What a sheet metal gauge number means
Ask for 16 gauge and you can be handed three different sheets. A gauge number is not one thickness: the old US gauge for iron and steel was defined by weight, steel sheet follows its own revised column, galvanized sheet is gauged by weight with the zinc included, and aluminium and copper use a different gauge altogether, defined by thickness. Here is what each one means, from the documents that define them.
Published 2026-10-31. Rather have the numbers worked out? Sheet Metal Gauge Chart and Converter does it in your browser.
1. The 1893 law: a weight
The US Standard Gauge for sheet and plate iron and steel was set by an Act of Congress in 1893 (15 U.S.C. §206). It defines each gauge by its weight per square foot and gives an “approximate thickness”, with 2½ % either way allowed (§208). The thicknesses are the weight at 40 lb per square foot per inch — wrought iron (NBS Circular 391, p. 4). Under the law, 16 gauge is 2.50 lb/ft², about 0.0625 in.
2. Steel sheet today: the revised column
Steel is about 2 % denser than wrought iron — 40.80 lb/ft² per inch (NBS Circular 391, p. 4) — so on density alone 16 gauge steel would be about 0.0613 in. The steel column this site uses, the “United States Standard (Revised)” column of the USAF / US Navy metals handbook (TO 1-1A-9, Table A-34), is thinner again: 16 gauge is 0.0598 in (1.52 mm). Its rows follow about 41.82 lb/ft² per inch; the handbook does not say why. So steel 16 gauge is neither the law’s 0.0625 nor the density-only figure.
3. Galvanized: sold by weight, zinc included
Galvanized sheet is gauged by the weight of the coated sheet, which includes the zinc, so the gauges are heavier for the same number and the source gives no thickness (NBS Circular 391, pp. 10–11). Measure the sheet, or ask for the coated thickness, rather than reading one off.
4. Aluminium, copper, brass: a different gauge
Nearly all non-ferrous sheet is gauged by the American Wire Gauge (Brown & Sharpe), a thickness gauge defined as a geometric series in NBS Handbook 100: 16 gauge is 0.0508 in (1.29 mm) — thinner again (NBS Circular 391, pp. 14–15, for its use on sheet).
The same number, side by side
| Gauge | Steel (revised) | 1893 law | AWG (aluminium, copper) | Galvanized, lb/ft² |
|---|---|---|---|---|
| 10 | 0.1345 (3.42) | 0.1406 (3.57) | 0.1019 (2.59) | 5.781 |
| 12 | 0.1046 (2.66) | 0.1094 (2.78) | 0.0808 (2.05) | 4.531 |
| 14 | 0.0749 * (1.90) | 0.0781 (1.98) | 0.0641 (1.63) | 3.281 |
| 16 | 0.0598 (1.52) | 0.0625 (1.59) | 0.0508 (1.29) | 2.656 |
| 18 | 0.0478 (1.21) | 0.0500 (1.27) | 0.0403 (1.02) | 2.156 |
| 20 | 0.0359 (0.91) | 0.0375 (0.95) | 0.0320 (0.81) | 1.656 |
| 22 | 0.0299 (0.76) | 0.0313 (0.79) | 0.0253 (0.64) | 1.406 |
| 24 | 0.0239 (0.61) | 0.0250 (0.64) | 0.0201 (0.51) | 1.156 |
* Printed as shown in the handbook. Every other row of the steel column equals its weight divided by 41.82 lb/ft² per inch, to four places; for 14 gauge that gives 0.0747, the figure more often quoted. Use the one your supplier quotes (see the gauge chart).
Common mistakes
- Reading a steel gauge off an aluminium chart, or the reverse.
- Using the 1893 law’s “approximate thickness” for modern steel sheet.
- Treating a nominal thickness as exact: the law itself allows 2½ % either way, and mills set their own tolerances.
- Ordering by gauge when the drawing needs a thickness: give the thickness, or the gauge and the system.
How this matches the tool
The sheet metal gauge chart lists every gauge in all four systems, looks a thickness up the other way, and prints. Every figure on this page comes from its data, which names the source of each table.
Sources
- 15 U.S.C. §206 (Act of March 3, 1893), US Standard Gauge for sheet and plate iron and steel; §208, the 2½ % allowance (US Code 2024, govinfo)
- 15 U.S.C. §208: the 2½ % allowance either way
- TO 1-1A-9 / NAVAIR 01-1A-9, USAF / US Navy general-data handbook for metals (Change 3, 2020), Table A-34, pp. A-51–A-52: steel sheet, US Standard (revised)
- NBS Circular 391, Standard Thicknesses, Weights, and Tolerances of Sheet Metal (1931): the 1893 basis (p. 4), galvanized weights (pp. 10–11), AWG for sheet (p. 15)
- NBS Handbook 100, Copper Wire Tables (1966), §2.2, p. 6: the American Wire Gauge definition
Skip the arithmetic
Sheet Metal Gauge Chart and Converter — What a gauge number means in inches and millimetres for steel and aluminium, what a galvanized gauge weighs, both ways — every value traced to a US government source.
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