Duct Static Pressure Calculator
Work out the static pressure a duct run costs — the resistance the fan has to overcome. Enter the airflow and duct, and get the air velocity, friction rate and total static pressure, with a check against good-practice design ranges.
Static Pressure: What the Fan Fights
Static pressure is the resistance air meets as it moves through a duct — the push the fan or air handler must supply to keep it flowing. It is measured in inches of water gauge (in.wg) or pascals. Add up the static pressure of every duct run, fitting, filter and coil and you get the total external static pressure the blower has to overcome; if it exceeds the blower’s rating, airflow collapses. This tool finds the friction loss of a single duct run.
Friction Rate and Duct Sizing
Friction rate is the pressure lost per 100 feet of duct. Residential and light-commercial systems are usually designed around 0.08 to 0.10 in.wg per 100 ft. Too high and the duct is undersized — noisy, hungry for fan power; too low and it is oversized and wasteful. Reading the friction rate back from airflow and duct size is the quickest way to check whether a duct is the right size.
Velocity, Velocity Pressure and Total Pressure
Air velocity matters as much as pressure. Keep supply ducts roughly in the 600 to 1200 fpm range for quiet homes, higher in commercial trunks. Velocity pressure is the kinetic part of the air’s energy, VP = ½ρV²; total pressure is static plus velocity pressure. Fans are rated on the static pressure they can develop, which is why this calculator leads with it.
Fittings and Equivalent Length
Elbows, takeoffs and transitions add loss too. The simplest way to include them is equivalent length: each fitting behaves like a certain extra run of straight duct — often 10 to 50 diameters for an elbow. Add that to the straight length and the friction calculation captures the whole path.
Frequently Asked Questions
What is a good static pressure for a duct system?
Total external static pressure usually should stay within the blower rating, commonly around 0.5 in.wg for residential air handlers. Per-run friction rate is typically designed near 0.08 to 0.10 in.wg per 100 ft.
How do you calculate duct static pressure?
Find the air velocity from airflow and duct area, then apply the Darcy friction equation with a friction factor based on Reynolds number and duct roughness, multiplied by the run length plus fitting equivalent length.
What air velocity should a duct have?
About 600 to 1200 fpm for residential supply ducts, up to 1500 to 2000 fpm in commercial trunks. Above roughly 2000 fpm ducts get noisy.
Why convert a rectangular duct to a round equivalent?
Friction correlations are based on round ducts, so a rectangular duct is converted to the round diameter that gives the same friction and flow before the loss is computed.
Related calculators
- CFM & Air Changes Calculator — set the airflow first.
- Duct Size Calculator — pick a duct size for the flow.
- Pressure Drop Calculator — friction loss for liquids in pipe.
- Pipe Flow Calculator — velocity and flow in a duct or pipe.