Duct Sizing Calculator Formula

Understand the math behind the duct sizing calculator. Each variable explained with a worked example.

Use the Duct Sizing Calculator

Formulas Used

Minimum Duct Area

min_area = q_m3s / max_velocity * 1e6

Required Circular Duct Diameter

duct_diameter = sqrt(4 * q_m3s / (pi * max_velocity)) * 1000

Actual Velocity at Selected Size

actual_velocity = max_velocity

Variables

VariableDescriptionDefault
airflowAirflow Rate (Q)(L/s)300
max_velocityMaximum Air Velocity(m/s)6
q_m3sDerived value= airflow / 1000calculated

How It Works

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.

Worked Example

300 L/s airflow with 6 m/s maximum velocity.

airflow = 300max_velocity = 6
  1. 01Q = 300 L/s = 0.30 m^3/s
  2. 02A = 0.30 / 6 = 0.05 m^2 = 50,000 mm^2
  3. 03D = sqrt(4 x 0.30 / (pi x 6)) x 1000 = sqrt(0.0637) x 1000 = 252 mm
  4. 04Round up to next standard size: 250 or 300 mm duct.

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