Rectangular Duct Flow and Hydraulic Diameter Calculator
For a rectangular duct or channel, find the velocity from the flow, the hydraulic diameter used in friction calculations, the equivalent round duct diameter and the Reynolds number.
- Dh = 4A ÷ P = 2ab ÷ (a + b) = 480 mm
- De = 1.30 (ab)0.625 ÷ (a + b)0.25 = 532.8 mm
- v = Q ÷ A = 5.787 m/s
Duct Equations
De = 1.30 × (ab)0.625 ÷ (a + b)0.25
Re = ρ v Dh ÷ µ
a and b are the inside width and height, A the area, P the wetted perimeter, Dh the hydraulic diameter used in Reynolds number and friction factor, and De the round duct diameter with the same friction loss at the same flow (Huebscher equation, used in HVAC duct design).
Worked example
A 600 × 400 mm duct carrying 5000 m³/h of air has a velocity of 5.79 m/s, a hydraulic diameter of 480 mm and an equivalent round size of about 532 mm.
Frequently Asked Questions
What is hydraulic diameter?
Four times the flow area divided by the wetted perimeter. It lets round pipe formulas be used for non circular ducts.
Is equivalent diameter the same as hydraulic diameter?
No. The equivalent diameter gives the same friction loss at the same flow, which is larger than the hydraulic diameter for rectangles.
What velocity is normal in HVAC ducts?
Typically about 3 to 8 m/s for supply ducts in buildings; higher in industrial systems.
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