Ventilation Duct Sizing
Input Data
Enter your project data
System Settings
ASHRAE velocity limits based on occupancy
Galv. steel: 0.045 | Aluminium: 0.0015 | Flex duct: 0.9
Air Conditions
0 = auto
Duct Segments
Calculation Results
Calculated instantly from your inputs
Ventilation Duct Sizing
Proper duct sizing is critical for energy-efficient HVAC systems. This calculator uses the Darcy-Weisbach equation with the Colebrook-White friction factor (Swamee-Jain explicit form) to determine pressure losses in air distribution systems. Rectangular ducts are analysed with the hydraulic diameter (4A/P) and the actual duct velocity, and all calculations follow ASHRAE Fundamentals guidelines. Pressure losses include both friction (along duct length) and dynamic losses (fittings).
Key Points:
- Darcy-Weisbach equation is the most accurate method for duct pressure drop calculation
- Rectangular ducts: friction uses hydraulic diameter Dh = 4A/P; Huebscher Deq is the equivalent round size
- Swamee-Jain approximation provides explicit friction factor without iteration
- ASHRAE velocity limits: Residential 2.5-4 m/s, Commercial 4-6 m/s, Industrial 6-10 m/s
- Fitting losses can exceed friction losses in systems with many bends and branches
- Air density decreases with altitude and increases with temperature decrease
- Noise level increases with velocity - NC rating should be checked for occupied spaces
Calculation Steps:
- Enter all required parameters
- Click "Update Calculation"
- Review results and analysis
Formula Variables:
ΔP
Pressure Loss
(Pa)
f
Friction Factor
(-)
Dh
Hydraulic Diameter
(m)
Re
Reynolds Number
(-)
V
Air Velocity
(m/s)
ρ
Air Density
(kg/m³)
Q
Flow Rate
(m³/s)