Mechanical

Duct Sizing Calculator

Size or check circular and rectangular ducts from airflow and velocity.

Use this calculator for preliminary duct sizing and checking based on airflow, target velocity and duct geometry. It can be used either to size a new duct from airflow and target velocity, or to check velocity in an existing duct.

Inputs

Calculation Mode

Moderate velocity used for general branch distribution.

Circular sizing selects the next standard diameter up. Rectangular sizing rounds width and height up to the nearest 50 mm.

Result

Duct Size
-
Enter values and click calculate.

Method

Required duct area is determined from airflow and design velocity:

A = Q ÷ v

For circular ducts:

d = √(4A ÷ π)

For rectangular ducts:

A = W × H

Velocity pressure may be estimated from:

Pᵥ = 0.5 × ρ × v²
  • A = duct area (m²)
  • Q = airflow rate (m³/s)
  • v = air velocity (m/s)
  • d = circular duct diameter (m)
  • W = rectangular duct width (m)
  • H = rectangular duct height (m)
  • Pᵥ = velocity pressure (Pa)
  • Calculated result = -

Reference

  • An air density of 1.2 kg/m³ is assumed for velocity pressure checks.
  • Duct sizing is commonly undertaken using guidance contained within CIBSE Guide B2, CIBSE Guide C and methods presented in CIBSE Guide C4.
  • The velocity presets in this calculator represent indicative design conventions only, used in practice to balance duct size, noise and pressure drop.
  • Where applicable, final design should align with Approved Document F, project specifications, ductwork standards and manufacturer requirements.
Design note: This calculator provides an indicative duct size or velocity check only. Final design should consider noise criteria, pressure drop, space constraints, fan selection, duct classification, leakage and project-specific requirements.

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