Sheet Steel Gauge to Inch and mm Chart

The table below is the Manufacturers’ Standard Gauge for UNCOATED sheet steel, and only that. A gauge number does not mean one thickness: 16 gauge is 0.0598 in in plain steel, 0.0635 in galvanised and 0.065 in aluminium. Confirm the material before you read a gauge number off a drawing, because the three scales are not interchangeable and nothing on the sheet tells you which one was meant.

Gauge is a weight, not a thickness

The Manufacturers’ Standard Gauge is defined by weight per square foot rather than by dimension. 16 gauge steel is the sheet that weighs 2.5 lb per square foot; because steel weighs 41.82 lb per square foot per inch of thickness, that works out at 0.0598 in. Every row in the table below is that same arithmetic, and the weight column is included so the derivation is visible rather than taken on trust.

Two consequences follow, and both catch people out. First, the sequence is backwards — a higher gauge number is a thinner sheet, because it is a lighter one. Second, the gauge scale is material-specific by construction: a square foot of aluminium weighing 2.5 lb is far thicker than a square foot of steel weighing the same, so the same gauge number has to mean a different thickness in a different metal.

Sheet steel gauge conversion

Method and precision

Thicknesses are the Manufacturers’ Standard Gauge for uncoated sheet steel. Each is reproduced from its own defining weight — pounds per square foot divided by 41.82, the weight of a square foot of steel one inch thick — rather than transcribed, so a wrong digit cannot survive. Millimetres are exact conversions at 25.4 mm to the inch, rounded to three decimal places for display. The galvanised column below is the same steel weight plus 0.15625 lb per square foot of zinc coating. The aluminium column is a separate published scale with no derivation shown here; it is included to demonstrate that the scales differ, not as a substitute for a material specification.

The same gauge number in three materials

This is the table to check before cutting. The three columns are three different standards that happen to share a numbering scheme, and the difference is not trivial: at 20 gauge the galvanised sheet is about 10 per cent thicker than the plain steel one, and the gap widens as the sheet gets thinner because the zinc is a fixed weight added to a shrinking base.

When to measure instead of looking up

A gauge number is a nominal figure with a rolling tolerance around it, so a sheet that measures a few thousandths off its table value is normal stock rather than a problem. If a fit depends on the thickness — a slip joint, a bend allowance, a fastener grip length — measure the sheet you have with a micrometer rather than working from the gauge. If it only has to be strong enough, the table is fine.

Micrometers and calipers are in measuring equipment, sheet cutting is in cutting tools, and holding it while you work is in clamping tools.