Learn why sheet-metal gauge must be looked up in a material-specific table before converting the listed thickness to millimeters. The same gauge number can mean different thicknesses.
Open the gauge to mm calculator · More Physics and Math videos
The calculation
Gauge + material + gauge system → nominal thickness; inches × 25.4 = mm
Worked example
In the supplier chart used here, 18-gauge mild steel is 0.0478 in. Multiplying by exactly 25.4 gives 1.21412 mm, about 1.214 mm. The same chart lists 18-gauge aluminum at 0.04030 in, or 1.02362 mm, about 1.024 mm.
A generic linear subtraction does not reproduce a sheet-metal gauge table. Use the correct material and gauge system, distinguish nominal thickness from manufacturing tolerance, and keep electrical wire gauge systems separate.
Video chapters
- 0:00 — 18 gauge of which material?
- 0:14 — Choose the table before reading the row
- 0:29 — 0.0478 in × 25.4 ≈ 1.214 mm
- 0:48 — Same gauge, different thickness
- 1:06 — Gauge steps are not a fixed mm subtraction
- 1:24 — A table gives a nominal thickness
- 1:41 — A measured thickness needs a matching table
- 2:00 — Material → table → thickness → units
Read the full transcript
Someone asks for eighteen-gauge sheet metal. How thick is that in millimeters? The gauge number alone is not enough. First identify the material and the gauge system, because the same number can describe different thicknesses in different tables.
Think of gauge as a lookup label, not a metric length. Locate the correct material column, then the gauge row. On the supplier chart used here, eighteen-gauge mild steel is listed as zero point zero four seven eight inches thick.
If your table gives inches, multiply that thickness by twenty-five point four millimeters per inch. Using the listed inch thickness gives about one point two one four millimeters. To two decimal places, that is one point two one millimeters. Keep the unrounded intermediate value if you need a different display precision.
Now keep the gauge at eighteen but choose aluminum in the same chart. Its listed thickness is zero point zero four zero three inches, approximately one point zero two four millimeters. The smaller thickness is not a calculation mistake. The material column changed.
Do not treat every gauge step as an equal millimeter change. A generic straight-line subtraction can produce incorrect values. Read the actual series for the material. Also keep wire-gauge systems separate from sheet-metal gauges; an electrical wire formula answers a different measurement question.
A listed gauge thickness is nominal. Real material has manufacturing tolerances, and coatings or the measurement method can affect what a caliper reads. If an exact fit matters, obtain the supplier's specification and tolerance, then verify the actual material with an appropriate measurement.
For the reverse direction, compare a measured thickness with the entries in the correct table. A value near one point two one four millimeters suggests the eighteen-gauge mild-steel entry in our example. Closeness alone does not prove the material, gauge system, or compliance with a specification.
The reliable method is to identify the material and gauge system, read the matching thickness, and convert units if needed. State the nominal thickness and its source. For the chart in this example, eighteen-gauge mild steel is about one point two one four millimeters.