Equal-Friction Duct Design Explained
Equal friction is the default sizing method for commercial low-pressure ductwork: pick one friction rate, then size every section to that rate. This guide walks the method, builds a full reducing-trunk schedule at 0.10 in. w.g. per 100 ft, and is explicit about what the method does not do for you.
Last updated September 2026
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The method in four steps
- Establish the airflow in every section by totalling the terminals downstream of it.
- Choose a single design friction rate for the system — ideally derived from the available fan static pressure divided by the effective length of the critical path.
- Size each section to the diameter (or equivalent diameter) that produces that friction rate at its airflow, then round up to the next stock size.
- Total the critical path — straight duct plus fittings — and confirm it fits inside the fan's available static pressure. Iterate the rate if it does not.
Worked reducing trunk at 0.10 in. w.g./100 ft
| Section | Airflow | Exact size | Stock size | Velocity | Actual rate |
|---|---|---|---|---|---|
| Trunk A — unit to first tee | 8,000 CFM | 29.9 in | 30 in | 1,630 fpm | 0.099 |
| Trunk B — after first tee | 6,000 CFM | 26.8 in | 28 in | 1,403 fpm | 0.081 |
| Trunk C — after second tee | 4,000 CFM | 23.0 in | 24 in | 1,273 fpm | 0.081 |
| Trunk D — final trunk section | 2,000 CFM | 17.7 in | 18 in | 1,132 fpm | 0.092 |
| Branch to VAV box | 800 CFM | 12.6 in | 14 in | 748 fpm | 0.059 |
Spiral galvanized round duct, 70 °F standard air. Sizes rounded up to the next stock diameter, which is why the actual rate is at or below 0.100.
Note what equal friction does automatically: velocity falls as the trunk reduces, from around 1,400 FPM at the unit down toward branch velocities at the far end. That self-tapering behaviour is the method's main attraction — it produces acoustically sane systems without the designer assigning velocities section by section.
Build your own schedule in the duct sizer, convert any section to a rectangular size in the rectangular to round converter, and read how to choose the friction rate first.
Where equal friction falls short
| Situation | What goes wrong | What to do instead |
|---|---|---|
| Very long supply mains at high velocity | Regain in the downstream sections is ignored, so the far end is oversized in pressure terms | Static regain sizing for the main, equal friction for the branches |
| Wildly unequal branch lengths | Near branches over-deliver badly; balancing wastes the fan energy you saved | Reduce the near-branch sizes deliberately or use a velocity-reduction pass |
| Exhaust with particulate | A minimum transport velocity must be held; equal friction may let velocity fall below it | Size to the transport velocity requirement, then check the friction |
| Very short systems | The fittings dominate so completely that the friction rate barely matters | Size on velocity and total the fittings directly |
Method and assumptions
- Method
- Equal-friction sizing with Darcy-Weisbach / Altshul-Tsal friction; ASHRAE Fundamentals Chapter 21 basis.
- Design rate used above
- 0.10 in. w.g. per 100 ft, chosen only to demonstrate the method.
- Material and air
- Spiral galvanized steel, ε = 0.09 mm, 70 °F at sea level.
- Rounding
- Exact diameters rounded up to the next standard stock size, so actual friction rates land at or below the design rate.
Limitations of this method
What the calculation on this page does not account for. Read these before using a number on a drawing or a submittal.
- Equal friction equalises loss per foot, not total loss per path. Every real system still needs balancing dampers, and the balancing loss is not in this schedule.
- The schedule above contains no fittings. Fitting losses on the critical path routinely equal or exceed the straight-duct loss and must be added before selecting a fan.
- Rounding up to stock sizes accumulates. A trunk of many short sections each rounded up will deliver less total loss than designed, shifting the balance point between branches.
- The method assumes constant volume in each section. Variable-volume systems need the sizing checked at both design and turndown conditions, where velocity and regenerated noise both change.
- No acoustic, thermal, structural, fire, smoke or code check is implied. This is preliminary sizing; a qualified engineer must complete and stamp the design.
Frequently asked questions
What is equal-friction duct design?
Equal-friction design sizes every duct in a system so the pressure loss per 100 feet is the same throughout. You choose one friction rate, then pick the diameter that produces that rate at each section's airflow.
What are the alternatives to equal friction?
Velocity reduction, where you assign a target velocity to each section, and static regain, where downstream sections are sized so the pressure recovered by slowing the air offsets the friction in the next run. Static regain suits long, high-velocity supply mains; equal friction suits most commercial low-pressure work.
Why do equal-friction systems need balancing dampers?
Because equal friction equalises loss per foot, not total loss per path. A short branch near the fan sees far less total resistance than the far branch, so it takes more than its share of the air until a damper adds the difference.