Fabric duct calculator — inlet velocity and diameter
Fabric duct is sized on one number above all others: the velocity of the air entering it. Too fast and the fabric ripples, breathes and wears at the inlet; too slow and the duct is bigger than it needs to be. Enter the airflow and the inlet condition, and this returns the minimum diameter, the next practical size, and the inlet velocity and velocity pressure that size produces.
Fabric duct diameter, round
26 in ø
Minimum 24.7 in at 1,200 fpm (6.10 m/s) · inlet velocity at this size 1,085 fpm (5.51 m/s)
| Inlet velocity at the selected size | 1,085 fpm (5.51 m/s) |
| Velocity limit applied | 1,200 fpm (6.10 m/s) |
| Inlet cross-sectional area | 0.343 m² · 3.69 ft² |
| Velocity pressure at the inlet | 18.6 Pa · 0.075 in.wg |
| Airflow released per unit length (average) | 50.0 CFM per ft |
| Air density used | 1.2229 kg/m³ |
| Inlet limit | Round diameter | Actual inlet velocity |
|---|---|---|
| 1500 fpm | 24 in | 1,273 fpm (6.47 m/s) |
| 1200 fpm | 26 in | 1,085 fpm (5.51 m/s) |
| 1000 fpm | 28 in | 935 fpm (4.75 m/s) |
| 800 fpm | 32 in | 716 fpm (3.64 m/s) |
Fabric ducts are typically designed to run at a static pressure somewhere between 0.15 and 1.6 in.wg (40–400 Pa), with many systems near 0.5 in.wg (about 120 Pa) — enough to keep the fabric inflated and drive air through the weave and orifices. The orifice layout, fabric permeability and throw come from the manufacturer's selection software; this tool sets the diameter that feeds them.
Inlet velocity is the number that matters
A metal duct holds its shape whatever the air does inside it. A fabric duct holds its shape only because the static pressure inside is higher than the room — and everything that disturbs that, starting with fast, turbulent air arriving at the inlet, shows up as movement in the fabric. A duct that ripples and breathes at the inlet end is noisy, looks wrong in an exposed ceiling and wears faster at exactly the point where the loads are highest.
That is why fabric duct is sized on inlet velocity rather than on a friction rate. Along the run the velocity falls away on its own, because air is leaving through the fabric and the orifices the whole way down; the inlet is the only place where the full airflow meets the smallest margin. Get the inlet right and the rest of the run usually behaves.
The upstream fittings decide how much velocity the inlet can take. A clean straight approach delivers evenly distributed air; an elbow a metre before the inlet throws the air to one side, and the fabric feels that as an uneven, pulsing load. Every published limit below is really a statement about how disturbed the incoming air is likely to be.
How to use the fabric duct calculator
- Enter the airflow at the fabric inlet. The air entering this fabric run — the full supply for a single-inlet system, or the share of it feeding each run on a multi-run layout.
- Choose the shape. Round for a suspended duct, half-round for a D-shaped duct fixed to a wall or ceiling. A half-round duct of the same diameter has half the area, so it needs a larger diameter for the same air.
- Pick the inlet condition. A clean straight inlet tolerates the highest velocity. An elbow close upstream, a top inlet, or a noise-sensitive space all call for slower air. If the manufacturer gives a different limit, choose the custom option and enter it.
- Add the length if you want the release rate. Optional. With the length entered, the tool reports the average airflow released per foot or per metre, which is the figure the orifice and fabric selection starts from.
- Read the size and the actual inlet velocity. The selected diameter is rounded up, so the real inlet velocity is always at or below the limit. The table shows the size each alternative limit would give, which is useful when the space overhead is tight.
Published inlet velocity limits
| Inlet condition | fpm | m/s |
|---|---|---|
| Straight inlet, no fittings within 3 diameters | 1500 | 7.6 |
| Elbow or offset within 3 diameters of the inlet | 1200 | 6.1 |
| Sound-sensitive spaces | 1000 | 5.1 |
| Top inlet | 1000 | 5.1 |
Representative of the limits fabric duct manufacturers publish in their design guides. Some product lines are rated higher with internal frames or inlet hardware, and long, fast runs often need extra provisions such as intermediate zippers — follow the product manual for the system actually specified.
