A gauge number does not give a thickness on its own. The same “16 gauge” is 0.0598 inch in steel on one current commercial chart and 0.0625 inch in the U.S. statutory steel table. To turn a gauge into a thickness, first identify the material and the chart the number comes from, then read that chart’s column for that material.
How gauge numbers relate to thickness
Gauge is a numbered thickness convention, and it runs in reverse: a higher gauge number means thinner sheet. Ten gauge is thicker than 16 gauge, and 20 gauge is thinner than 16 gauge. The number tells you where a sheet sits in a series, not how many inches or millimeters it measures until you consult the table that defines that series.
Why one gauge number can have more than one thickness
Gauge has no single universal conversion. Two factors change the answer:
- The material. NIST’s historical Circular 391 describes different gauges used for ferrous and nonferrous sheet metals. Steel, stainless steel and aluminum should never share one column.
- The chart convention. Even for steel, a statutory table and a current commercial chart can list different values for the same gauge number.
Converting a gauge to thickness, step by step
- Identify the material before you open any chart. Confirm whether it is carbon steel, stainless steel or aluminum, because a gauge number has no meaning without it.
- Find the chart’s title, source and units. Note whether it is a statutory table or a commercial chart, and whether it reports inches or millimeters.
- Read the column for your material in the row for your gauge. That value is the nominal thickness for that chart.
- Convert between units if needed. Multiply inches by 25.4 to get millimeters.
- Check the thickness your supplier states on the order or specification. If the supplier uses a different gauge series or a stated thickness, use that value for the job.
- If fit or dimensions are critical, measure the actual sheet (see the measurement section below).
Reading the chart for each material
Carbon steel
Steel has two references in common use that do not match exactly. The U.S. Code text of 15 U.S.C. § 206 (accessed in 2026) establishes a standard gauge for “sheet and plate iron and steel in the United States of America.” It is the only standard gauge for that scope under that section. A current commercial chart from Welders Supply, updated June 23, 2026, lists a different steel value for 16 gauge. The difference is 0.0027 inch, or about 0.07 mm. Whether that matters depends on the tolerance your part requires. Name the chart you used in any drawing or specification and do not average the two values.
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Stainless steel
Stainless has its own column in commercial charts and should be read from that column. The statutory steel table does not cover stainless steel, so its values cannot be used to confirm a stainless gauge. Confirm the stainless thickness with the supplier’s stated value for the stock you are buying.
Aluminum
Aluminum has its own column and should be read from it. Its 16 gauge value is substantially thinner than steel’s at the same gauge number. Never apply a steel value to aluminum, or the reverse, when converting a gauge to thickness.
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Comparing 16 gauge across the references
The table below uses 16 gauge because it is the only size for which the sources in this guide give values for all three materials. Millimeter values are conversions of the listed inch values, rounded to two decimals.
| Material | Reference | Nominal thickness (in) | Nominal thickness (mm) | Notes |
|---|---|---|---|---|
| Iron and steel | 15 U.S.C. § 206 statutory table (U.S. Code, accessed 2026) | 0.0625 | 1.59 | Statute covers sheet and plate iron and steel only |
| Steel | Welders Supply commercial chart (2026; published June 17, updated June 23) | 0.0598 | 1.52 | Commercial convention; differs from the statutory table |
| Stainless steel | Welders Supply commercial chart (2026) | 0.0625 | 1.59 | Not stated in 15 U.S.C. § 206, which covers iron and steel only |
| Aluminum | Welders Supply commercial chart (2026) | 0.0508 | 1.29 | Not stated in 15 U.S.C. § 206, which covers iron and steel only |
Nominal thickness is not guaranteed thickness
A gauge chart gives a reference value, not a measurement of the sheet in your hands. NIST’s Circular 391, Standard Thicknesses, Weights, and Tolerances of Sheet Metal (Customary Practice), describes usual American manufacturer practice, including customary thicknesses and permissible variations. The circular states that it “does not promulgate these data as standards.” Treat its figures as a description of common practice, and rely on the supplier’s stated thickness and tolerance for any job where dimensions are critical.
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Measuring the actual sheet
Mitutoyo describes sheet-metal micrometers designed to measure sheet-metal thickness, and it notes deep-throat models for measurements farther into a sheet. Use this method to check stock against its nominal gauge:
- Use a sheet-metal micrometer whose measuring range covers the sheet you are checking.
- Clean the anvil and spindle faces, then confirm the instrument reads zero when closed.
- Measure at several points away from edges, bends, and any visible damage, and record each reading.
- Compare the average of your readings with the nominal value for the material and gauge, and with the supplier’s stated thickness.
- If readings fall outside the supplier’s stated thickness or tolerance, raise the question with the supplier before fabrication.
Limits of these values
The figures here come from a small set of sources: the statutory steel table, NIST’s historical circular, and one current commercial chart. They do not establish values for every metal, every regional practice or every supplier. For a specific order, the convention that governs is the one your supplier or specification names.
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