One number, four thicknesses
“18 gauge” on a drawing has not specified a thickness. It has specified a number that four different standards interpret four different ways.
The same gauge number across the four common sheet materials, in millimetres| Gauge | Steel | Galvanised | Stainless | Aluminium | Spread |
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| 10 | 3.416 | 3.510 | 3.572 | 2.588 | 38.0% |
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| 14 | 1.897 | 1.994 | 1.984 | 1.628 | 22.5% |
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| 16 | 1.519 | 1.613 | 1.588 | 1.291 | 24.9% |
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| 18 | 1.214 | 1.311 | 1.270 | 1.024 | 28.0% |
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| 20 | 0.912 | 1.006 | 0.952 | 0.813 | 23.9% |
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| 24 | 0.607 | 0.701 | 0.635 | 0.511 | 37.3% |
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The spread never falls below about twenty percent anywhere the four series overlap, and at the extremes it approaches forty. That is not a rounding disagreement; it is a different piece of metal.
Aluminium is the thinnest at every shared gauge number, without exception — at 16 gauge it is fifteen percent thinner than steel. This is the substitution that causes trouble, because aluminium is already about a third the stiffness of steel for a given thickness, and taking fifteen percent off the thickness on top of that compounds it. Swapping “16 gauge steel” for “16 gauge aluminium” is not a like-for-like change in any sense.
Galvanised runs thickest, which surprises people who expect a coating to be negligible. It is not negligible relative to a thin sheet: the zinc allowance is 0.0037 of an inch, which on 24 gauge is fifteen percent of the whole thickness.
Four standards that are not even alike
These are not four versions of one idea with slightly different numbers. They are four different kinds of definition, and only one of them has a rule you can compute.
- Uncoated steel uses the Manufacturers’ Standard Gauge, which is a weight series. The gauge numbers were set from pounds per square foot — 3 gauge is about ten pounds, 24 gauge about one — and the thicknesses are whatever those weights work out to. That is why they are unround numbers like 0.0598 rather than anything a person would choose.
- Galvanised steel takes the uncoated series and adds a flat 0.0037-inch allowance for the zinc, at every gauge in the table. It is the only one of the four defined in terms of another.
- Stainless runs in exact fractions of an inch. Gauges 7 to 14 step down by precisely one sixty-fourth each: 11 gauge is one eighth, 16 gauge is one sixteenth, 22 gauge is one thirty-second. It is the tidiest of the four and has nothing to do with the carbon steel series.
- Aluminium uses Brown & Sharpe, which is a formula: thickness is 0.005 × 92^((36 − n)/39) inches. It is the same standard as American Wire Gauge, which means 16 gauge aluminium sheet and 16 AWG wire are the same dimension — the only place the wire and sheet worlds actually coincide.
That last point cuts both ways. Because aluminium sheet shares its gauge with wire, and steel sheet does not, a shop that works in both will find the numbers agreeing in one material and not in the other, for reasons that are entirely historical.
Backwards, and not evenly spaced
A higher gauge number is thinner. That much most people know. What catches them is that the steps are nothing like equal.
What one gauge number is worth, in uncoated steel| At gauge | One step is | As a proportion |
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| 8 | 0.379 mm | about 10% |
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| 14 | 0.188 mm | about 10% |
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| 20 | 0.076 mm | about 8% |
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| 28 | 0.036 mm | about 9% |
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One gauge at the thick end is worth ten times one gauge at the thin end. So “go two gauges heavier” means adding three quarters of a millimetre on 8 gauge and seven hundredths on 28 — the same instruction, two orders of magnitude apart in effect.
As a proportion the steps are far more consistent, sitting between roughly eight and seventeen percent throughout. That is the useful mental model: a gauge number is a rough tenth, not a fixed amount.
There is a further difference in structure. Aluminium’s Brown & Sharpe series is genuinely geometric — the ratio between adjacent gauges is a constant 92^(1/39), about 1.1229, at every single step. Steel and stainless are arithmetic instead, with a constant absolute step over a range and a ratio that drifts from about 1.10 to 1.20 across the same span. Two materials, two different kinds of progression, under one set of numbers.
Where the numbers came from
None of this was designed. Gauge numbers come from wire drawing, where a wire was pulled through successively smaller dies and the number counted the number of drawings — which is why the scale runs backwards. More passes, thinner wire, higher number.
Sheet metal borrowed the idea and each trade borrowed it separately, at different times, from different starting points, for different metals. By the late nineteenth century there were dozens of competing gauge systems in Britain and America, and the confusion was bad enough that the British Standard Wire Gauge was fixed by an Act of Parliament in 1883 to try to end it. It did not end it.
What actually happened is that the material standards moved on and the gauge numbers stayed as habit. ASTM A480 specifies stainless sheet by decimal thickness. ASTM B209 specifies aluminium by decimal thickness. Neither is written in gauge numbers at all — the numbers survive in ordering, in conversation and on drawings, which is exactly where they do the most damage.
Outside the US and UK the question mostly does not arise. Sheet is ordered in millimetres across Europe and India, and the metric sizes are round numbers chosen by people rather than derived from a weight series: 0.5, 0.6, 0.8, 1.0, 1.2, 1.5, 2.0, 2.5, 3.0 mm. Notice that none of those is a gauge in any of the four systems. A 2 mm sheet is 5.4% thicker than 14 gauge steel and 5.5% thinner than 13 — it sits between them, belonging to neither.
Specifying it so nobody has to guess
The practical conclusion of everything above is short: write the thickness, not the gauge.
A specification reading 1.5 mm 304 stainless, 2B finish or 0.060″ aluminium 5052-H32 cannot be misread. One reading 16 gauge can be misread four ways, and the person misreading it will be confident, because in their trade that number has always meant one thing.
Three habits that prevent most of the trouble:
- Put the material next to the number, always. If a gauge number has to appear, it should never appear alone —
16 ga (1.52 mm) mild steel is unambiguous and takes four extra characters. - Give the decimal thickness in brackets. It also survives translation, which a gauge number does not: a supplier working in millimetres has no way to interpret a bare gauge number without knowing which of the four tables you meant.
- Check what you actually received. A caliper reading is worth more than the label. If a measured thickness does not land within a couple of percent of any gauge in the relevant series, it is a metric sheet and calling it by a gauge number will mislead the next person.
For quoting and for shipping, the areal weight matters as much as the thickness — sheet is sold by weight, freight is charged by weight, and a structure has to carry it. That is why the calculator gives kilograms per square metre and pounds per square foot beside every thickness. At 16 gauge, steel is 11.9 kg/m² and aluminium is 3.5 kg/m²: less than a third, from two sheets carrying the same number.
Sources and methodology
The material standards are what actually govern a purchase, and each one specifies thickness in its own terms — which is precisely why the gauge numbers diverge. The gauge tables themselves are trade convention rather than a standard anyone maintains, so the ASTM references below are the documents that supersede them, and the reason the last section recommends not using gauge numbers at all.