Duct Size Calculator
Enter airflow and your design friction rate to get the round duct diameter, the next nominal size up, and the resulting air velocity — with the ASHRAE equation documented.
Last updated: 2026-09-15
How the calculation works
- The tool solves the ASHRAE equal-friction chart equation for diameter by iteration — the same equation behind the classic ductulator.
- It reports the exact diameter and snaps it up to the next standard nominal size (4, 5, 6, 7, 8... inches).
- Air velocity is checked because noise and pressure drop rise sharply above ~1000 fpm in residential systems.
Formula
Q = 0.455 · A · D^0.61 · ΔP^0.5 solved for D (galvanized round duct, standard air)
| Symbol | Meaning | Unit |
|---|---|---|
Q | Airflow | CFM |
A | Duct cross-section area | ft² |
D | Duct diameter | in |
ΔP | Friction rate | in. w.c. per 100 ft |
Worked example
Interpreting the result
The nominal size is what you install. If velocity exceeds about 900–1000 fpm in occupied spaces, go one size up — noise complaints are common above that. Trunk ducts are usually designed at the same friction rate so the whole system balances; each branch takes its CFM share.
Assumptions
- Round galvanized steel duct, standard air density, clean interior.
- Equal-friction design method with a constant friction rate.
Limitations
- Flexible duct has 2–4× the resistance of smooth metal — upsize significantly or avoid long flex runs.
- Rectangular ducts need equivalent-diameter conversion; this tool sizes round only.
- Fittings (elbows, takeoffs) dominate real system pressure loss and are not counted here.
Frequently asked questions
What friction rate should I use?
0.08 in. w.c./100 ft is the common residential default. Use 0.06 for quiet systems or long runs, and higher only for short industrial runs where noise doesn't matter.
Does this work for flex duct?
Not directly. Fully stretched smooth flex is close, but typical compressed flex can double the friction. Size flex one or two nominal sizes larger than the metal equivalent.