How to calculate duct pressure loss
Before you can pick a fan you need the friction loss of each duct run. Enter airflow in CFM and the rectangular duct size, and this tool finds the velocity, converts the rectangle to the round duct that loses the same pressure, and reports both the friction rate in inches of water gauge per 100 ft and the total loss for the run.
The equivalent round diameter uses the Huebscher relation De = 1.30 x (a x b)^0.625 / (a + b)^0.25, and the friction rate comes from Darcy-Weisbach with galvanized duct roughness of 0.0003 ft and standard air at 0.075 lb/ftĀ³, the same basis as the ASHRAE Fundamentals friction chart. Fittings are handled by the equivalent length method used in SMACNA practice, and this tool counts one smooth 90Ā° elbow as 15 equivalent diameters, which matches a loss coefficient of about 0.25.
Low-velocity commercial design usually targets a friction rate near 0.08 to 0.10 in. w.g. per 100 ft, with main duct velocity around 1,000 to 1,800 fpm and branches lower. Going much above that drives noise and fan power up quickly, so upsizing the duct is often cheaper than the extra static. Fitting losses vary widely with radius, takeoff angle, and turning vanes, so check manufacturer or SMACNA tables for unusual fittings and always size the fan on the worst path in the system, with a margin confirmed by the design engineer.
Frequently Asked Questions
Equivalent length converts each fitting into a length of straight duct and is quick for early sizing. The loss coefficient method multiplies velocity pressure by a C value and is more accurate for final calculations and unusual fittings.
No. Add the losses of filters, coils, grilles, and diffusers plus discharge velocity pressure to this duct loss. Most designers then add a 10 to 15 percent margin before selecting the fan.