Quick answer: Most HVAC ducts are sized by the equal-friction method — pick a friction rate of about 0.8–1.0 Pa/m (0.08–0.1 in.wg per 100 ft) and size every duct to it, keeping velocity below the noise limit for the space. Do it instantly with the duct size calculator (online ductulator).
Why duct size matters
Undersize a duct and air velocity climbs — you get noise, high friction and a bigger fan bill for the life of the building. Oversize it and you waste sheet metal, ceiling space and money. Good duct sizing threads between the two: enough area to move the air quietly, no more.
The three duct-sizing methods
| Method | How it works | Best for |
|---|---|---|
| Equal friction | Constant friction rate (Pa/m) on every run | Most systems — the default |
| Velocity | Pick a target velocity, size for it | Quick / industrial exhaust |
| Static regain | Resize so regained static offsets friction | Large, high-velocity systems |
Equal-friction, step by step
- Choose a friction rate — 0.8–1.0 Pa/m for comfort HVAC.
- For each duct's airflow, read the round diameter that gives that friction (chart, ductulator, or calculator).
- Check the velocity against the table below — drop the friction rate if a duct is too fast/noisy.
- Convert round to rectangular using the equivalent diameter if needed.
Maximum air velocity (noise limit)
| Application | Max velocity (m/s) | (fpm) |
|---|---|---|
| Residential / low-noise | 3 – 5 | 600 – 1000 |
| Commercial main duct | 5 – 7.5 | 1000 – 1500 |
| Commercial branch | 3 – 5 | 600 – 1000 |
| Industrial | 7.5 – 12+ | 1500 – 2500 |
Worked example
Airflow = 1000 CFM (472 L/s), friction rate = 0.9 Pa/m Equal-friction → round duct ≈ 350 mm (14") Velocity ≈ 4.9 m/s ✓ within commercial-branch limit Rectangular equivalent ≈ 400 × 250 mm
Change the friction rate and every duct resizes — that's the beauty of the ductulator. Once sized, check total resistance with the duct pressure drop calculator.
FAQ
What friction rate should I use? 0.8–1.0 Pa/m (0.08–0.1 in.wg/100 ft) for comfort HVAC; lower it for very quiet spaces.
Round or rectangular duct? Round is more efficient and quieter for the same area; rectangular fits tight ceilings. Size round first, then convert.
How do I size ductwork quickly? Use the online ductulator — enter airflow and friction rate and read the size.