A Guide to Sheet Metal Gauges and Conversions 

Sheet metal is produced by rolling metal into flat sections of varying thickness. Thinner material is often described as foil, while heavier flat material is generally referred to as plate. Sheet metal has been used throughout history for walls, roofs, containers, and even suits of armour. Today, sheet metal has almost unlimited applications across the construction, manufacturing, automotive, aerospace, and marine industries, as well as for protection and decoration.

At metals4U, sheet metal is supplied and sold by thickness, not by gauge. However, gauge references are still widely used across industry, which is why understanding how gauge relates to actual thickness can be helpful when comparing materials or working from drawings or specifications.

What Are Sheet Metal Gauges?

Sheet metal thickness is sometimes described using a measurement system known as the gauge system. Gauge numbers do not follow a straight scale, and they were not designed to convert neatly into millimetres. There is no single calculation that works, which is why thickness is usually checked against a chart.

For this reason, gauge is better viewed as a point of reference rather than a precise measurement. For this reason, metals4U lists all sheet metal products by their actual thickness in millimetres, ensuring clarity and consistency when ordering.

As sheet metal is produced, the rolling process can introduce slight variations in thickness. The allowable variation within each thickness range is known as tolerance. This is another reason why relying solely on the gauge can be misleading.

In the UK, gauge references usually relate to Standard Wire Gauge, while other systems are used elsewhere. These systems do not correlate exactly, and the meaning of  ‘gauge’ can also change depending on the metal.

Gauge references also vary by material, with different metals following their own standards. A 20 gauge mild steel sheet, for example, does not have the same thickness as 20 gauge aluminium. This difference is because ferrous and non-ferrous metals are measured against different reference values.

For sheet metal, relying solely on gauge can cause issues when the material is cut, formed, or fitted. These issues are why metals4U supplies sheet metal by stated thickness in millimetres, allowing customers to order with confidence and avoid confusion.

Standard Wire Gauge Conversion Chart 

Gauge conversion charts show what a gauge reading equates to in real terms. They list the corresponding thickness in millimetres and inches, making it easier to interpret specifications that reference gauge rather than measurement.

Gauge values do not follow a straight or predictable scale. As the gauge measurement gets bigger, the material actually becomes thinner, but not by the same amount each time. Because of this, charts are essential for checking equivalents or translating gauge-based specifications into measurable thickness.

1 SWG = 0.300” / 7.620 mm	16 SWG = 0.064” / 1.626 mm
2 SWG = 0.276” / 7.010 mm	17 SWG = 0.056” / 1.422 mm
3 SWG = 0.252” / 6.401 mm	18 SWG = 0.048” / 1.219 mm
4 SWG = 0.232” / 5.893 mm	19 SWG = 0.040” / 1.016 mm
5 SWG = 0.212” / 5.385 mm	20 SWG = 0.036” / 0.914 mm
6 SWG = 0.192” / 4.877 mm	21 SWG = 0.032” / 0.813 mm
7 SWG = 0.176” / 4.470 mm	22 SWG = 0.028” / 0.711 mm
8 SWG = 0.160” / 4.064 mm	23 SWG = 0.024” / 0.610 mm
9 SWG = 0.144” / 3.658 mm	24 SWG = 0.022” / 0.559 mm
10 SWG = 0.128” / 3.251 mm	25 SWG = 0.020” / 0.508 mm

Sheet metal gauge chart (Steel, Aluminium, Stainless Steel)

Common sheet metal gauge thicknesses (Approximate)

Because different metals follow different gauge standards, a single gauge number cannot be applied consistently across all materials. Mild steel, stainless steel, aluminium, and copper or brass each have their own reference thicknesses for the same gauge.

For this reason, gauge charts should be treated as material-specific reference tools, not as a basis for ordering sheet metal.

Important: The values in this table are approximate and may vary depending on the gauge system used and manufacturing tolerance.

When comparing sheets made from different metals, it is vital to check the stated thickness in millimetres or inches. At metals4U, sheet metal is always sold by thickness, giving better control over strength, fit, and fabrication requirements.

How to choose the right sheet metal thickness

There is no single thickness that suits every job. The right choice depends on what the sheet needs to do once it is in use. A thicker sheet tends to hold its shape better, but it also weighs more and is harder to work with during cutting or forming. Thinner material is easier to handle and shape, although it is not always suitable where strength is required.

The way the sheet will be processed also makes a difference. Heavier material usually needs more space when bending, and equipment that can apply greater force. Thinner material can be folded more tightly, but it is more prone to distortion if not adequately supported. Welding behaviour, stiffness once fitted, and how well the sheet holds its shape are all factors worth considering.

Where fit is key, such as during folding, welding, or installation, it is clearer to work to a stated thickness in millimetres.

A note on tube gauge vs sheet gauge

It is worth noting that the tube gauge is different from the sheet gauge. While metals4U supplies sheet metal by thickness, certain tube products are specified and sold by gauge. Tube gauge refers to wall thickness and follows different conventions from sheet gauge, so the two should not be compared directly.

When ordering tube, always refer to the product description to confirm whether the size is stated by gauge or by wall thickness.

We stock a full range of sheet metal in a wide choice of thicknesses and available in aluminium, brass, copper, mild steel, and stainless steel.