Formula & worked example
ΔP/L=f·(ρV²/2)/D; Re=ρVD/μ; f=64/Re laminar, otherwise Swamee–Jain.
1,000 CFM through a 12 in round duct at standard air conditions reports velocity, Reynolds number, friction factor and pressure loss per 100 ft.
HVAC engineering calculator · Updated Sep 17, 2026
Duct Friction Rate Calculator — Pressure Loss, Reynolds & Equal-Friction Check
Calculate straight round-duct friction rate with Darcy-Weisbach, Reynolds number and Swamee-Jain friction factor, plus total run loss and common HVAC units.
Inputs
Live result
Velocity 6.468057 m/s (1,273.2396 fpm)
Reynolds number 131,140.46
Flow regime turbulent
Darcy friction factor 0.018728211
Friction rate 1.5474816 Pa/m
Friction rate 0.18935905 in.w.g./100 ft
Straight-run pressure loss 47.167239 Pa (0.18935905 in.w.g.)
Assumptions & limits
- Straight round duct is modeled; fittings and components are separate losses.
- Roughness, density and dynamic viscosity are explicit inputs so material and air-condition assumptions remain auditable.
Frequently asked questions
What does this Duct Friction Rate Calculator calculate?
Calculate straight round-duct friction rate with Darcy-Weisbach, Reynolds number and Swamee-Jain friction factor, plus total run loss and common HVAC units.
Why are the assumptions shown?
HVAC results depend on air properties, measured conditions and design conventions. MAXScanner exposes the governing assumptions so the result can be checked instead of hiding them behind a single number.
Can this replace final HVAC design?
No. Use it for transparent sizing and cross-checking, then verify final selections against measured conditions, equipment data and applicable design standards.