Duct Pressure Calculator

What is Duct Pressure Drop?

Duct pressure drop, also called static pressure loss, is the reduction in air pressure as air flows through ductwork due to friction and resistance. It's measured in inches of water column (in. w.c.) or Pascals (Pa). Understanding pressure drop is crucial for HVAC design because fans must overcome this resistance to move air effectively. Excessive pressure drop leads to reduced airflow, poor system performance, and increased energy costs. The pressure drop depends on duct size, length, material roughness, air velocity, and number of fittings.

Pressure Drop Formulas

• Friction Loss: ΔP = f × (L/D) × (ρ × V²/2)
• Velocity Pressure: Pv = (V/4005)² (for standard air)
• Total Pressure: ΔP_total = Friction Loss + Fitting Losses

Factors Affecting Pressure Drop

• Duct size: Smaller ducts increase pressure drop
• Air velocity: Higher velocity causes exponential increase
• Duct length: Longer runs accumulate more friction loss
• Material roughness: Rough surfaces increase friction
• Fittings: Each elbow/transition adds resistance

Design Tips

• Use larger ducts to reduce velocity and friction
• Minimize bends and fittings in duct runs
• Choose smooth materials over flexible ducts
• Keep total system pressure under fan rating
• Add 10-20% safety margin to calculations

Applications

• Fan selection and sizing
• HVAC system design optimization
• Troubleshooting airflow issues
• Energy efficiency analysis
• Ventilation system planning