Commercial Duct Friction Loss Calculator
A fast commercial duct sizing tool and RTU air velocity calculator: enter CFM and a friction rate to get round equivalent diameter, rectangular duct dimensions, air velocity (FPM), and velocity pressure for field and design work.
in. w.g. per 100 ft
Calculations use the ASHRAE-derived approximation for round duct diameter at a given friction rate and equivalent rectangular dimensions. Always verify against local codes and project specifications.
No saved runs yet. Calculate a run, then save it.
- No sign-up required for calculations.
- Works on desktops, tablets, and mobile.
- Engineer-friendly friction rate sizing.
Quick Cost Estimator
Auto-detected from your ZIP code; change it any time.
Estimated installed cost — U.S. National Average
$140,000 – $220,000
$14.00 – $22.00 per sq ft
Rooftop Units (RTUs) with medium ducts (12"–24"). Regional index 1.00× (Baseline mechanical cost index); project-size factor 1.00×. Rough planning figure only — request a formal quote below.
Worked example: a 4,000 CFM office supply trunk
A rooftop unit serves an open-plan office floor with a 4,000 CFM supply trunk. The designer has 0.08 in. w.g. per 100 ft of friction rate available for the trunk after reserving static pressure for the VAV boxes, diffusers, and fittings. Entering 4,000 CFM at 0.08 in. w.g./100 ft with a 1.4:1 rectangular aspect ratio, the calculator returns a rounded-up round duct diameter of 24 inches at 1,273 FPM with a velocity pressure of 0.101 in. w.g.
That velocity falls inside the 1,200–1,800 FPM range commonly used for commercial supply trunks, but the project's acoustic criteria still need checking. If ceiling depth forces a rectangular duct, the 1.4:1 aspect-ratio setting gives preliminary dimensions of 27 × 19 in. from the same equivalent-diameter calculation. Both dimensions are rounded up to whole inches, so the final width-to-height ratio may differ from the requested 1.4:1; verify the selected rectangular duct's velocity and friction loss separately.
Before committing the size to a drawing, run the critical path through the fan static pressure calculator to confirm the fan can deliver 4,000 CFM against the full system total, and check the trunk's elbows and take-offs in the fitting loss calculator — fittings on the first 20 feet after the unit often add more loss than the straight duct itself.
Duct sizing and friction loss FAQ
How does this commercial duct friction loss calculator work?
Enter design airflow in CFM and a target friction rate in inches of water gauge per 100 feet. The tool returns equivalent round diameter, rectangular duct dimensions, air velocity in FPM, and velocity pressure using the ASHRAE-derived friction chart approximation.
What friction rate should I use for commercial duct sizing?
Most commercial low-pressure supply systems are sized between 0.06 and 0.10 in. w.g. per 100 ft. Lower rates reduce fan energy and noise; higher rates save sheet metal and space.
Can I use it as an RTU air velocity calculator?
Yes. Enter the rooftop unit's design CFM and your friction target to get duct velocity in FPM. Keep main trunks near 1,200-1,800 FPM and branches under about 1,000 FPM to control noise.
Is this commercial duct sizing tool free?
Yes, it is free and requires no sign-up. It is intended for rapid field calculations by engineers, HVAC technicians, and facility managers.
This calculator is for preliminary design guidance only. Final duct sizing, construction, and installation must be performed by qualified professionals and comply with applicable codes and standards.