SMACNA duct gauge chart
Gauge selection tables for rectangular and round duct, indexed by pressure class and the governing dimension, with the nominal thickness of each gauge in millimetres and inches. Reinforcement spacing and joint class drive the final selection, so the tables state plainly where they stop and the construction standard takes over.
Read this before using the tables. Gauge alone does not define compliant duct construction. SMACNA selection pairs sheet thickness with a reinforcement type and spacing and a transverse joint class, and several combinations satisfy the same duty — a lighter gauge with closer reinforcement, or a heavier gauge with wider spacing. The tables below give the indicative gauge for estimating and for checking a fabricator's proposal. For construction, work from the SMACNA HVAC Duct Construction Standards tables for the specified pressure class and reinforcement class.
Rectangular duct — gauge by pressure class and largest side
| Largest side (mm) | 250 Pa (1 in.wg) | 500 Pa (2 in.wg) | 750 Pa (3 in.wg) | 1000 Pa (4 in.wg) | 1500 Pa (6 in.wg) | 2500 Pa (10 in.wg) |
|---|---|---|---|---|---|---|
| Up to 300 | 26 ga (0.551 mm) | 26 ga (0.551 mm) | 24 ga (0.701 mm) | 24 ga (0.701 mm) | 22 ga (0.853 mm) | 22 ga (0.853 mm) |
| 301 – 450 | 26 ga (0.551 mm) | 26 ga (0.551 mm) | 24 ga (0.701 mm) | 24 ga (0.701 mm) | 22 ga (0.853 mm) | 22 ga (0.853 mm) |
| 451 – 750 | 24 ga (0.701 mm) | 24 ga (0.701 mm) | 22 ga (0.853 mm) | 22 ga (0.853 mm) | 20 ga (1.006 mm) | 20 ga (1.006 mm) |
| 751 – 1200 | 22 ga (0.853 mm) | 22 ga (0.853 mm) | 20 ga (1.006 mm) | 20 ga (1.006 mm) | 18 ga (1.311 mm) | 18 ga (1.311 mm) |
| 1201 – 1500 | 20 ga (1.006 mm) | 20 ga (1.006 mm) | 18 ga (1.311 mm) | 18 ga (1.311 mm) | 16 ga (1.613 mm) | 16 ga (1.613 mm) |
| 1501 – 2100 | 18 ga (1.311 mm) | 18 ga (1.311 mm) | 16 ga (1.613 mm) | 16 ga (1.613 mm) | 16 ga (1.613 mm) | 16 ga (1.613 mm) |
| Over 2100 | 16 ga (1.613 mm) | 16 ga (1.613 mm) | 16 ga (1.613 mm) | 16 ga (1.613 mm) | 16 ga (1.613 mm) | 16 ga (1.613 mm) |
The governing dimension is the largest side, because that is the side that bows. A 1500 × 300 duct is selected on the 1500, not on the 300.
Round duct — gauge by pressure class and diameter
| Diameter (mm) | 250 Pa (1 in.wg) | 500 Pa (2 in.wg) | 750 Pa (3 in.wg) | 1000 Pa (4 in.wg) | 1500 Pa (6 in.wg) | 2500 Pa (10 in.wg) |
|---|---|---|---|---|---|---|
| Up to 200 | 26 ga (0.551 mm) | 26 ga (0.551 mm) | 26 ga (0.551 mm) | 26 ga (0.551 mm) | 24 ga (0.701 mm) | 24 ga (0.701 mm) |
| 201 – 350 | 24 ga (0.701 mm) | 24 ga (0.701 mm) | 24 ga (0.701 mm) | 24 ga (0.701 mm) | 22 ga (0.853 mm) | 22 ga (0.853 mm) |
| 351 – 600 | 24 ga (0.701 mm) | 24 ga (0.701 mm) | 24 ga (0.701 mm) | 24 ga (0.701 mm) | 22 ga (0.853 mm) | 22 ga (0.853 mm) |
| 601 – 900 | 22 ga (0.853 mm) | 22 ga (0.853 mm) | 22 ga (0.853 mm) | 22 ga (0.853 mm) | 20 ga (1.006 mm) | 20 ga (1.006 mm) |
| 901 – 1250 | 20 ga (1.006 mm) | 20 ga (1.006 mm) | 20 ga (1.006 mm) | 20 ga (1.006 mm) | 18 ga (1.311 mm) | 18 ga (1.311 mm) |
| 1251 – 1500 | 18 ga (1.311 mm) | 18 ga (1.311 mm) | 18 ga (1.311 mm) | 18 ga (1.311 mm) | 16 ga (1.613 mm) | 16 ga (1.613 mm) |
| Over 1500 | 16 ga (1.613 mm) | 16 ga (1.613 mm) | 16 ga (1.613 mm) | 16 ga (1.613 mm) | 16 ga (1.613 mm) | 16 ga (1.613 mm) |
Round duct carries pressure in hoop tension rather than plate bending, so it holds its shape with lighter metal than a rectangular duct of the same duty — one more reason round is cheaper to build wherever the void allows it.
