Key takeaways
- Lower gauge number means thicker steel. The scale is inverse and not linear.
- Steel gauge, aluminum gauge and stainless gauge are three different tables for the same number.
- Bending stiffness rises with the cube of thickness, so 16 gauge is roughly twice as stiff as 18 gauge.
- Geometry beats gauge: a formed return or a brace does more than one gauge step.
Why gauge is an inverse scale
Gauge predates the ability to measure thin sheet accurately. The Manufacturers Standard Gauge for sheet steel defines each gauge number by weight per square foot, and the numbers count how many sheets it took to make up a fixed weight. More sheets to the stack means thinner sheets, so the number goes up as the steel gets thinner. Thickness is derived from the weight using the density of steel, which is why the inch figures look arbitrary.
The full table
| Gauge | Inches | mm | lb / sq ft | kg / sq m |
|---|---|---|---|---|
| 7 | 0.1793 | 4.55 | 7.500 | 36.6 |
| 8 | 0.1644 | 4.18 | 6.875 | 33.6 |
| 9 | 0.1495 | 3.80 | 6.250 | 30.5 |
| 10 | 0.1345 | 3.42 | 5.625 | 27.5 |
| 11 | 0.1196 | 3.04 | 5.000 | 24.4 |
| 12 | 0.1046 | 2.66 | 4.375 | 21.4 |
| 13 | 0.0897 | 2.28 | 3.750 | 18.3 |
| 14 | 0.0747 | 1.90 | 3.125 | 15.3 |
| 15 | 0.0673 | 1.71 | 2.813 | 13.7 |
| 16 | 0.0598 | 1.52 | 2.500 | 12.2 |
| 17 | 0.0538 | 1.37 | 2.250 | 11.0 |
| 18 | 0.0478 | 1.21 | 2.000 | 9.8 |
| 19 | 0.0418 | 1.06 | 1.750 | 8.5 |
| 20 | 0.0359 | 0.91 | 1.500 | 7.3 |
| 21 | 0.0329 | 0.84 | 1.375 | 6.7 |
| 22 | 0.0299 | 0.76 | 1.250 | 6.1 |
| 23 | 0.0269 | 0.68 | 1.125 | 5.5 |
| 24 | 0.0239 | 0.61 | 1.000 | 4.9 |
| 25 | 0.0209 | 0.53 | 0.875 | 4.3 |
| 26 | 0.0179 | 0.45 | 0.750 | 3.7 |
The cube rule, and its limit
Bending stiffness of a flat panel scales with the cube of its thickness. That produces satisfying numbers: 16 gauge is about 1.96 times as stiff in bending as 18 gauge, and 14 gauge is about 3.8 times.
It also explains why gauge is oversold. The cube rule applies to a flat panel. The moment you form a return, add a hem, roll a bead or spot weld a brace behind the panel, you are changing the section, and section geometry moves stiffness by far more than one gauge step does. A 20 gauge drawer front with a formed double return and a stiffener can be more rigid than a flat 18 gauge one, at less weight and less cost.
The practical reading guide for what gauge means on a tool box, and which panel the number usually refers to, is in steel gauge on tool boxes, decoded.