Sheet Metal & Wire Gauge Converter
Gauge to thickness for steel, galvanised, stainless, aluminium and wire — and why 16 gauge spans a quarter depending on which metal you meant.
The same gauge number on every scale
This is the whole problem in one table. Ordering by gauge alone specifies a range, not a dimension.
Why a gauge number isn't a thickness
"16 gauge" is a thickness only once you also say what the metal is. Galvanised steel is 0.0635″, stainless 0.0625″, mild steel 0.0598″ and aluminium 0.0508″ — the thickest is a quarter thicker than the thinnest. Order "16 gauge" without naming the material and you have specified a range, which is why steel standards bodies now recommend quoting decimal thickness and treating the gauge number as a nickname.
Gauge numbers run backwards, and their steps are uneven. Higher numbers are thinner, which everyone learns quickly. What catches people later is that "two gauges thicker" means different things in different parts of the scale: 20 to 18 gauge steel adds 0.0119″, while 10 to 8 gauge adds 0.0299″ — two and a half times as much for the same two numbers.
And sheet gauge is not wire gauge. American Wire Gauge is a separate scale that happens to share the numbers, and it measures a diameter. It is also the only one here defined by a formula rather than a table — diameter = 0.005 × 92(36−n)/39 inches — which is exactly where the familiar rules come from: six gauges doubles the diameter, three gauges doubles the cross-section, ten gauges multiplies the cross-section by about ten. Those hold everywhere on the scale because they are properties of the exponent.
- The galvanised offset is a constant, not a proportion. Zinc adds about 0.0037″ at every gauge, because it's a coating thickness. So it's under 3% on 10 gauge and over 10% on 20 gauge — negligible on plate, significant on thin sheet.
- Stainless lands on tidy fractions in a few places. 11 gauge is exactly ⅛″, 16 gauge exactly 1/16″, 18 gauge exactly fifty thou. Useful to remember, and a coincidence of how the scale was set rather than a pattern to extrapolate.
- Mill tolerance sits on top of all of this. Sheet is supplied to a thickness tolerance of a few per cent either way, so a measured sheet rarely matches the table exactly. Where the dimension matters, specify decimal thickness and tolerance.
How to use
- Pick the material — this is the part that actually decides the thickness.
- Enter the gauge number, or switch to measured thickness to work backwards.
- Check the comparison table before ordering anything by gauge alone.
- Where the dimension matters, quote the decimal thickness instead.
Frequently asked questions
How thick is 16 gauge?
That depends entirely on the metal, which is the whole problem with gauge numbers. Galvanised steel is 0.0635 inches, stainless 0.0625, mild steel 0.0598 and aluminium 0.0508. The thickest of those is a quarter thicker than the thinnest, so ordering sixteen gauge without naming the material specifies a range rather than a dimension. This is exactly why steel standards bodies now recommend quoting decimal thickness and treating the gauge number as a nickname.
Why do higher gauge numbers mean thinner metal?
The scales descend from wire drawing, where the number counted how many times the wire had been pulled through successively smaller dies. More passes meant thinner wire, so a bigger number meant a smaller diameter. Sheet gauge inherited the convention. It catches people out permanently because it runs opposite to almost every other measurement they use.
Are the steps between gauges even?
No, and assuming they are is a common source of error. On mild steel, going from twenty to eighteen gauge adds 0.0119 inches while going from ten to eight adds 0.0299 — two and a half times as much for the same two numbers. So "a couple of gauges thicker" means very different amounts depending on where you are in the scale, and it is worth checking rather than estimating.
Is sheet gauge the same as wire gauge?
No. American Wire Gauge is a separate scale that happens to share the numbers, and it measures a diameter rather than a sheet thickness. The overlap is a historical accident and a permanent source of confusion, particularly since non-ferrous sheet uses the same Brown and Sharpe formula that AWG does — so aluminium sheet and wire of the same gauge really are the same figure, while steel sheet of that gauge is not.
Where do the AWG rules of thumb come from?
From the formula itself. AWG is defined as diameter equals 0.005 times 92 to the power of 36 minus n over 39, in inches — unlike the sheet steel scales, which are tables of historical values. Because it is a smooth exponential, six gauges doubles the diameter, three gauges doubles the cross-sectional area, and ten gauges multiplies the area by about ten. Those relationships hold everywhere on the scale rather than approximately in places.
Why is galvanised steel thicker than plain steel at the same gauge?
Because the zinc coating is counted in the thickness. It adds about 0.0037 inches, and crucially that offset is very nearly constant across the whole scale, since it is a coating thickness rather than a proportion. So it is under three per cent on ten gauge and over ten per cent on twenty gauge — negligible on heavy sheet, and quite enough on thin sheet to stop a part fitting a slot it was designed for.
Why does stainless land on round numbers?
Coincidence of how that particular scale was set. Eleven gauge stainless is exactly an eighth of an inch, sixteen gauge exactly a sixteenth, and eighteen gauge exactly fifty thousandths. It is a genuinely useful thing to remember for those three, but it is not a pattern you can extrapolate to the gauges in between, which are not round at all.
Will a sheet actually measure what the chart says?
Rarely exactly. Sheet is supplied to a thickness tolerance that can be a few per cent either way, so the chart value is nominal rather than guaranteed. Measure at several points and away from the edges, since coil is thinner at the edges than the middle and a burr or coating will read high. Where the dimension genuinely matters, specify the decimal thickness and the tolerance rather than a gauge number.
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