Formulas and a worked example
- Round: A = πD² ÷ 4, so D = √(4Q ÷ πV)
- Half-round (D-shape): A = πD² ÷ 8, so D = √(8Q ÷ πV)
- Inlet velocity at the chosen size: V = Q ÷ A
- Velocity pressure: pv = ρV² ÷ 2
4000 CFM entering a suspended round fabric duct, with an elbow just upstream so the limit is 1200 fpm. Required area = 4000 ÷ 1200 = 3.33 ft², giving a minimum diameter of 24.7 in. Rounded up, a 26 inch duct carries the air at 1085 fpm. Mounted as a half-round against the ceiling instead, the same air needs a 35.0 inch minimum — a 36 inch half-round at about 1130 fpm.
Things to keep in mind
- Match the metal to the fabric. Where the layout allows, run the supply metal at the fabric diameter for about three diameters before the inlet, with smooth transitions and long-radius elbows upstream.
- Start the fan gently. A fabric duct that collapses when the fan stops snaps open when it restarts. A soft start or a VFD ramp, or internal hoops or tensioning hardware, keeps that from becoming a daily shock load.
- The diameter is not the whole selection. Fabric permeability, orifice size and position and the throw needed to reach the occupied zone all come from the manufacturer's selection — this tool fixes the diameter that feeds them.
- Check condensation risk on cold supply. In humid spaces a non-porous fabric at low supply temperature can sweat on the outside. A permeable fabric keeps a film of air moving over the surface.
- Diameter ranges vary by maker. Sizes here round up in 2 in or 50 mm steps; confirm the nearest size actually offered.
Frequently asked questions
What is the maximum velocity for fabric duct?
It is set at the inlet and depends on what is upstream. Published manufacturer guidance is around 1500 fpm (7.6 m/s) for a straight inlet with no fittings within three diameters, about 1200 fpm (6.1 m/s) where an elbow or offset sits close to the inlet, and about 1000 fpm (5.1 m/s) for noise-sensitive spaces and top inlets. Some product ranges go higher with extra hardware, so the product manual has the last word.
Why is fabric duct sized on inlet velocity instead of friction rate?
Because the fabric is held in shape by static pressure, and the threat to it is the dynamic energy of the air arriving at the inlet. Too much velocity makes the fabric ripple, flap and breathe at the inlet end, which is audible and wears the material. Friction along a fabric run is small and falls away quickly, because the air is discharged progressively along the length.
What size fabric duct do I need?
Divide the airflow by the allowable inlet velocity to get the area, then convert to a diameter: D = √(4Q ÷ πV) for round, D = √(8Q ÷ πV) for half-round. For 4000 CFM at 1200 fpm that is a 24.7 inch minimum, so a 26 inch round duct, which then runs at about 1085 fpm.
What static pressure does fabric duct need?
Enough to keep the fabric fully inflated and drive the design airflow through the weave and the orifices. Systems typically operate somewhere between about 0.15 and 1.6 in.wg (40–400 Pa), with many running near 0.5 in.wg (around 120 Pa). The exact figure comes from the manufacturer, because it depends on the fabric permeability and the orifice pattern.
Does fabric duct have a pressure drop along its length?
A little at the inlet end, but less than most people expect. As air leaves through the fabric and the orifices, the velocity inside falls along the run and some of that velocity pressure is regained as static pressure — the same static regain effect a designer works hard to achieve in metal duct happens naturally here. That is why long fabric runs can still discharge evenly.
Can I connect fabric duct straight off an elbow?
It is best avoided. Turbulence from an elbow just upstream of the inlet makes the fabric move and is the reason the allowable velocity drops when a fitting sits within three diameters of the inlet. Where the layout forces it, use a long-radius elbow, a smooth transition and a straight metal section of about three diameters before the fabric starts.
Last updated: 24 October 2026