A gauge number is not a thickness
It is a position in a table, and there is more than one table. Carbon steel sheet uses the manufacturers standard gauge, which was defined from weight per square foot rather than from a measurement. Stainless has its own series. Aluminum, brass and copper use the Brown and Sharpe gauge, which is the same series as American wire gauge and comes from a completely different lineage. Galvanized steel uses the steel series with the zinc coating added on top, conventionally 0.0037 inches for the standard coating weight.
The practical consequence is on the ordering line. Sixteen gauge means 0.0598 inches in carbon steel, 0.0635 in galvanized, 0.0625 in stainless and 0.0508 in aluminum. If you design a bracket in steel, prototype it in aluminum because that is what was on the rack, and then order sixteen gauge without saying which metal, you will get a part fifteen percent thinner than the one you tested. Specify the thickness in decimals whenever anything depends on it, and treat the gauge number as a shorthand for talking to the supplier rather than as an engineering dimension.
Where the numbers come from
The steel series has an odd historical origin. It was defined so that each gauge corresponds to a nominal weight per square foot, with sixteen gauge landing on 2.5 pounds per square foot, and the thickness was then derived by dividing that weight by an assumed density. The assumed density used in that derivation is slightly higher than the modern figure for mild steel, which is why the thickness in the table times a real steel density comes out around 2.44 pounds per square foot rather than the nominal 2.5. This page uses actual densities, so its weights are the physics rather than the historical convention. The difference is about two percent, which matters for a shipping quote across a pallet of sheet and does not matter for anything else.
The Brown and Sharpe series used for the non-ferrous metals is a geometric progression: each gauge is a fixed ratio thinner than the one before, running from 0.46 inches at gauge zero-zero-zero-zero down by a constant factor. That is why the aluminum numbers look tidier as ratios and never coincide with the steel ones except by accident.
Weight per square foot, and why it is the useful number
Thickness times density times 144 gives pounds per square foot, and that is the figure to carry around, because sheet is bought by the sheet and priced by the pound. Multiply by the sheet area in square feet and you have what will come off the truck.
| Gauge | Steel (in) | Steel lb/sq ft | Aluminum (in) | Aluminum lb/sq ft |
|---|---|---|---|---|
| 10 | 0.1345 | 5.49 | 0.1019 | 1.43 |
| 12 | 0.1046 | 4.27 | 0.0808 | 1.13 |
| 14 | 0.0747 | 3.05 | 0.0641 | 0.90 |
| 16 | 0.0598 | 2.44 | 0.0508 | 0.71 |
| 18 | 0.0478 | 1.95 | 0.0403 | 0.57 |
| 20 | 0.0359 | 1.47 | 0.0320 | 0.45 |
| 22 | 0.0299 | 1.22 | 0.0253 | 0.36 |
The aluminum column is the reason aluminum ductwork, trailer skins and truck bodies exist. Sixteen gauge aluminum weighs less than a third of sixteen gauge steel, and even matched on thickness rather than gauge it is about a third of the weight.
Tolerance, and when the table stops being true
The thicknesses here are nominal. Real sheet is rolled to a tolerance, and the allowed band is a percentage rather than a fixed amount, so at 26 gauge and thinner the tolerance is a noticeable fraction of the thickness itself. Galvanized adds a second variable, because the coating weight varies within its own specification and the 0.0037 inch allowance used here is a standard G90 figure rather than a measurement of your sheet.
None of that matters for ordering material or estimating a load. It matters when the part is a press fit, when a bend allowance is being calculated to a thousandth, or when a laser or plasma program is written against a specific thickness. In those cases put a micrometer on the actual sheet. It takes ten seconds and it settles the question that no chart can.
Once you know the thickness
Thickness is the input to most of the rest of the work. The press brake tonnage calculator takes it and a die opening and tells you whether the bend fits the machine, the bend allowance calculator turns it into a flat pattern, and the cutting cost calculator costs getting that pattern out of the sheet. For bar, tube, plate and angle rather than sheet, the metal weight calculator covers every shape from its dimensions.
Questions people ask
How thick is 16 gauge steel?
It is 0.0598 inches, or about 1.52 mm, in carbon steel on the manufacturers standard gauge. In galvanized steel it is 0.0635 inches because of the zinc coating, in stainless 0.0625, and in aluminum only 0.0508. The steel figure is close enough to a sixteenth of an inch, which is 0.0625, that people conflate them, and for most purposes that approximation is harmless. It stops being harmless when a slot has to fit the sheet or a bend allowance is being worked to a thousandth.
Why does a higher gauge number mean thinner metal?
Because the numbering came from wire drawing, where the gauge counted how many times the wire had been pulled through progressively smaller dies. More passes meant more reduction and thinner wire, so the count runs backwards relative to thickness. Sheet gauge inherited that convention. Above the top of the range the numbering runs out and the sequence goes to zeroes, so 0, 00, 000 and 0000 are progressively thicker still, and above that the industry gives up on gauges and quotes plate in decimal inches, which is what everyone should have been doing all along.
Is galvanized steel the same gauge as plain steel?
The base steel is, and the finished sheet is not. Galvanized gauge is defined as the steel gauge plus the zinc coating, which for the common G90 coating adds about 0.0037 inches across both faces. So 16 gauge galvanized measures around 0.0635 inches while 16 gauge plain steel measures 0.0598. That difference is enough to matter in a tight die, in a slip joint, and in a laser or plasma program written for the bare thickness. It also means a micrometer reading on galvanized sheet does not correspond to any entry in the plain steel table.
What gauge should I use for a project?
That depends on span, load and how it is supported, none of which a thickness chart can tell you. As orientation for what people commonly build: 22 to 26 gauge for ductwork and flashing, 18 to 20 gauge for enclosures and light brackets, 14 to 16 gauge for tool bodies and anything that gets handled, 11 to 12 gauge for structural brackets and weldments, and heavier than that is usually quoted as plate in decimal inches rather than as a gauge. Stiffness is dominated by bends and ribs rather than thickness, so a formed 20 gauge panel will beat a flat 16 gauge one by a wide margin.
How do I convert gauge to millimetres?
Look up the thickness in inches for the right material and multiply by 25.4. There is no direct gauge to millimetre relationship because gauge is not a metric system and never was, which is why the results are awkward numbers like 1.52 mm for 16 gauge steel. Most of the world outside North America specifies sheet in millimetres directly, and a metric supplier will stock 1.5 mm rather than anything that corresponds to a gauge. If you are working from a metric drawing, order by decimal thickness and ignore the gauge column entirely.