HVAC duct sizing calculator (velocity method)
Duct size is the first thing an MEP drafter needs and the last thing anyone writes down. The velocity method sizes a duct directly from the airflow it carries and the speed the specification allows — low velocity in occupied areas to keep noise down, higher in risers and plant rooms. This calculator gives the required area, the equivalent round diameter, and a rectangular option for the depth you have available in the ceiling void.
HVAC duct sizing calculator
- Round duct diameter
- 357mm
- Rectangular option
- 333 x 300 mm
- Cross-sectional area
- 0.1m²
- Airflow
- 1,059 CFM (1,800 m³/h)
- Velocity pressure
- 15Pa
- Aspect ratio
- 1.11:1 — acceptable
Formula
A = Q / v, d = √(4A/π), w = A / h, pv = 0.5ρv²Q in m³/s, v in m/s, air density taken as 1.2 kg/m³ at 20 °C. Round the diameter up to the next standard size (100, 125, 160, 200, 250, 315, 400, 500 mm) and keep rectangular aspect ratios at or below 4:1 to limit friction and material cost.
Worked examples
| Input | Result |
|---|---|
| 500 L/s at 5 m/s | 357 mm round → specify 400 mm, or 333 x 300 mm |
| 250 L/s branch at 3 m/s | 326 mm round → 315 mm, or 278 x 300 mm |
| 2000 L/s riser at 8 m/s | 564 mm round → 630 mm, or 833 x 300 mm |
Questions
What duct velocity should I design to?
Typical guidance: 2–3 m/s in branches serving occupied rooms, 4–6 m/s in main ducts, and 6–10 m/s in risers and plant rooms. Lower velocities cost more in sheet metal and save noise and fan energy.
Why does aspect ratio matter?
A flat duct has more surface area for the same airflow, so friction, heat loss and material cost all rise. Keep it at or below 4:1 wherever the void allows.
Is the velocity method enough on its own?
It sizes the duct. You still need a pressure-loss run — equal friction or static regain — along the index circuit to select the fan.
How do I convert CFM to L/s?
Divide CFM by 2.11888. 1000 CFM is about 472 L/s.
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