Work out the round duct diameter — or the width of a rectangular duct — needed to carry a required airflow at a sensible design velocity.
Round up to the next standard duct size. Real duct sizing also balances velocity, pressure loss and noise across the whole system — use a friction chart or design software for the full job.
traqR keeps duct schedules, quotes and job records together so what you size ends up on the material list and the invoice.
Start free trialDividing the required airflow by the target velocity gives the cross-sectional area the duct must have.
For a round duct, the diameter is √(4 × area ÷ π). For a rectangular duct with a fixed height, the width is the area divided by that height.
Duct comes in standard sizes, so round the calculated dimension up to the next available size and re-check the resulting velocity.
Pick a target air velocity, then divide the required airflow by it to get the duct area, and convert that area to a round diameter or rectangular size. For 300 L/s at 4 m/s you need about 0.075 m², which is roughly a 310 mm round duct. Then round up to a standard size.
Lower velocities mean quieter systems and less fan energy but larger ducts; higher velocities save space at the cost of noise and pressure. Branch ducts in occupied spaces are often designed around 3–5 m/s, with main ducts a little higher, but it depends on the application.
Round duct is more efficient for airflow and easier to seal, while rectangular duct fits better in tight ceiling spaces. This tool sizes a round duct, or the width of a rectangular duct for a height you choose, so you can compare.
No. It sizes a single duct for a target velocity. A complete design balances velocity, pressure loss and noise across the whole system using friction charts or software, to a standard such as AS 1668.
This calculator sizes a single duct from flow and a target velocity and is a general aid only. Full duct design must balance velocity, pressure loss and noise across the system to the relevant standards, by a competent HVAC designer.
traqR keeps duct schedules, quotes and jobs together so what you size ends up on the invoice. Try it free for 14 days.