Ask ten suppliers for "16 gauge" and you will get four different thicknesses. Converting sheet metal gauge to mm only works if you also say which metal you mean: 16 gauge mild steel is 1.52 mm, galvanized is 1.61 mm, stainless is 1.59 mm, aluminium is 1.29 mm. That is a 25% spread hiding inside one number, and it lands on your buyer's laser cutter, not on yours.
A sheet metal gauge is a legacy index number inherited from weight tables, not a unit of thickness. The same gauge number maps to a different millimetre value depending on the metal, which is why every quotation that travels across a border should carry millimetres first and the gauge number second, if at all.
Sheet Metal Gauge to mm: The Four Tables Nobody Puts Side by Side
Most gauge charts online show one column. The problem only becomes visible when you put the four common ones next to each other.
| Gauge | Mild steel (MSG) | Galvanized steel | Stainless steel | Aluminium (B&S) |
|---|---|---|---|---|
| 26 | 0.455 mm | 0.551 mm | 0.475 mm | 0.404 mm |
| 24 | 0.607 mm | 0.701 mm | 0.635 mm | 0.511 mm |
| 22 | 0.759 mm | 0.853 mm | 0.795 mm | 0.643 mm |
| 20 | 0.912 mm | 1.006 mm | 0.953 mm | 0.813 mm |
| 18 | 1.214 mm | 1.311 mm | 1.270 mm | 1.024 mm |
| 16 | 1.519 mm | 1.613 mm | 1.588 mm | 1.291 mm |
| 14 | 1.897 mm | 1.994 mm | 1.984 mm | 1.628 mm |
| 12 | 2.657 mm | 2.753 mm | 2.779 mm | 2.052 mm |
| 11 | 3.038 mm | 3.134 mm | 3.175 mm | 2.304 mm |
| 10 | 3.416 mm | 3.510 mm | 3.571 mm | 2.588 mm |
Two things jump out. First, the number runs backwards — a higher gauge is thinner metal. Second, the four columns drift apart as the metal gets thicker: at 26 gauge the aluminium-to-galvanized gap is 0.15 mm, at 10 gauge it is 0.92 mm. If your buyer specifies "10 gauge" and assumes steel while you quote aluminium, you have quoted a part that is a quarter thinner than the one they costed.
Why One Number Means Four Thicknesses
The explanation is historical and it is worth knowing, because it tells you which numbers to trust.
Steel gauge is a weight table, not a thickness table
The Manufacturers' Standard Gauge for sheet steel assigns each gauge number a weight per square foot, working from 41.82 lb per square foot per inch of thickness. Thickness is what falls out of the weight, not what was specified. That is also why the increments look irregular — they were never designed to be round millimetre values.
The older US Standard Gauge is older still and sits in federal law: the Act of 3 March 1893, now codified at 15 U.S.C. § 206, declares a single legal standard gauge for sheet and plate iron and steel, defined by weight per square foot of wrought iron. A gauge number is a nineteenth-century weight index that survived into twenty-first-century purchase orders.
Galvanized adds a zinc allowance
Galvanized sheet runs on its own table, in which each gauge number equals the bare-steel thickness plus 0.0037 in (0.094 mm) for the zinc coating. Check the table above: 16 gauge bare is 0.0598 in, 16 gauge galvanized is 0.0635 in. The difference is exactly the coating allowance. Coating class matters here too — ASTM A653/A653M covers hot-dip galvanized sheet and specifies coating designations such as G60 or G90, which change the mass of zinc and therefore the finished thickness.
Stainless and aluminium never used the steel table at all
Stainless has its own gauge series, and thickness tolerances for stainless flat products come from ASTM A480/A480M, not from any gauge chart. Aluminium in North America is usually quoted on the Brown & Sharpe (American Wire Gauge) series, which is a geometric progression designed for wire drawing. There is no reason for a wire-drawing series and a steel weight series to agree, and they do not.
The takeaway is short enough to put on a sticky note: gauge is a name, millimetres are a measurement.
What Your Quotation Should Actually Say
Once you accept that gauge is a label, the fix is mechanical. Every sheet thickness that leaves your office should carry three things: the number, the tolerance, and the standard that owns the tolerance.
| Weak line on a quote | What the buyer can act on |
|---|---|
| 16 ga steel | 1.5 mm cold-rolled steel, ±0.13 mm, EN 10131 |
| 1.6 mm alu | 1.6 mm aluminium 5052-H32, ±0.10 mm, EN 485-4 |
| 18 ga stainless | 1.27 mm stainless 304 2B, ±0.10 mm, ASTM A480/A480M |
| 20 ga galv sheet | 1.0 mm hot-dip galvanized, Z275 (G90), ±0.09 mm, EN 10346 |
Note what changed. The nominal number moved to millimetres, a tolerance appeared, and a standard name tells the buyer's engineer where the tolerance came from. A buyer who can read those three fields does not need to email you.
