Sheet Metal Gauge to mm: A Complete Conversion Guide

Why Gauge Numbers Don't Mean One Fixed Thickness

Gauge numbers go back to wire drawing, where the number roughly counted how many times stock had been pulled through a die, so a higher gauge means a thinner sheet, the opposite of what most people expect the first time they see it. For steel, that early system was later formalised into the Manufacturers' Standard Gauge, defined from weight rather than a direct thickness measurement, which is why the resulting millimetre values look like an odd, uneven sequence rather than a clean scale. Aluminium, brass and copper use a separate system, generally the Brown and Sharpe gauge, and stainless steel runs its own chart that sits close to mild steel's but doesn't quite match it. None of these three systems were designed to line up with each other, and they don't.

Older British drawings sometimes reference the Birmingham Wire Gauge (BWG) or Standard Wire Gauge (SWG) rather than the American-originated Manufacturers' Standard Gauge that most current charts, including this one, are built around, so a gauge number pulled from a decades-old drawing isn't guaranteed to match a modern conversion chart even for the same material. If a gauge figure is coming from an old specification rather than a current supplier list, it's worth double-checking which system it's actually using before converting it.

A Few Gauges Worth Memorising

A handful of these are worth remembering without reaching for the chart. 16 gauge steel is roughly 1.5mm, 18 gauge is roughly 1.2mm, and 20 gauge is roughly 0.9mm. Those three cover a large share of everyday fabrication work, and knowing them by memory is often enough to sanity-check a quote or a drawing before you need to look anything else up. At the heavier end, 12 gauge (roughly 2.7mm) and 10 gauge (roughly 3.4mm) turn up regularly on brackets, heavier enclosures and light structural work, while at the lighter end, 22 gauge (roughly 0.76mm) and 24 gauge (roughly 0.61mm) are common for ductwork, cladding and casings where weight matters more than rigidity.

Does Galvanising Change the Thickness?

Galvanised sheet complicates the picture slightly, since the quoted thickness normally includes the zinc coating on top of the bare steel core. The coating typically adds somewhere around 0.1mm across both faces combined, so a galvanised sheet at a given gauge measures very slightly thicker overall than the same gauge in plain mild steel, even though the steel underneath is the same. For anything where the steel's own thickness matters structurally, such as a load calculation or a weld specification, it's the core thickness that counts, not the coated total. Our guide to galvanised sheet covers how the coating itself is applied and what it adds beyond corrosion protection.

BM Steel's Sheet and Plate Range

BM Steel specifies and sells sheet directly in millimetres rather than by gauge, in line with how flat steel is generally quoted in the UK, so once you've converted from a gauge figure you can order straight from the range without translating it again. Our cold reduced sheet gives a smooth, precision finish across common gauge-equivalent thicknesses for fabrication and engineering work, our galvanised sheet adds corrosion protection for anything going outdoors, and our aluminium treadplate covers the lighter, non-ferrous end of the same conversion table. For anything thicker than sheet range, our wider sheets and plates range moves into structural and heavy fabrication territory.