Gauge is a legacy measurement system used to express sheet metal thickness as a dimensionless number, originally derived from the weight of a sheet per square foot. Millimetres are a direct SI unit of measurement.

There is no fixed mathematical relationship between the two: the conversion depends entirely on the material type and the gauge system in use, which is why gauge charts are material-specific and why this tool always requires a material selection first.

Because gauge is based on the weight of a sheet per unit area, and different metals have different densities. A given weight of aluminum occupies a different volume than the same weight of steel. As a result, gauge 16 in steel (1.519 mm) is thicker than gauge 16 in aluminum (1.290 mm).

If you order 16-gauge aluminum expecting steel thickness, you will receive material approximately 15% thinner than anticipated. Always use the correct gauge chart for your material, and always specify both gauge and material together on purchase orders.

Thinner, for steel, aluminum, copper, and brass. A higher gauge number corresponds to a thinner sheet across these materials. This is the most common point of confusion for engineers new to working with sheet metal specifications.

The one exception is zinc, which uses an inverted scale where a higher gauge number means a thicker sheet. This tool flags that exception clearly in the material selector.

Three separate gauge systems are in common use, and the same gauge number gives different thicknesses across each. This is the root cause of most gauge-related specification errors in international supply chains.

As a general rule, sheet metal ranges from approximately 0.5 mm (thinnest practical) to 6 mm (thickest). Material below 0.5 mm is typically classified as foil. Material above 6 mm is generally classified as plate rather than sheet, though the exact boundary varies by material, manufacturer, and regional standard.

Nominal gauge thickness is not exact. Under ASTM A568 and EN 10131, the acceptable tolerance for cold-rolled sheet metal is typically ±0.03–0.08 mm depending on nominal thickness, sheet width, and steel grade.

For example, 16-gauge steel at a nominal 1.519 mm may have an acceptable range of approximately 1.47–1.57 mm under standard mill tolerances. This tool shows the tolerance band for each conversion result. For critical applications, verify the nominal thickness against your drawing specification before ordering.

Yes, but best practice is to also specify the exact thickness in millimetres alongside the gauge callout, along with the gauge system and applicable standard. For example: “16 ga carbon steel per ASTM A568 — 1.519 mm nominal”. This removes ambiguity if the supplier uses a different gauge standard or operates in a different region.

For critical tolerances, always include the nominal mm value and define acceptable tolerance limits explicitly on the drawing.

Gauge affects corrosion durability but not inherent corrosion resistance. A thicker gauge means more material for corrosion to work through before the part fails structurally, so it will last longer under the same exposure conditions. It does not change the material’s inherent resistance to corrosion: that is determined by alloy composition, surface finish, and any protective coatings applied.

SMACNA standards specify ductwork thickness based on pressure class and duct width. The most common gauges are 20 gauge (0.912 mm / 0.0359 in) for low-pressure rectangular duct under 305 mm wide, and 16 gauge (1.519 mm / 0.0598 in) for medium-pressure duct and round spiral duct in higher-pressure applications. Galvanized carbon steel is the dominant material for commercial HVAC, with aluminum used in corrosion-sensitive environments.

Identify the applicable gauge standard for your material first. MSG applies to carbon and galvanized steel; AWG (Brown and Sharpe) applies to aluminum, copper, and brass; stainless steel uses a fractional-inch standard. Once the standard is confirmed, the conversion is a direct table lookup.

Do not use generic gauge conversion formulas that assume a single system: this produces significant errors across material families. This calculator covers all three standards for gauges 3 through 30.