. A short 20 foot branch and a long 150 foot trunk at the same friction rate produce very different total losses, 0.10 in.wc/100ft over 20 feet is 0.02 in.wc total, the same rate over 150 feet is 0.15 in.wc, more than seven times as much. Long runs matter more than their per-foot number alone suggests, which is why this calculator reports total loss whenever a length is entered, not just the rate.
How these numbers are derived
Velocity comes directly from CFM divided by the duct's cross-sectional area (V = Q/A), for a rectangular duct this uses the rectangle's own area, not its equivalent round diameter's area. Friction rate comes from the Darcy-Weisbach equation with the Swamee-Jain approximation of the Colebrook-White friction factor, the same equations and the same solver used across every sizing chart on this site, run here forward from a known size instead of solved backward from a target. Rectangular duct converts to its Huebscher equivalent diameter first, exactly as on the rectangular sizing chart. Roughness is whichever material is selected above; standard air is assumed throughout (ρ = 1.204 kg/m³, μ = 1.825×10⁻⁵ Pa·s). Nothing here is looked up from a table, every figure is computed live so the method stays visible.
Frequently asked questions
What is a good air velocity for ductwork?
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. Lower velocities are quieter but need a larger duct for the same CFM; this calculator flags whether your input is under or over those ceilings.
How do you calculate duct friction loss?
Friction loss over a run is the friction rate (in inches of water column per 100 feet, from the Darcy-Weisbach equation with the Swamee-Jain friction factor) multiplied by the actual run length divided by 100. A run at 0.10 in.wc per 100 ft that is 150 feet long loses 0.10 x (150/100) = 0.15 in.wc total.
What is friction rate in HVAC?
Friction rate is the pressure a duct loses to airflow resistance per 100 feet of length, expressed in inches of water column per 100 ft (in.wc/100ft). It depends on duct diameter (or equivalent diameter for rectangular), CFM, and the surface roughness of the duct material.
What velocity is too high for a duct?
There is no single hard cutoff, but velocities meaningfully above 900 fpm in supply duct or 700 fpm in return duct commonly produce noticeable noise at registers and along the run. Above roughly 1500 to 2000 fpm, whistling and rushing air noise become hard to avoid regardless of duct material.
Does duct material change the friction rate for the same size and CFM?
Yes, substantially. The same diameter and CFM through smooth bare galvanized duct, installed galvanized duct with joints, and rough flexible duct liner produce three different friction rates, because friction rate depends on surface roughness. Velocity does not change between materials, only friction rate does; see the comparison in the calculator above.