A gauge number is not a thickness. It is a position on a table, and there are four tables in common use - US Standard Gauge for steel, a separate table for galvanised sheet, a third for stainless steel and a fourth (Brown & Sharpe) for aluminium. The same gauge number means a different wall on each of them, which is why a drawing that specifies "18 gauge" and nothing else is ambiguous.
What is 18 gauge sheet metal in mm?
18 gauge is 1.214 mm as US Standard Gauge for carbon steel, 1.310 mm for galvanised sheet, 1.270 mm for stainless steel and 1.024 mm as Brown & Sharpe gauge for aluminium. The number is a position in a table, not a measurement, so the thickness depends entirely on which table is meant. That is why we quote and manufacture in millimetres: 1.2 mm sheet, not 18 gauge. When a drawing only carries a gauge number, we convert it against the material named and confirm the millimetre value back to the customer before the strip is ordered.
Why the same gauge gives four different thicknesses
The gauge systems were built around the manufacturing process, not around metrology. US Standard Gauge was set by the weight of a square foot of sheet, which is why it drifts once the alloy changes: steel, galvanised steel, stainless steel and aluminium all weigh different amounts for the same volume, so the same number lands on a different thickness. Aluminium sheet in the United States is normally ordered on Brown & Sharpe gauge instead, which is a different sequence again.
Two other traps come with the number. A gauge is nominal: the mill tolerance on cold-rolled sheet is not zero, so a "1.2 mm" coil can arrive a few hundredths either side of 1.2 mm. And gauge says nothing about the material condition or the direction of the strip, both of which affect how the part forms more than a 0.03 mm thickness difference does.
Gauge to millimetre chart
| Gauge | Steel (US Std) | Galvanised | Stainless | Aluminium (B&S) |
|---|---|---|---|---|
| 10 | 3.416 mm (0.1345 in) | 3.510 mm (0.1382 in) | 3.571 mm (0.1406 in) | 3.175 mm (0.1250 in) |
| 12 | 2.732 mm (0.1076 in) | 2.753 mm (0.1084 in) | 2.779 mm (0.1094 in) | 2.052 mm (0.0808 in) |
| 14 | 1.897 mm (0.0747 in) | 2.108 mm (0.0830 in) | 1.984 mm (0.0781 in) | 1.628 mm (0.0641 in) |
| 16 | 1.519 mm (0.0598 in) | 1.613 mm (0.0635 in) | 1.588 mm (0.0625 in) | 1.290 mm (0.0508 in) |
| 18 | 1.214 mm (0.0478 in) | 1.310 mm (0.0516 in) | 1.270 mm (0.0500 in) | 1.024 mm (0.0403 in) |
| 20 | 0.912 mm (0.0359 in) | 1.006 mm (0.0396 in) | 0.953 mm (0.0375 in) | 0.813 mm (0.0320 in) |
| 22 | 0.759 mm (0.0299 in) | 0.836 mm (0.0329 in) | 0.787 mm (0.0310 in) | 0.643 mm (0.0253 in) |
| 24 | 0.607 mm (0.0239 in) | 0.686 mm (0.0270 in) | 0.635 mm (0.0250 in) | 0.511 mm (0.0201 in) |
| 26 | 0.455 mm (0.0179 in) | 0.533 mm (0.0210 in) | 0.457 mm (0.0180 in) | 0.404 mm (0.0159 in) |
| 28 | 0.378 mm (0.0149 in) | 0.457 mm (0.0180 in) | 0.396 mm (0.0156 in) | 0.321 mm (0.0126 in) |
| 30 | 0.305 mm (0.0120 in) | 0.381 mm (0.0150 in) | 0.318 mm (0.0125 in) | 0.254 mm (0.0100 in) |
Values are the nominal thicknesses of the standard tables. Mill tolerances and the coating weight on galvanised sheet sit on top of them - a hot-dip galvanised sheet at "18 gauge" is thicker than the steel table says because the zinc is included.
What thickness does to the process
Thickness is the first number we look at when a part is quoted, because it drives the press tonnage, the die clearance, the radius and the number of drawing stages. In a deep drawn part the wall is not the blank thickness anyway: it thins on the wall and thickens at the flange, so the drawing has to state where the wall is measured and what it may be at that point.
- Cutting and blanking - thin sheet (0.2–0.8 mm) cuts cleanly but distorts easily; the same profile in 3 mm steel needs roughly four times the tonnage and a different die clearance.
- Bending - the minimum inside radius scales with thickness, so a tight radius that works in 1 mm sheet cracks in 2 mm.
- Deep drawing - draw ratio, die radius and blank holder force are all functions of thickness; a specification that says "18 gauge" changes the process when it is read as 1.21 mm instead of 1.27 mm.
- Welding and assembly - a 0.05 mm mismatch is invisible in 3 mm plate and is a gap in 0.6 mm sheet.
How to specify thickness on a drawing
- State the thickness in millimetres to two decimals, not as a gauge number.
- State the material and grade next to it (DC04, 304, 430F, C11000, 5052).
- State the thickness tolerance or the standard you want it held to, for example EN 10131 normal or special tolerances.
- Say whether the thickness is measured on the finished part after coating, and where on the part it is measured.
- If the part is drawn, say where the wall thickness is checked - most disputes about a drawn part are about where the wall was measured.
Key point
Convert the gauge to millimetres against the material named on the drawing, then confirm the number back before strip is ordered. A gauge number that gets read against the wrong table is a 0.05 mm error that shows up as a cracked bend or a part that will not fit the fixture.
Frequently asked questions
What is 20 gauge in mm?
20 gauge is 0.912 mm as US Standard Gauge for steel, 1.006 mm for galvanised, 0.953 mm for stainless and 0.813 mm as Brown & Sharpe gauge for aluminium.
Is gauge the same in the UK and the US?
No. The US uses US Standard Gauge for steel, while the UK traditionally used Imperial Standard Wire Gauge (SWG), where 18 gauge is about 1.22 mm. Modern drawings in both countries specify millimetres, which removes the ambiguity.
Why does galvanised sheet show a different thickness for the same gauge?
Because the galvanised table includes the zinc coating. The zinc adds 0.02–0.05 mm depending on the coating class, so the same gauge number lands on a thicker wall.
Can you stamp 3 mm sheet?
Yes - our presses run from 25 t to 350 t, so the thickness limit depends on the part geometry and the required tonnage rather than on a fixed maximum. Send the drawing and we will confirm the press and the tooling route.