Sheet Metal Gauge Chart: Gauge to mm and Inches, and Why One Gauge Number Has Four Answers

1. Why One Gauge Number Has Five Answers

The word gauge arrived from wire drawing, where a number counted the passes a rod made through a die on its way to a thinner wire. More passes meant thinner metal, which is why every gauge scale on this page runs backwards: a larger number is a thinner sheet. When sheet metal replaced hand-drawn stock, each industry kept the numbering it already had, and because the industries did not talk to each other, the scales never converged.

The result is five bases behind one word. Carbon steel gauge is a weight scale. Galvanized steel gauge is a weight scale with a zinc allowance added. Stainless steel gauge is a ladder of sixty-fourths of an inch. Aluminum, brass and copper sheet use a geometric progression inherited from wire. And architectural copper is not sold by gauge at all but by ounces per square foot, a fifth basis that is not a gauge number at all.

The four values of sixteen gauge

The spread between the thickest and the thinnest of those four rows is 323 micrometers, or about 25 percent. That is a larger error than most shop tolerances, and it arrives without a warning, because every one of the four numbers looks like a normal answer.

2. Gauge to Millimeters, All Metals Side by Side

This is the table to keep open while ordering. Every figure is the thickness of that gauge number in that metal, rounded to the nearest thousandth of a millimeter. Read across a row to see how far apart the metals are at the same gauge number.

The widest gaps are at the light end, where each scale was tuned for a different product. At 30 gauge galvanized steel is 0.399 mm and aluminum is 0.254 mm, a difference of 145 micrometers on a sheet that is less than half a millimeter thick. Order aluminum to the galvanized callout and the part arrives noticeably softer and easier to form. The sections below give the basis for each column, and the reason the numbers are what they are.

3. Carbon Steel on the Manufacturers Standard Gage

Carbon steel sheet uses the Manufacturers Standard Gage, usually written MSG. The system is a weight scale rather than a thickness scale, and that single fact explains why the millimeter figures look arbitrary. The standard takes the density of steel as 489.6 lb/ft³, which is 40.80 lb/ft² per inch of thickness. A further adjustment of about 2.5 percent then produces the figure actually used for sheet: 41.82 lb/ft² per inch. The adjustment exists because sheet weights are calculated from the specified width and length with all shearing taken on the over side, and because a rolled sheet is a little thicker at the center than at the edges.

Each gauge number is therefore a nominal weight, and the thickness is that weight divided by 41.82. Sixteen gauge is 40 oz/ft², which is 2.5 lb/ft², which is 0.0598 inch. Every row below is derived that way, and the weights column is the definition, not a conversion of the thickness.

Two things follow from a weight basis. First, the steps are uneven: the drop from 10 gauge to 11 gauge is 0.378 mm, while the drop from 24 gauge to 25 gauge is 0.076 mm. There is no tidy formula and the ladder cannot be interpolated. Second, the weight is exact by definition while the thickness is derived, so if a supplier quotes you a gauge and a weight that disagree, the weight is the figure to trust.

Practically, 16 gauge is the general fabrication workhorse, 10 to 12 gauge is structural sheet and bracket territory, and 22 to 26 gauge is roofing, flashing and light duct. To turn any of these thicknesses into a length measurement in another unit, see the length conversion guide or use the inches to millimeters converter directly.

4. Galvanized Steel, and the Zinc Allowance

Galvanized steel has the oldest and least trustworthy of the scales. The Galvanized Sheet Gage was set in the early days of galvanizing, before modern wiping rolls could control coating weight, so the zinc thicknesses it assumes are heavier than what a mill produces today. That is why a chart built on the old galvanized scale can be a percent or two generous on a modern coil.

The scale itself is simple: take the base steel gauge and add a zinc allowance. The allowance used in the standard charts is 2.5 oz/ft² of zinc in total, split as 1.25 oz on each face. Expressed in the same weight basis as the steel, that allowance adds about 0.0037 inch at every gauge number, which is why the galvanized column sits consistently above the steel column rather than drifting away from it.

