Duct sizing without the guesswork
Equal-friction duct sizing comes down to two numbers — the CFM a run carries and the friction rate you design to. Here is how it works, the velocities to stay inside, and how to size a round duct in seconds.
Undersized duct is behind more callbacks than most techs want to admit — high static pressure, noisy registers, a blower fighting for its life, and rooms that never reach setpoint. The fix is not eyeballing it off the last job. Equal-friction sizing turns duct design into two inputs and a repeatable answer, and you can run it on every branch before you ever cut metal.
The two numbers that decide a duct size
- Airflow (CFM) — how much air that run has to carry. For a branch, that is the airflow the room needs; for a trunk, it is the sum of everything downstream of it.
- Friction rate — how much static pressure you are willing to spend per 100 feet of duct, in inches of water gauge. This is the design dial: a lower friction rate gives bigger, quieter duct; a higher one gives smaller, faster, louder duct.
"Equal friction" means you pick one friction rate and size every run to it, so the whole system loses pressure at the same rate per foot. That keeps the design consistent and the airflow balanced without a dozen one-off decisions.
Where the friction rate comes from
On a real job the friction rate is not a guess — you derive it from the available static pressure. Start with the blower's rated external static, subtract the coil, filter, and registered fittings, then divide the pressure that is left over by the total effective length of the longest run. For typical residential systems that lands around 0.08 to 0.10 inWG per 100 ft, which is why those values are a sane default when you do not have the full Manual D worked out.
How the math works
Equal-friction sizing comes straight out of the ASHRAE duct-design relationship — the Huebscher form of the Colebrook-White equation for galvanized round duct. You do not have to do it by hand, but it helps to know it is real engineering and not a nomograph someone sketched once:
The relationship
Pressure loss per 100 ft is proportional to CFM raised to the 1.82 power, divided by diameter to the 4.97 power. Solve it for diameter and you get the exact round size for a given CFM and friction rate — then round up to the next stock duct size, because metal comes in whole inches.
That strong dependence on diameter is why going up one duct size buys so much: a small bump in diameter drops the friction loss dramatically. It is also why shaving a duct down "to make it fit" is so punishing — the static penalty climbs fast.
Stay inside the velocity window
Sizing to a friction rate gets you a diameter; checking velocity tells you whether that diameter will behave. Velocity is just the airflow divided by the duct's cross-sectional area, and for residential supply duct you want to land in a sensible band:
- Above ~900 FPM: you are into noise territory for residential supply — registers whistle and the run gets loud. Size up.
- Below ~400 FPM: the duct is likely oversized for the flow. It will be quiet, but you are paying for metal you do not need and air can stratify in long horizontal runs.
- In between: the sweet spot for residential supply trunks and branches — moves the air without making a scene.
Round vs. rectangular
These numbers are for galvanized round duct. Rectangular and flex behave differently — flex especially eats far more static when it is not pulled tight and run straight. If you size in round and convert to rectangular, use an equivalent-diameter table so you keep the same friction loss.
Try the tool
Size it in seconds with the Round Duct calculator
Enter the CFM and your friction rate and get the round duct size — rounded to stock — plus the resulting velocity and exact diameter, with high- and low-velocity flags. ASHRAE methodology, runs offline.
Open the Duct Sizing tool