Air Ducts - Friction Loss Diagram

Friction loss (head loss) in standard air ducts:

Download and print Air Ducts - Friction Loss vs. Air Flow and Duct Size Chart

The diagram is based on standard air 0.075 lb/ft³ in clean round galvanized metal ducts.

• 1 inch water = 248.8 N/m²(Pa) = 0.0361 lb/in² (psi) = 25.4 kg/m² = 0.0739 in mercury
• 1 ft³/min (cfm) = 1.7 m³/h = 0.47 l/s
• 1 ft/min = 5.08×10^-3 m/s
• 1 inch = 25.4 mm = 2.54 cm = 0.0254 m = 0.08333 ft

Example - Friction Loss in Air Duct

The friction loss in a 20 inches duct with air flow 4000 cfm can be estimated to approximately 0.23 inches water per 100 feet duct as shown in the diagram above. The air velocity can be estimated to approximately 1850 feet per minute.