Duct Friction Loss Calculator
Friction rate determines how much of the blower's available static each duct uses.
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Standard air, clean galvanized round duct; flex duct has far higher losses.
Formula
- ΔP/100 ft ≈ 0.109136 × CFM^1.9 ÷ D^5.02
Example calculation
400 CFM in 10 in round → about 0.077 inWC per 100 ft.
Practical HVAC use
Compare against the design friction rate used in Manual D.
Common mistakes
- Forgetting fitting equivalent lengths.
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Educational tool. Results are estimates from the stated formulas; always confirm with manufacturer data and the code adopted in your jurisdiction. All calculators