Duct Gauge Chart — Minimum Sheet Metal Thickness by Size
How thick the duct wall must be: the mechanical code's minimum sheet-metal thicknesses for round and rectangular duct by size and static-pressure class, per IMC Table 603.4 (2021 edition), with the approximate galvanized gauge each value corresponds to. The table governs single dwelling units — everything else, anything over 20 inches, and anything over 1" w.g. builds to the SMACNA standards instead.
Round duct — minimum thickness (inches)
| Duct size | Galv @ 1/2" w.g. | Gauge | Alum @ 1/2" | Galv @ 1" w.g. | Gauge | Alum @ 1" |
|---|---|---|---|---|---|---|
| Less than 12" | 0.013" | ≈ 30 ga | 0.018" | 0.013" | ≈ 30 ga | 0.018" |
| 12" to 14" | 0.013" | ≈ 30 ga | 0.018" | 0.016" | ≈ 28 ga | 0.023" |
| 15" to 17" | 0.016" | ≈ 28 ga | 0.023" | 0.019" | ≈ 26 ga | 0.027" |
| 18" | 0.016" | ≈ 28 ga | 0.023" | 0.024" | ≈ 24 ga | 0.034" |
| 19" to 20" | 0.019" | ≈ 26 ga | 0.027" | 0.024" | ≈ 24 ga | 0.034" |
Rectangular duct — minimum thickness (inches)
| Duct size | Galv @ 1/2" w.g. | Gauge | Alum @ 1/2" | Galv @ 1" w.g. | Gauge | Alum @ 1" |
|---|---|---|---|---|---|---|
| 8" and less | 0.013" | ≈ 30 ga | 0.018" | 0.013" | ≈ 30 ga | 0.018" |
| 9" to 10" | 0.013" | ≈ 30 ga | 0.018" | 0.016" | ≈ 28 ga | 0.023" |
| 11" to 12" | 0.016" | ≈ 28 ga | 0.023" | 0.019" | ≈ 26 ga | 0.027" |
| 13" to 16" | 0.019" | ≈ 26 ga | 0.027" | 0.019" | ≈ 26 ga | 0.027" |
| 17" to 18" | 0.019" | ≈ 26 ga | 0.027" | 0.024" | ≈ 24 ga | 0.034" |
| 19" to 20" | 0.024" | ≈ 24 ga | 0.034" | 0.024" | ≈ 24 ga | 0.034" |
Where SMACNA takes over
Table 603.4 covers ducts within SINGLE DWELLING UNITS only — it is the code's exception. Everything else is commercial: IMC 603.4 sends metallic ducts to the SMACNA HVAC Duct Construction Standards (the 2021 code references ANSI/SMACNA 006-2020, the 4th edition, per its Chapter 15 listing). SMACNA rectangular gauge is not a single gauge-by-size ladder: the standard trades sheet gauge against reinforcement class and joint spacing (its Tables 2-x by pressure class in the current edition). Cite the standard — this site does not reproduce those schedules. What gauge callouts translate to in inches and pounds is on the sheet metal gauge chart, and the support side of duct construction is on the hanger spacing chart. As always, the mechanical code and amendments your jurisdiction enforces govern.
Common questions
What gauge sheet metal is used for residential ductwork?
Under IMC Table 603.4, most residential duct is 0.013" galvanized — approximately 30 gauge — up through 14" round or 8"-wide rectangular at low static. Larger sizes and 1" w.g. systems step through 0.016" (≈28 ga) and 0.019" (≈26 ga) to 0.024" (≈24 ga) at the 19–20" top of the table.
Do these thickness minimums apply to commercial ducts?
No — the table is the code's single-dwelling-unit exception. Commercial metallic duct is constructed per the SMACNA HVAC Duct Construction Standards (the 2021 IMC references the 2020 fourth edition), where sheet gauge trades against reinforcement class and joint spacing rather than following one gauge-per-size ladder.
Why does the code list a thinner number than the gauge chart?
The code prints minimum acceptable thickness after rolling tolerance, not the nominal. Nominal 30-gauge galvanized is 0.0157", but a delivered sheet can legally run to about 0.013" — which is exactly the code's number. That's why the gauge equivalents on this page are labeled approximate.
How are round duct joints fastened?
IMC 603.4.1: at least three sheet metal screws or rivets, spaced equally around the joint. Where a joint is partially inaccessible, the three fasteners go equally spaced on the exposed portion so the joint can't hinge open.
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