Duct Static Pressure Calculator: Friction Loss per 100 ft

Static pressure is what the blower has to push against. Each run adds friction in straight duct, which depends on airflow, size and material, plus a penalty for every elbow, tee and transition. The easy way to count the penalty is as extra straight duct, the equivalent length. Add the run, the coil and the filter and you have the total external static pressure (TESP) to compare with the blower rating.

Duct static pressure and friction loss of a run

Units Airflow Airflow unit Shape Diameter (in) Material Straight length (ft) 90° elbows, smooth (count) 90° elbows, mitered (count) 45° elbows (count) Tees, straight through (count) Tees, branch (count) Transitions, gradual (count) Transitions, abrupt (count)

Friction rate: 0.0887 in.wc/100 ft
Velocity: 935 fpm
Equivalent diameter (rectangular): 14.0 in
Equivalent length of fittings: 87.5 ft
Total length: 137.5 ft
Run static pressure (in.wc): 0.1220 in.wc
Run static pressure (Pa): 30.35 Pa

Total external static pressure (TESP) (in.wc)

Supply ducts Return ducts Coil Filter Blower rating at this airflow

Total: 0.480 in.wc
Margin to the rating: +0.020 in.wc
Share of the rating: 96 %
Verdict: Close to the blower's limit

Showing the worked example. Turn on JavaScript to calculate your own numbers.

Standard air (1.2 kg/m³ density), Darcy-Weisbach friction with the Colebrook-White factor. Rectangular ducts use the Huebscher equivalent diameter De = 1.30·(a·b)^0.625 ÷ (a+b)^0.25 (ASHRAE Fundamentals) for friction, and the real velocity Q ÷ (a·b). Fittings are added as equivalent length, count × (Le/D) × De; the Le/D values are rounded planning estimates from ASHRAE fitting loss coefficients, not a design or code substitute. Check the blower's fan table and your local code.

How do you calculate duct static pressure in the app?

Open the Airflow tab and tap the Static pressure card.
Type the Airflow, for example 1000 CFM, and pick Round duct or Rectangular duct.
Enter the Diameter, or the Width and Height, then choose the Material and the Straight length.
Set the Fittings steppers: 90° elbow, smooth, 90° elbow, mitered, 45° elbow, Tee, straight through, Tee, branch, Transition, gradual and Transition, abrupt.
Read Friction rate, Velocity, Equivalent length of fittings, Total length and Run static pressure.
In the TESP card tap Use the run for supply (or Use the run for return), then fill Coil, Filter and Blower rating and read Total, Margin and the verdict.

What static pressure do these runs lose?

Standard air, Colebrook friction, equivalent-length fittings
Run	Friction rate	Velocity	Run static pressure
1,000 CFM, 14 in round galvanized, 50 ft + 3 smooth 90° elbows, 1 branch tee, 1 gradual transition (fittings 87.5 ft, total 137.5 ft)	0.0887 in.wc/100ft (0.724 Pa/m)	935 fpm	0.122 in.wc (30.4 Pa)
400 CFM, 8 in round galvanized	0.2563 in.wc/100ft	1,146 fpm	-
2,000 CFM, 20 x 10 in rectangular, 60 ft (equivalent diameter 15.23 in)	0.2121 in.wc/100ft	1,440 fpm	0.127 in.wc (31.7 Pa)
500 L/s, 315 mm round galvanized, 20 m (metric)	1.464 Pa/m	6.42 m/s	29.3 Pa

Each fitting is converted to straight duct. A smooth 90° elbow counts as 10 diameters of duct, a branch tee as 40 and a gradual transition as 5, so the 14 in example adds 3 x 10 + 40 + 5 = 75 diameters, which is 87.5 ft. Rectangular ducts use the equivalent diameter for friction but the real air speed, which is why the 20 x 10 in duct reads 1,440 fpm.

How do you add up total external static pressure?

Add the supply ducts, the return ducts, the coil and the filter. Say supply 0.12, return 0.06, coil 0.20 and filter 0.10 in.wc: the total is 0.48 in.wc. Against a blower rated 0.50 in.wc at that airflow that is 96 %, and the app shows Close to the blower's limit. Under 90 % it reads Within the blower's rating and over 100 % Over the blower's rating.

Common mistakes

Counting only the straight length and forgetting the fittings. In the example they add 87.5 ft to a 50 ft run.
Using the nominal blower rating at the wrong speed tap. The rating that matters is the one at your airflow.
Leaving the material on Galv. steel for a Flex duct run. Flex is rougher, so its friction rate is clearly higher.
Mixing units. The app converts when you switch between imperial and metric, but check that the pressures you type in the TESP card use the same unit.

The run is the single longest supply or return path. Sizing every branch, balancing and the room-by-room design belong to a full duct design.

Questions and answers

How do I calculate duct static pressure?

Multiply the friction rate by the total length of the run, where the total length is the straight duct plus the equivalent length of the fittings. At 0.0887 in.wc/100ft and 137.5 ft the run is 0.122 in.wc.

What is total external static pressure?

It is the pressure the blower works against outside the equipment: supply ducts, return ducts, coil and filter added together. Compare it with the blower rating at your airflow.

What is a good friction loss per 100 ft?

The Reference tab lists 0.08 to 0.10 in.wc/100ft for supply ducts and 0.05 to 0.08 for returns as starting points. The run static pressure then tells you if the total fits the blower.

How to calculate duct static pressure and TESP