Convert standard steel sheet-metal gauge to thickness in inches and millimeters.
Metal • gauge
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Sheet-metal thickness is specified by "gauge", where a higher gauge number means a thinner sheet — and the actual thickness differs by metal. This chart gives standard (uncoated) steel gauge in both inches and millimeters.
Fabricators, HVAC installers, auto-body workers and DIYers use it to read drawings and order the right material. Note that aluminum and stainless steel use slightly different gauge thicknesses than the steel values shown.
Sheet metal gauge is a legacy numbering system where higher numbers mean thinner material, and — critically — the gauge means different thicknesses for different metals. A 16-gauge steel sheet is 1.519 mm while 16-gauge aluminium is 1.290 mm, a 15% difference from the same number. This is because the systems evolved separately: steel uses the Manufacturers' Standard Gauge, derived from weight per square foot, while non-ferrous metals use the Brown & Sharpe scale. There is no formula that spans both, only lookup tables, and specifying gauge without naming the metal is meaningless.
No single formula — gauge is a lookup table, and it differs per metalSteel: Manufacturers' Standard Gauge, derived from weight per square footAluminium, brass, copper: Brown & Sharpe (AWG) scaleHigher gauge number = thinner materialwhere:
Assumptions: Always specify the metal alongside the gauge, or better, specify thickness in millimetres or thousandths of an inch. Galvanised steel is thicker than plain steel at the same gauge because the coating is included.
Look up the same gauge number in two metals and quantify the difference.
Result16 ga steel = 1.519 mm; 16 ga aluminium = 1.290 mm
A 15% thickness difference materially changes stiffness, which scales with the cube of thickness — so 16-gauge aluminium is roughly 39% less stiff than 16-gauge steel before even accounting for the difference in elastic modulus. Specify millimetres on drawings.
Gauge numbers run backwards because they originally counted how many times a piece was drawn through progressively smaller dies: more passes meant thinner material and a higher number. Nothing about the number is a measurement, which is why it cannot be converted arithmetically and why every gauge conversion is a lookup in a table rather than a calculation.
The consequence that costs money is that gauge is metal-specific. Sixteen gauge is about 0.0598 inch in standard steel, 0.0625 inch in stainless and only 0.0508 inch in aluminium — a spread of more than 20% across three materials sharing one number. Ordering by gauge alone without naming the metal is therefore ambiguous, and a drawing that specifies gauge for a part later re-quoted in a different alloy is a well-worn route to parts that do not fit or fold as intended.
For anything structural or dimensional, the decimal or millimetre thickness is the figure to specify. Modern practice increasingly quotes gauge only as a familiar shorthand alongside an actual dimension, and international suppliers generally work in millimetres regardless. Where a job crosses between shops or countries, giving the thickness removes an entire category of misunderstanding at no cost.