Gauge is material-specific, not one scale
"16 gauge" sounds like a single, precise measurement. It isn't. Several different, unrelated numbering systems all use the word "gauge," and each one assigns a different thickness to the same gauge number. This table shows five of them side by side, steel (Manufacturers' Standard Gauge), galvanized, aluminum/brass/copper (Brown & Sharpe), Birmingham, and zinc gauge, so the gauge number you were given can actually be resolved to a real thickness once you know which material it applies to. Reading a gauge number without knowing the governing standard is reading half a specification.
How to read this table
Find your gauge number in the leftmost column, then read across to the material you're actually working with. Each column is an independent standard: the Steel column and the Aluminum/Brass/Copper column at the same row are not different ways of expressing the same thickness, they are two different thicknesses that happen to share a gauge number. If you only know "16 gauge" and not the material, you do not yet have a usable thickness. A drawing, spec sheet, or supplier catalog that calls out a bare gauge number without naming a material or standard is leaving out information you need before you can cut, order, or bend the right stock.
Where the numbers come from
The Manufacturers' Standard Gauge (steel and galvanized) is defined by weight, not thickness directly: one inch of thickness is defined as 41.82 pounds per square foot of steel, and every gauge's thickness is back-calculated from that weight standard. That's a historical artifact of how steel was originally bought and sold by weight, and it's why steel gauge thicknesses don't fall into a clean mathematical pattern the way Brown & Sharpe does. Brown & Sharpe (aluminum, brass, copper) is a true geometric series: thickness equals 0.324861 times 0.890525 raised to the power of the gauge number, a formula that reproduces every value in this table to within a fraction of a thousandth of an inch. Birmingham gauge and zinc gauge are separate, independently defined standards, listed here as published rather than derived from a formula.
Why galvanized is thicker than plain steel at the same gauge
Galvanized sheet in this table runs slightly thicker than plain steel at the same gauge number, for example 0.0635" versus 0.0598" at 16 gauge, because galvanized gauge includes a zinc coating applied over the base steel. The gauge number describes the finished, coated sheet, not the bare steel underneath it, so a small, fairly consistent thickness difference shows up across the whole galvanized column.
Why some cells read "n/a"
Galvanized and zinc gauge are not defined across the entire gauge range in this table. Galvanized sheet in this table is defined from 8 gauge through 32 gauge; zinc gauge is defined from 3 gauge through 24 gauge. Outside those ranges, "n/a" in this table means that standard simply does not define a value for that gauge number, not that a measurement is missing or unknown. Birmingham gauge, by contrast, is defined across the full 3 through 38 range shown here, so it has no blank cells.
A note on stainless steel
We do not list a stainless steel column. Stainless sheet is commonly specified by decimal thickness rather than gauge in current practice, and the sources that do assign stainless a gauge disagree with each other on which standard applies. Rather than publish a number we cannot stand behind, we've left it out. If you need a stainless thickness, specify it directly in decimal inches or millimeters rather than by gauge.
For thread and drill references built from the same sourcing standard, see the drill bit size chart.