Commercial Duct Sizing Examples
Five reusable worked examples with the inputs, the output, the design interpretation and what changes when one variable moves: a 10,000 CFM office trunk, a VAV branch, a kitchen exhaust run, a before/after elbow swap, and a round to rectangular conversion.
Last updated September 2026
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Example 1 — 10,000 CFM office supply trunk
Inputs: 10,000 CFM, 200 ft of straight trunk, spiral galvanized duct, 70 °F, sea level. Question: what does the choice of friction rate cost?
| Design rate | Exact size | Stock size | Velocity | Loss over 200 ft |
|---|---|---|---|---|
| 0.08 in. w.g./100 ft | 34.0 in | 36 in | 1,415 fpm | 0.121 |
| 0.10 in. w.g./100 ft | 32.6 in | 34 in | 1,586 fpm | 0.161 |
| 0.15 in. w.g./100 ft | 30.0 in | 32 in | 1,790 fpm | 0.218 |
Interpretation. Moving from 0.08 to 0.15 in. w.g./100 ft saves several inches of diameter and a meaningful amount of metal, but pushes velocity up and roughly doubles the straight-duct loss over the run. In an open-plan office, the 0.08 to 0.10 band keeps trunk velocity near 1,400 FPM and avoids attenuation; in a warehouse the 0.15 option is the better commercial decision.
Example 2 — branch duct serving a VAV box
Inputs: 1,200 CFM branch, 12 in round, 25 ft of duct, conical take-off (C 0.5), two smooth elbows (C 0.15 each), open volume damper (C 0.2), VAV box 0.25 in. w.g.
| Item | Value |
|---|---|
| Branch velocity | 1,528 fpm |
| Velocity pressure | 0.145 in. w.g. |
| Straight duct, 25 ft | 0.066 |
| Fittings, ΣC = 1.0 | 0.145 |
| VAV box | 0.250 |
| Branch total | 0.461 |
What changes if it is flexible duct? The same 12 in at 70% extension has a friction rate of 0.602 against 0.263 in spiral steel — roughly 2.3 times the loss per foot. Details are in the flex versus rigid guide.
Example 3 — kitchen or laboratory exhaust run
Inputs: 3,000 CFM exhaust, welded duct, minimum transport velocity 1,500 FPM. Here the velocity requirement drives the size and the friction rate is simply whatever falls out.
| Size | Velocity | Friction rate | Verdict |
|---|---|---|---|
| 14 in round | 2,806 fpm | 0.692 | Meets a 1,500 FPM transport minimum |
| 16 in round | 2,149 fpm | 0.354 | Meets a 1,500 FPM transport minimum |
| 18 in round | 1,698 fpm | 0.197 | Meets a 1,500 FPM transport minimum |
Interpretation. Sizing this run at a comfortable 0.10 in. w.g./100 ft would give a larger duct and drop the velocity below the transport minimum — acceptable on a supply trunk, a code and maintenance problem on grease or contaminated exhaust. Size on velocity first, then accept and budget the higher friction rate.
Example 4 — before and after: mitred versus smooth elbow
Inputs: the 36 in trunk from Example 1, carrying 10,000 CFM at 1,415 FPM, velocity pressure 0.125 in. w.g. Four 90° turns on the critical path.
| Elbow type | C | Four elbows (in. w.g.) | Change |
|---|---|---|---|
| Mitred, no vanes | 1.20 | 0.598 | Baseline |
| Mitred with turning vanes | 0.30 | 0.149 | −0.448 |
| 3-piece segmented | 0.34 | 0.169 | −0.428 |
| Smooth radius, r/D 1.5 | 0.15 | 0.075 | −0.523 |
Interpretation. Changing nothing but the elbow detail recovers 0.523 in. w.g. on this path — comparable to removing a long stretch of duct, and usually cheaper than upsizing the trunk. Price your own swap in the fitting loss calculator.
Example 5 — round to rectangular conversion
Inputs: the 10,000 CFM trunk again, but the ceiling void will not take a 36 in round. Three rectangular candidates carrying the same air:
| Section | Aspect ratio | Equivalent diameter | Actual velocity | Friction rate | Perimeter per ft |
|---|---|---|---|---|---|
| 44 × 20 in | 2.2:1 | 31.8 in | 1,636 fpm | 0.112 | 10.7 ft |
| 36 × 24 in | 1.5:1 | 32.0 in | 1,667 fpm | 0.109 | 10.0 ft |
| 60 × 14 in | 4.3:1 | 29.8 in | 1,714 fpm | 0.155 | 12.3 ft |
Perimeter per foot of run is a direct proxy for sheet metal cost, insulation area and hanger loading.
Interpretation. All three move the same air, but the 60 × 14 option at over 4:1 uses substantially more metal per foot and loses more pressure than the near-square 36 × 24. Take the flattest section the void genuinely forces, not the flattest that fits. Convert your own section in the rectangular to round converter, and the background is in round versus rectangular sizing.
Method and assumptions
- Method
- Darcy-Weisbach with the Altshul-Tsal friction factor; fittings by the C × Pv loss coefficient method; Huebscher equivalent diameter for rectangular sections.
- Air properties
- 70 °F standard air at sea level throughout.
- Material
- Spiral galvanized steel, ε = 0.09 mm, except where flexible duct is stated (ε = 6 mm at 70% extension).
- Rounding
- Round sizes rounded up to the next standard stock diameter; pressures shown to three decimals.
- Terminal data
- VAV box and terminal losses are representative figures, not manufacturer selections.
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.
- These are teaching examples with clean, round numbers. Real projects bring elevation, temperature, lining, leakage class and coordination constraints that change every figure.
- None of the examples include the full critical path, the return side, or the fan's own system effect — they size sections, not systems.
- Component losses quoted for VAV boxes, dampers and terminals are illustrative. Use the scheduled equipment's published data.
- Exhaust transport velocities vary by contaminant and by jurisdiction. The 1,500 FPM figure used here is illustrative and does not substitute for the applicable code or standard.
- Everything here is preliminary design guidance. Final duct sizing, construction and installation must be performed by qualified professionals and comply with applicable codes and standards.
Frequently asked questions
What size duct do I need for 10,000 CFM?
At a design friction rate of 0.10 in. w.g. per 100 ft in spiral galvanized duct, 10,000 CFM needs roughly a 34 to 36 inch round duct, running near 1,400 to 1,600 FPM. A 44 by 20 inch rectangular section is the equivalent if headroom rules out round.
How do I size a branch to a VAV box?
Size the branch on velocity rather than friction rate — commonly 800 to 1,200 FPM for the branch and under 800 FPM in the flexible runout — then confirm the branch pressure loss including the take-off and box against the trunk static available at that point.
Why do kitchen and lab exhaust runs get sized differently?
They must hold a minimum transport velocity so grease or contaminants stay entrained, typically 1,500 to 2,000 FPM or more. That velocity requirement sets the size; the resulting friction rate is an outcome, not a design choice.