The practical consequence: if you are designing for a minimum thickness, order on the base steel thickness and treat the zinc as extra, not as structural. Inspectors measure coating weight separately, and a coil that meets its gauge on base steel can still fail a coating specification. The conventional millimeter figure for 16 gauge galvanized is 1.613 mm, of which 1.519 mm is steel.

5. Stainless Steel and the Sixty-Fourths Ladder

Stainless is the one scale that is a clean fraction ladder. It descends from the United States Standard Gage of 1893, and its numbers land on exact divisions of an inch: 11 gauge is exactly 1/8 inch and 16 gauge is exactly 1/16 inch. From 3 gauge down to 14 gauge the ladder steps by a whole sixty-fourth of an inch each time, and below that it steps in half and then smaller fractions.

Because these values are exact, they terminate in decimal, which is also why every published stainless table agrees to five or six places and then disagrees in the fourth. The standard published form rounds the exact value to four decimals, and a value that lands exactly on a half, such as 2/64 of an inch, can be rounded down by one table and up by the next. The table below shows the fraction, the exact decimal and the rounded form, so you can see where a printed chart is likely to differ from this one.

Note the arithmetic behind the last digit. 7 of the rows below land exactly on a half at the fifth decimal place, which is where the two nearest published figures separate. If your supplier's chart shows 0.0312 inch for 22 gauge and this page shows 0.03125, neither number is wrong; one is a rounding of the other. The difference is under two thousandths of a millimeter and well inside the thickness tolerance of the material.

6. Aluminum, Brass and Copper on the Brown and Sharpe Scale

The non-ferrous sheet metals do not use a weight scale. They use the Brown and Sharpe gauge, drawn up for wire by J. R. Brown and Lucian Sharpe in 1856 and adopted by the Association of Brass Manufacturers in February 1857, which is the same scale the electrical trade calls American Wire Gauge. It is a geometric progression: each size is a fixed ratio smaller than the last, that ratio being the thirty-ninth root of 0.460, or 1.1229322. Size 0000 is defined as 0.4600 inch and size 36 as 0.005 inch, with thirty-eight sizes between them.

A useful property of a geometric scale is that six sizes heavier is almost exactly twice the weight per unit area, because the sixth power of the ratio is 2.0050. Gauge numbers for sheet therefore move in steps that a chart can approximate without arithmetic. In millimeters the definition is compact: diameter in millimeters equals 0.127 multiplied by 92 to the power of (36 minus the gauge number) divided by 39.

Because aluminum, brass and copper sheet share this scale with wire, the site's own wire converters double as sheet converters: AWG to square millimeters gives the cross-sectional area of a wire at that gauge, and square millimeters to AWG runs the other way. For sheet you want the diameter rather than the area, but the gauge number is defined by the same series.

One detail worth knowing before you compare this table with a supplier's. The sheet tables round the inch figure to four decimals first and then convert to millimeters, while a wire table converts the unrounded diameter straight to millimeters. At 16 gauge that difference is between 1.290 mm from the sheet convention and 1.291 mm from the wire convention. Both are correct; they simply round at different points in the chain.

7. Copper by Weight, Not by Gauge

Architectural copper is the exception that catches people out. It is not specified by gauge but by ounces per square foot, so that 16 ounce copper means one pound of copper spread over one square foot of sheet. Because the density of copper is fixed at 555 lb/ft³, which is 8.89 grams per cubic centimeter and the value adopted for copper by the ASTM and the IEC, the ounce figure converts to a thickness with no ambiguity at all.

