Gauge to MM: Understanding Sheet Metal Thickness
4 Aug 2025
Sheet metal’s been around for centuries – used in everything from suits of armour to satellite panels. These days, you’ll find it everywhere too – cladding buildings, lining walls, shaping furniture, even wrapped around your car. But despite being so common, there’s one thing that still confuses almost everyone…
Sheet metal gauges.
What’s 16 gauge thickness in mm? How thick is 10 gauge steel? Why does 18 gauge in mm not match up across materials? And what’s SWG, anyway?
Let’s cut through the confusion and break it down – gauge by gauge.
Article in brief
Sheet metal gauges are used to describe thickness – but they’re not straightforward
SWG (standard wire gauge) is the most common system in the UK
One gauge number doesn’t equal one universal thickness (material matters)
Converting to mm or inches is always safer and more accurate
There are tools to help you measure thickness precisely
1
What is a sheet metal gauge?
A sheet metal gauge is a number used to describe the thickness of a sheet. Higher number = thinner sheet. Lower number = thicker sheet. Sounds simple. It isn’t.
The scale isn’t linear and was originally based on the number of operations required to draw a wire or sheet down to size. That’s why 18 gauge thickness in mm isn’t just “2 more” than 20 gauge thickness in mm – it’s a completely different measurement.
Add to that different systems for ferrous and non-ferrous metals and it gets messy fast.
2
SWG meaning and why it matters
In the UK, the most common system is SWG, short for standard wire gauge. It’s also sometimes written as s.w.g., swg wire or even just “gauge” in shorthand. But don’t confuse it with AWG (American wire gauge) – that’s a different beast altogether.
SWG isn’t just used for sheet metal – it’s also used for wire sizing. But today, we’re focused on sheet metal.
3
Gauge to mm: Thickness in real numbers
Here's a handy sheet metal gauge to mm conversion chart for some of the most common sizes:
Gauge | Inches | Millimetres
10 | 0.128" | 3.251 mm
16 | 0.064" | 1.626 mm
18 | 0.048" | 1.219 mm
20 | 0.036" | 0.914 mm
22 | 0.028" | 0.711 mm
24 | 0.022" | 0.559 mm
So if you’re asking:
Gauge | Inches | Millimetres
10 | 0.128" | 3.251 mm
16 | 0.064" | 1.626 mm
18 | 0.048" | 1.219 mm
20 | 0.036" | 0.914 mm
22 | 0.028" | 0.711 mm
24 | 0.022" | 0.559 mm
So if you’re asking:
- How thick is 10 gauge steel? – It’s roughly 3.25 mm.
- How many mm is 18 gauge? – That’s 1.219 mm, assuming you’re using SWG.
- What is 22 gauge in mm? – Just about 0.711 mm thick.
Remember – always clarify what material and gauge system you're using. Steel sheet gauge to mm isn't the same as aluminium.
4
Why gauge doesn’t always cut it
One key thing to remember about the gauge vs mm debate – gauge isn’t exact.
Because of how metal is rolled, a bit of variation – called tolerance – is built in. So, while 16 gauge thickness in mm might be listed as 1.626 mm, the real-world thickness could be a touch more or less.
That’s why it's smarter to rely on mm when placing orders or comparing materials.
5
How to measure sheet metal thickness
There are several ways to check the thickness of metal gauges:
- Digital callipers – Easy to use, accurate to the decimal
- Manual callipers – Reliable, but need practice to read
- Gauge measuring tool – Has labelled holes or slots for quick checks
- Ultrasonic gauge tester – Ideal for non-destructive testing when you can’t reach the edges
Still unsure? You can always use an online gauge to mm converter or download a sheet metal thickness chart to keep nearby.
Related: Check out our SWG to mm conversion chart for steel tube
6
Common sheet metal thickness (by gauge)
Wondering what thickness works best for your job? Here’s a quick guide:
- 22 gauge thickness in mm – 0.71 mm, often used for casings, brackets
- 20 gauge thickness in mm – 0.91 mm, suitable for ducting and general enclosures
- 18 gauge to mm – 1.22 mm, a solid choice for structural panels
- 16 gauge into mm – 1.63 mm, often used for automotive or heavy-duty uses
- 10 gauge in mm – 3.25 mm, serious strength for frames and machinery
- 7 gauge steel thickness – Around 4.47 mm, moving into plate territory
Looking for the thickness of 8 gauge steel or even 5 gauge steel thickness? Those sizes are heavier and often labelled in mm directly, not by gauge.
When to ditch gauge and go metric
If all these numbers are starting to do your head in, you’re not alone.
That’s why many manufacturers (The Metal Store included) are moving toward standard metric sheet metal thickness. It’s cleaner, more precise and avoids the constant question: “What gauge is 3/16 steel?”
7
What are the standard sheet metal sizes?
When it comes to sheet metal sizes, standard formats include:
- 2,000 x 1,000 mm
- 2,500 x 1,250 mm
- 3,000 x 1,500 mm
But we’ll cut to size, so you only pay for what you need. That’s one less offcut to deal with in the workshop.
8
Use gauge if you must – but mm is better
Here’s the takeaway:
- Gauge is useful, especially in the trade, but it’s not exact.
- Always clarify the system (SWG, MSG, AWG, etc.)
- Use millimetres wherever possible for clarity
- Know your material – 20 gauge aluminium ≠ 20 gauge steel
At the end of the day, sheet metal’s only helpful if you know exactly what you’re getting – and how thick it really is.
Need sheet metal cut to size?
At The Metal Store, we stock a wide range of sheet metals, including: Mild steel, Aluminium, Stainless steel, Galvanised steel, Copper and Brass sheets.
Whether you’re working in 10 gauge stainless steel thickness, 18 ga to mm or just want it all in clean metric numbers – we’ve got you covered.
Free cutting. Fast delivery. No messing about.
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