Duct Sizing Calculator
Calculate the required duct diameter for a given airflow rate and maximum velocity.
Required Circular Duct Diameter
252 mm
Required Circular Duct Diameter vs Airflow Rate (Q)
Formule
## HVAC Duct Sizing Duct size is determined by the airflow requirement and an acceptable air velocity. ### Formula **A = Q / V** (required area) **D = sqrt(4Q / (pi V))** (circular duct diameter) where Q is airflow in m^3/s and V is maximum velocity in m/s. Higher velocity means smaller ducts but more noise and pressure drop.
Exemple Résolu
300 L/s airflow with 6 m/s maximum velocity.
- 01Q = 300 L/s = 0.30 m^3/s
- 02A = 0.30 / 6 = 0.05 m^2 = 50,000 mm^2
- 03D = sqrt(4 x 0.30 / (pi x 6)) x 1000 = sqrt(0.0637) x 1000 = 252 mm
- 04Round up to next standard size: 250 or 300 mm duct.
Questions Fréquentes
What air velocity is recommended for ducts?
Main supply ducts: 5-8 m/s. Branch ducts: 3-5 m/s. Near grilles/diffusers: 2-3 m/s. Return air: 3-5 m/s. Higher velocities save on duct size but increase noise and fan power.
What are standard round duct sizes?
Common diameters: 100, 125, 150, 200, 250, 300, 350, 400, 450, 500, 600, 700, 800 mm. Select the next standard size up from the calculated diameter.
How do I convert to rectangular duct?
For an equivalent rectangular duct, keep the same cross-sectional area. For a rectangular duct a x b with the same friction as a circular duct D, use the hydraulic diameter formula: D_eq = 1.3 (ab)^0.625 / (a+b)^0.25.
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