The ounce system has an advantage over gauge: it is proportional in the same direction as the number, so a heavier copper is a thicker copper, and the arithmetic is a straight ratio. A 20 ounce sheet is exactly one and a quarter times the mass of a 16 ounce sheet. Gauge gives you no such handle, which is precisely why the architectural trade abandoned it. Suppliers sometimes quote copper in gauge as an approximation, and the two scales do not line up: 16 ounce copper falls between 23 and 24 gauge on the Brown and Sharpe scale without equalling either, so always specify the ounces on a copper order.

8. What a Sheet Actually Weighs

Because the carbon steel gauge is defined by weight, the interesting question is which weight. There are two answers, and the gap between them is about 2.4 percent at every gauge.

The nominal weight is the one built into the gauge definition: the gauge number names a weight in ounces per square foot, and that weight is what appears on an order and an invoice. The true mass of the finished sheet is slightly less, because the 41.82 lb/ft² per inch figure carries the allowance for shearing on the over side and for the crown of a rolled sheet, while the actual density of steel is closer to 7,850 kilograms per cubic meter.

The constant gap is the point. It is not a rounding error and it does not disappear at some gauge number; it is the difference between the commercial basis and the physical basis, and it is about 2.4 percent on every row. For a quick estimate of a real part, use the true mass: for steel, kilograms per square meter equals the thickness in millimeters multiplied by 7.85. For the nominal quantity you will be billed for, use the weight column at the top of the chart.

To convert between the density units used above, the site has a density converter that handles kilograms per cubic meter to pounds per cubic foot and the reverse, and the density conversion guide explains why density is the one place where a unit change can also change the value of the material. Mass conversions themselves are covered in the weight and mass guide.

9. Why Two Charts Give Two Answers

Put three sheet metal gauge charts side by side and the numbers will not match everywhere. There are three ordinary reasons, and one that is a real error.

Rounding order. A chart can round the inch figure and then convert to millimeters, or convert to millimeters and then round. The two orders differ by up to one thousandth of a millimeter at ordinary sheet thicknesses, which is enough to make two correct charts disagree in the third decimal place. This page rounds the inch to four decimals first, then converts, which is what supplier charts do, and states the exact value alongside so the rounding is visible.

A column that has slipped a row. Galvanized charts are especially prone to this, because the galvanized column is a copy of the steel column with an offset added by hand. If a galvanized figure looks like the value for the next gauge down, that is usually what happened. Check the figure against the steel column above it before accepting it.

An obsolete zinc allowance. The oldest published galvanized tables still carry coating thicknesses from before modern wiping, which makes them a little thicker than mill product at the same callout. A chart copied from those tables will read high by one or two percent on galvanized, and correct on everything else.

The error that is not ordinary is a chart that mixes scales, quoting the carbon steel figures in a column labeled simply gauge and leaving the metal unnamed. That is where the five-answer problem starts, and it is why every table on this page names its standard.

10. Putting Gauge on a Drawing

The single most useful habit with gauge numbers is to stop using them on drawings. Specify thickness in millimeters or in decimal inches, and add the gauge in brackets if a shop floor habit needs it. A decimal thickness has one meaning across every metal, and it is the figure the material certificate will state, so there is nothing to reconcile at inspection.

Three rules of thumb are worth carrying into the shop, all of them derived from thickness rather than from gauge, which is the point:

• Gauge stops being used above about 3/16 inch, which is 0.1875 inch or 4.76 millimeters. Thicker material is bought by fraction or decimal, so a callout above that range in gauge is a sign the drawing came from somewhere else.
• Minimum inside bend radius for mild steel is commonly set near one times the thickness, growing larger for stainless and for hard aluminum tempers. Because it scales with thickness and not with gauge, the same rule works on every metal.
• Weight per unit area scales directly with thickness: thickness in millimeters times 7.85 gives kilograms per square meter for steel, and multiplying that by the area in square meters gives the part mass with no gauge table needed.

For the general method behind these conversions, and for the families where a factor does not exist, see How to Convert Units. For a single page of exact factors across every category, see the unit conversion chart.

Frequently Asked Questions