Duct Size Calculator
Calculate the round duct diameter needed from airflow and air velocity for HVAC design.
Sizing estimates are for preliminary planning only. Final duct design must account for pressure drop, fittings, noise criteria, and local HVAC codes. Consult a qualified mechanical engineer or HVAC professional before installation.
How it works
- 1Enter the design airflow in m³/h (or CFM) and choose an air velocity.
- 2The calculator divides airflow by velocity to find the required duct area.
- 3It then converts the area to an equivalent round duct diameter, d = √(4A ÷ π).
Use cases
- Sizing residential supply and return ducts for low-noise operation (3–5 m/s).
- Designing commercial trunk ducts where 5–10 m/s velocities are acceptable.
- Checking whether an existing duct can carry a new airflow without noise.
Frequently asked questions
What air velocity should I use?
For quiet residential ducts, 3–5 m/s (600–1000 ft/min); commercial trunks run at 5–10 m/s where noise matters less.
How is the round diameter calculated?
Area A = Q ÷ v, then d = √(4A ÷ π). 1000 m³/h (0.278 m³/s) at 5 m/s gives A = 0.0556 m² and d ≈ 266 mm.
Does it account for pressure drop or fittings?
No — it sizes the straight duct only. Add pressure drop for elbows, tees, and diffusers, and verify the total with a full friction calculation.
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