Quick answer: The most common way to size HVAC ducts is the equal-friction method: pick a friction rate of about 0.08–0.1 in.wg per 100 ft (or 0.8–1.0 Pa/m), then find the duct area from A = Q ÷ V (airflow ÷ velocity). Convert that area to a round diameter with D = √(4A ÷ π), or to an equivalent rectangular size.
Why duct sizing matters
Ducts that are too small create high velocity, noise and pressure loss — forcing the fan to work harder and burn more energy. Ducts that are too large waste sheet metal and space. Correct sizing delivers the required airflow quietly and efficiently.
Duct sizing methods
- Equal friction — keep a constant friction rate throughout; simple and the most widely used for comfort systems.
- Velocity reduction — set target velocities and reduce them downstream; good for controlling noise.
- Static regain — recover velocity pressure at each branch; used for large, high-velocity systems.
The duct sizing formula
Area: A = Q / V (Q = airflow, V = velocity)
Round dia: D = √(4A / π)
Rectangular equivalent:
De = 1.30 × (a·b)^0.625 / (a+b)^0.250Where Q is airflow (m³/s or CFM), V is design velocity (m/s or fpm), and a, b are the rectangular duct sides.
Recommended air velocities
| Duct section | Residential (m/s) | Commercial (m/s) |
|---|---|---|
| Main supply duct | 3.5–5.0 | 5.0–8.0 |
| Branch duct | 3.0–4.0 | 4.0–6.0 |
| Supply outlet / grille | 1.5–2.5 | 2.0–4.0 |
| Return air duct | 3.0–4.0 | 4.0–5.0 |
Enter airflow (CFM or m³/h) and your target velocity or friction rate — get round and rectangular duct sizes instantly.
Open the Duct Size Calculator →Worked example
Sizing a main supply duct for 1,000 CFM (0.472 m³/s) at a commercial velocity of 6 m/s:
- Area A = 0.472 ÷ 6 = 0.0787 m²
- Round diameter D = √(4 × 0.0787 ÷ π) ≈ 0.316 m ≈ 315 mm
- Equivalent rectangular duct ≈ 350 × 200 mm
Check the friction rate stays near 0.8–1.0 Pa/m; if it is higher, go one size up.
Common duct sizing mistakes
- Choosing velocity too high — the number-one cause of noisy systems.
- Ignoring fitting losses at elbows, tees and transitions.
- Forgetting to recalculate downstream as airflow drops off at each branch.
Related MEP calculators
Start upstream with the HVAC Heat Load Calculator and the AHU Sizing Calculator to get the airflow you need before sizing ducts.
Frequently Asked Questions
What is the formula for duct size?
Duct area A = Q / V, where Q is airflow and V is velocity. Convert area to a round diameter with D = root(4A/pi) or to an equivalent rectangular size.
What is the equal-friction method?
The equal-friction method sizes the whole duct system to a constant friction rate, typically 0.08-0.1 in.wg per 100 ft. It is simple and widely used for comfort HVAC systems.
What air velocity should I use for ducts?
For commercial main ducts use about 5-8 m/s, branches 4-6 m/s, and outlets 2-4 m/s. Lower velocities are used in residential systems to reduce noise.
How do I convert CFM to duct size?
Convert CFM to m3/s, divide by your design velocity to get area, then convert area to a round diameter or rectangular duct using the standard formulas.