Hong Life CalculatorsUnderstanding the sheet metal gauge chart — concept and principle
This chart converts a sheet metal gauge number, from 7 to 30 gauge, into thickness in inches and millimetres for steel and for aluminum. Gauge numbers are still common in US drawings, imported products, ductwork and car body parts, while Korean and European suppliers quote sheet thickness in millimetres, so the chart bridges the two.
How it is calculated
The value is looked up in two different gauge tables:
- Steel uses the Manufacturers' Standard Gauge for sheet steel. Each gauge stands for a weight per square foot, and the thickness follows from a nominal 41.82 lb per square foot per inch of thickness. For example 14 gauge = 3.125 lb/ft², and 3.125 ÷ 41.82 = 0.0747 in.
- Aluminum uses the Brown & Sharpe gauge, the same as American Wire Gauge (AWG): diameter = 0.005 in × 92^((36 − n) ÷ 39).
- Millimetres = inches × 25.4.
The principle
Gauge numbers come from the way metal was once made and sold: by drawing wire through successive dies or by weight per area, so a higher number means more reduction and thinner material. That is why gauge runs backwards compared with thickness. Because steel and aluminum were standardised by different systems, the same number gives different thicknesses. In the Brown & Sharpe system each gauge step changes the thickness by a fixed ratio of about 1.123, so every six gauges the thickness roughly halves.
Worked example
Default input: 14 gauge.
- Steel (Manufacturers' Standard Gauge): 0.0747 in.
- Convert to mm: 0.0747 × 25.4 = 1.897 mm.
- Aluminum (Brown & Sharpe): 0.0641 in.
- Convert to mm: 0.0641 × 25.4 = 1.628 mm.
- Result: 14 gauge = 0.0747 in (1.897 mm) in steel and 0.0641 in (1.628 mm) in aluminum.
How to read the result
A lower gauge is thicker and stiffer. As a rough orientation, 7–11 gauge (about 3–4.5 mm in steel) is structural plate-like sheet, 12–16 gauge (about 1.5–2.7 mm) is typical for brackets, enclosures and heavy panels, 18–22 gauge (about 0.75–1.2 mm) for car body panels, appliances and ducts, and 24 gauge and above for light flashing and covers. When ordering in Korea, state the thickness in millimetres; suppliers usually stock round values such as 1.6 mm or 2.0 mm rather than gauge sizes.
Common mistakes
- Using a steel gauge thickness for aluminum or the reverse; at 14 gauge the difference is about 0.27 mm.
- Applying the plain steel table to galvanized or stainless sheet, which use slightly different tables.
- Assuming a higher gauge is thicker; it is thinner.
- Forgetting that nominal thickness has a mill tolerance, so measured sheets vary slightly.
FAQ
Frequently asked questions
How thick is 16 gauge steel?
0.0598 in, or 1.519 mm.
How thick is 16 gauge aluminum?
0.0508 in, or 1.290 mm, thinner than 16 gauge steel.
Is 18 gauge thicker than 20 gauge?
Yes. 18 gauge steel is 1.214 mm and 20 gauge is 0.912 mm.
Sources: Manufacturers' Standard Gauge for Sheet Steel · ASTM B258, Standard Specification for Standard Nominal Diameters and Cross-Sectional Areas of AWG Sizes of Solid Round Wires