4,44 m/s
Design limit: 6 m/s
AIR DISTRIBUTION
Duct sizing calculatorDuct sizing
Find the right duct size for your airflow. Clear calculations, practical choices.
Design inputs
Starting values from the sample presets; adjust to suit your design.
m/s
Pa/m
mm
mm
m
°C
ⓘ Internal dimensions. Dry air at 101,325 Pa.
RECOMMENDED DUCT SIZEWithin limits
500 × 300 mm
Rectangular duct · Galvanized steel
Area 0,150 m²Equivalent Ø 420 mmAspect ratio 1,67 : 1
0,62 Pa/m
Design limit: 1 Pa/m
6,2 Pa
Straight duct friction only
Operating characteristics
Velocity vs. airflowRe = 110.000Friction factor f = 0,020
Calculation method & assumptions ＋
V = Q/A; Dh = 4A/P; Re = ρVDh/μ. Friction loss R = fρV²/(2Dh). The Darcy friction factor is solved iteratively with Colebrook for turbulent flow. Laminar flow uses f = 64/Re (an approximation for rectangular ducts); Re between 2,300 and 4,000 triggers a transitional-flow warning.
For rectangular ducts, De = 1.30(ab)^0.625/(a+b)^0.25. De is for equivalent round duct comparison only; friction calculations use hydraulic diameter and actual velocity. Air density follows the ideal gas law and viscosity follows Sutherland’s law. Up to 30 qualifying sizes closest to the maximum friction loss are retained. Dimensions range from 100 to 2,000 mm in 50 mm increments, with an aspect ratio no greater than 4:1.
Velocity and friction limits are user-defined design inputs, not universal standards. The standard and smoke extract presets reproduce the supplied sample settings. Results cover straight duct friction and exclude elbows, tees, terminals, equipment and other local losses. ASHRAE — Duct Design reference.