How do you calculate duct size from flow rate?

Use the formula d = √(4Q / πV) where Q is the flow rate in m³/s and V is the velocity in m/s. Then round up to the nearest standard duct size per DW144.

What is a typical duct velocity for supply air?

Typical supply duct velocities are 5-8 m/s for main ducts and 3-5 m/s for branch ducts, according to CIBSE Guide B.

Duct Sizing Calculator — Circular & Rectangular

Flow rate

Velocity (m/s)

Required diameter—

Nearest standard size—

Actual velocity—

Cross-sectional area—

Formula: d = √(4Q / πV)

Standard circular duct sizes per DW144 (BESA specification for sheet metal ductwork)

Flow rate

Velocity (m/s)

Aspect ratio

Fix width (mm)

Required area—

Suggested dimensions—

Equivalent circular diameter—

Actual velocity—

Formula: A = Q / V d_eq = 1.3(ab)^0.625 / (a+b)^0.25

Equivalent diameter formula from CIBSE Guide C, Section C4

Common duct velocities

Typical values from CIBSE Guide B, Table B2.1

Application	Typical Velocity (m/s)
Supply duct (main)	5 – 8
Supply duct (branch)	3 – 5
Extract duct (main)	5 – 9
Extract duct (branch)	3 – 5
Fresh air intake	2.5 – 4
Kitchen extract	7 – 10

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Common duct velocities

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