Gauge to thickness
| Gauge | mm | inches | kg/m² | lb/ft² |
|---|---|---|---|---|
| 26 ga | 0.551 | 0.0217 | 4.33 | 0.886 |
| 24 ga | 0.701 | 0.0276 | 5.50 | 1.127 |
| 22 ga | 0.853 | 0.0336 | 6.70 | 1.371 |
| 20 ga | 1.006 | 0.0396 | 7.90 | 1.617 |
| 18 ga | 1.311 | 0.0516 | 10.29 | 2.108 |
| 16 ga | 1.613 | 0.0635 | 12.66 | 2.593 |
Gauge numbers run backwards — 16 gauge is nearly three times the thickness of 26. Values are nominal for galvanised sheet; mill tolerance and zinc coating both move the delivered figure slightly. Turn thickness into a takeoff weight with the duct weight calculator.
Where these tables stop
- Reinforcement is not shown. Spacing and member size are chosen together with gauge. Two compliant answers can differ by a full gauge depending on how the fabricator reinforces.
- Negative pressure is the harder case. Duct on the suction side of a fan can collapse rather than bow, and extract systems often need construction heavier than the numerical pressure alone suggests.
- Unsupported span matters. A long run between hangers behaves differently from a well-supported one, whatever the sheet thickness.
- Non-ferrous is specified by thickness. Gauge is a galvanised steel convention; aluminium and stainless are called up in millimetres or inches.
- The specification wins. Where a project names a gauge, a reinforcement class or a joint type, that governs over any chart including this one.
Frequently asked questions
What gauge should ductwork be?
It depends on two things: the pressure class the duct is built to, and the governing dimension — the largest side for rectangular duct, the diameter for round. Typical low-pressure commercial rectangular duct runs 26 gauge up to about 300 mm, 24 gauge to 750 mm, 22 gauge to 1200 mm and heavier above that. Higher pressure classes step the whole scale up. The tables below give the indicative selection; the SMACNA construction standard gives the contractual one.
How thick is 24 gauge duct?
Nominally 0.701 mm, which is 0.0276 inches. The full range used in duct work runs from 26 gauge at 0.551 mm through 24, 22, 20 and 18 to 16 gauge at 1.613 mm. Note that gauge numbers run backwards — a smaller number is thicker metal — and that gauge is defined for galvanised sheet specifically, so aluminium and stainless thicknesses are specified directly rather than by gauge.
Why does the pressure class change the gauge?
Because pressure tries to deform the duct. A flat side under positive pressure bows outward, and under negative pressure it can collapse inward — which is why extract and return duct on the suction side of a fan often needs heavier construction than supply duct at the same numerical pressure. Thicker sheet resists that deformation, as does closer reinforcement, and the standard trades the two against each other.
Is gauge the whole story?
No, and this is the most common misreading of a gauge chart. SMACNA selection is a combination of sheet thickness, reinforcement type and spacing, and transverse joint class. A lighter gauge with closer reinforcement and a heavier gauge with wider spacing can both be compliant for the same duty, and fabricators choose between them on cost and shop practice. A chart that gives only gauge is a starting point for estimating, not a construction specification.
Does round duct need the same gauge as rectangular?
Generally lighter for the same duty. A circular section resists internal pressure by hoop tension rather than by bending a flat plate, so it holds its shape with far less material — which is one more reason round duct is cheaper to build where the void allows it. The spiral seam itself also stiffens the tube along its length.
What about aluminium and stainless duct?
Gauge numbers are defined for galvanised steel, so aluminium and stainless are normally specified by thickness in millimetres or inches directly. Aluminium is less stiff than steel, so equivalent construction usually needs a greater thickness than a straight gauge substitution would suggest. Where a project specifies these materials, work from the thickness in the specification rather than converting a steel gauge.
Last updated: 26 July 2026