Ventilation Duct Sizing

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

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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 to determine pressure losses in air distribution systems. The equivalent diameter method is used for rectangular ducts, 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
  • Equivalent diameter for rectangular ducts: Deq = 4A/P (hydraulic diameter)
  • 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:
  1. Enter all required parameters
  2. Click "Update Calculation"
  3. 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)