One more habit worth building: quote the nominal thickness you will actually roll, not the gauge equivalent rounded to a marketing number. If you are supplying 1.5 mm, say 1.5 mm. Calling it "16 gauge" invites the buyer to look up 16 gauge in whichever table sits on their desk and to expect 1.588 mm — a 0.088 mm difference that is invisible on a photo and very visible on a press brake.
Where the Number Goes Wrong on the Drawing
Thickness disputes rarely start in the spec sheet. They start on the image, because a photograph of a bent metal panel does not tell anyone whether the wall is 1.2 mm or 1.6 mm, and the eye cannot referee it.
The reliable fix is to put the measured number on the picture, at the edge it describes: a leader line from the actual sheet edge, the value in millimetres, the tolerance beside it, and the material callout in the same block. That is the difference between an image that decorates a catalogue and an industrial spec diagram a purchasing engineer can quote from. Software that snaps to the real edge and locks the measured value onto the image gets this right by construction; an AI image generator that paints a plausible-looking "1.5 mm" beside a rendered panel produces a number nobody measured, which is worse than no number at all.
The same discipline applies to the other numbers on the sheet — bend radius, hole pitch, edge distance. If you have not standardised how those land on the drawing, how to label metal product specifications covers the layout, and steel profile dimensions for export handles the extruded and rolled shapes that live next to the sheet on the same order.
Pre-Quote Thickness Checklist
Run this before any sheet metal quote leaves the building.
- Thickness stated in millimetres (decimal, not fraction), with inches only in brackets
- Material and temper/grade named (CR steel, 304 2B, 5052-H32, hot-dip Z275)
- Tolerance stated as a ± value, not "as per standard"
- The standard that owns the tolerance is named (ASTM A480, EN 10131, EN 10346, EN 485-4)
- For galvanized: coating designation given (Z275 / G90), and it is clear whether thickness is with or without coating
- Gauge number, if used at all, appears in brackets after the millimetre value — never alone
- The drawing carries the same thickness value as the spec table, at the same precision
- Any buyer-supplied gauge number confirmed in writing against a millimetre value before costing
The last line is the one that saves money. When a buyer's RFQ says "14 gauge", reply with "14 gauge = 1.90 mm mild steel in our quote; confirm if you mean 1.98 mm stainless." Two minutes of email beats a container of wrong-thickness panels.
Common Ground and Grey Areas
Not every part of this is a hard rule, and pretending otherwise is how suppliers lose credibility.
Hard: the tolerance standards. ASTM A480/A480M, EN 10131 and EN 10346 are published documents with numeric limits; a mill either meets them or does not.
Convention: the gauge tables themselves. The Manufacturers' Standard Gauge and the galvanized sheet gauge are industry conventions, widely followed and widely published, not law — except for the 1893 US Standard Gauge, which is law and which almost nobody actually uses for modern sheet.
Varies by market: which system your buyer has in mind. North American buyers reach for gauge numbers reflexively. European and most Asian buyers work in millimetres and treat a gauge number as a foreign-language word they will translate wrongly. UK legacy drawings sometimes carry Standard Wire Gauge (SWG), which is a fifth table and disagrees with all four above.
FAQ
Is 16 gauge thicker than 18 gauge?
Yes. Gauge numbers run backwards — the lower the number, the thicker the metal. In mild steel, 16 gauge is 1.52 mm and 18 gauge is 1.21 mm, so 16 gauge is about 25% thicker.
How do I convert sheet metal gauge to mm?
You cannot convert without knowing the metal. Use the Manufacturers' Standard Gauge table for uncoated steel, the galvanized sheet gauge table for coated steel, the stainless gauge table for stainless, and the Brown & Sharpe (AWG) table for aluminium. For 16 gauge, that gives 1.519 mm, 1.613 mm, 1.588 mm and 1.291 mm respectively.
Is sheet metal gauge the same for stainless and mild steel?
No. Stainless runs on a separate gauge series. At 16 gauge the difference is 1.588 mm versus 1.519 mm — small enough to survive a phone call, large enough to fail an incoming inspection that checks against a nominal with a ±0.10 mm tolerance.
What thickness tolerance should I quote with sheet metal?
Quote the tolerance from the standard that governs the product, not a number you invented. Typical cold-rolled steel at 1.5 mm sits around ±0.13 mm under EN 10131; stainless flat products follow ASTM A480/A480M. Always name the standard beside the tolerance so the buyer's QC team knows which limit they are testing against.
Should I stop using gauge numbers entirely with overseas buyers?
Use millimetres as the primary value and put the gauge number in brackets for buyers who think in gauge. Removing it entirely confuses North American customers; leading with it confuses everyone else.