Sheet Metal & Wire Gauge Chart: AWG, SWG, BWG to mm

A gauge number alone does not tell you the actual thickness — it depends entirely on which gauge system you're using. An 18 gauge measurement in AWG (American Wire Gauge) works out to 1.024 mm. The same number in SWG (British Imperial Standard Wire Gauge) gives 1.219 mm, and in BWG (Birmingham Wire Gauge) it's 1.245 mm — nearly 22% thicker than the AWG equivalent for exactly the same gauge marking.

Australia specifies sheet metal and wire by millimetre thickness for fabrication and purchasing, but imported materials, legacy drawings, and some trade catalogues still use gauge references. This chart covers AWG, SWG, and BWG side by side from gauge 4/0 (heaviest) through 40 (finest), with application guidance for each range.

Gauge to Millimetres — Complete Cross-Reference

AWG values are mathematically defined by ASTM B258 — a strict geometric progression with a 92:1 diameter ratio over 39 steps from 4/0 to 36 AWG. SWG values are from BS 3737:1964. BWG values are from Stubs Iron Wire Gauge published tables in Machinery's Handbook. Each value has been verified against at least two published sources. [VERIFY: confirm ASTM B258 current edition year; confirm BS 3737:1964 has not been superseded; BWG table values beyond gauge 35 are undefined in most published sources.]

Why Multiple Gauge Systems Exist (and Why It Matters)

The three gauge systems developed independently in different industrial traditions, which is why the same number refers to a completely different physical dimension depending on which system is being used.

AWG — American Wire Gauge

AWG (also called Brown & Sharpe Wire Gauge) emerged from the American wire-drawing industry in the 19th century. It uses a mathematically rigorous geometric progression now codified in ASTM B258: the diameter ratio between any two consecutive gauges is constant (the 39th root of 92, approximately 1.1229). This means each step changes cross-sectional area by approximately 26%, which is convenient for electrical engineers calculating resistance and current capacity — but the counter-intuitive numbering (higher = thinner) regularly causes confusion in mechanical and fabrication work.

SWG — British Imperial Standard Wire Gauge

SWG (defined in BS 3737:1964) emerged from the British wire and sheet metal industries. Its values do not follow the same mathematical progression as AWG, so the same gauge number refers to a physically different size. SWG 18 is 1.219 mm; AWG 18 is 1.024 mm — a 0.195 mm difference from the same marking. SWG was the dominant system in the UK, Australia, and other Commonwealth countries through the 20th century, and persists on older drawings and in some heritage trade catalogues.

BWG — Birmingham Wire Gauge

BWG (also called Stubs Iron Wire Gauge) originated in the Birmingham steel industry. It is most commonly encountered in tube wall thickness specifications — a marking of "16 BWG wall" means 1.651 mm wall thickness. BWG values are tabulated (not mathematically generated), and most published tables end at BWG 35 (0.127 mm). BWG differs from both AWG and SWG: BWG 16 is 1.651 mm, AWG 16 is 1.290 mm, SWG 16 is 1.626 mm.

The practical takeaway: never assume a gauge number means the same thickness across systems. Always confirm which system applies and convert to millimetres for any precision work.

Sheet Steel Gauge — What Australia Uses

Modern Australian sheet steel specifications use millimetre thickness exclusively. AS 1397:2021 [VERIFY: edition year] covers continuous hot-dip metallic coated steel sheet and strip (Zincalume, Galvabond, and similar products used in roofing, cladding, and general fabrication). AS/NZS 1594 [VERIFY: current edition] covers hot-rolled steel flat products for structural, manufacturing, and general engineering use — both specify thickness in millimetres, not gauge.

Common AS 1397-range sheet thicknesses available from Australian distributors, with approximate gauge equivalents for reference:

For any new design work or purchasing, specify the actual millimetre thickness as the primary dimension. Use gauge references only as supplementary information when communicating with overseas suppliers or reading legacy drawings. The approximate equivalents above are guides only — there is rarely an exact gauge match to a metric thickness.

AWG (American Wire Gauge) — Wire and Light Sheet

AWG is the standard wire sizing system in North America and appears on US-origin cable, wire rope, and light sheet products imported into Australia. The AWG scale runs from 4/0 (0000) at 11.684 mm diameter (the heaviest) down to gauge 40 at 0.0799 mm (the finest in common engineering use). Heavier conductors are expressed as kcmil (thousand circular mils) rather than AWG.

AWG is primarily a wire conductor sizing system. When AWG appears on a wire mesh, expanded metal, or woven product, it typically refers to the wire diameter used in construction, not the sheet thickness. For solid sheet metal from US sources, many suppliers use a separate US Sheet Metal Gauge system based on weight per area rather than wire diameter — confirm with your supplier which system applies before ordering.

For electrical wire selection, cross-sectional area (mm²) is the Australian standard per AS/NZS 3000:2018 Wiring Rules, but AWG is used universally for imported cable and conductor cross-referencing.

SWG (British Imperial Standard Wire Gauge) — Wire and Sheet

SWG is defined by BS 3737:1964 and covers both wire and thin sheet. It was the dominant gauge system in the UK, Australia, New Zealand, and other Commonwealth countries through the mid-20th century, and remains in use on older engineering drawings and in some sector-specific trade catalogues.

SWG covers a wider practical range for sheet metal work than AWG — gauges 8 through 24 SWG span thicknesses from 4.064 mm down to 0.559 mm, covering most general fabrication applications. Gauges 25 through 36 SWG cover fine wire applications. Unlike AWG, SWG extends to gauge 50 in the BS 3737 standard (not shown here as gauges above 40 are rarely used in fabrication).

When a drawing or specification from the UK era or a Commonwealth country uses a gauge reference without specifying a system, SWG is the most likely intent. Always confirm before ordering material.

BWG (Birmingham Wire Gauge) — Sheet Steel and Tube Walls

BWG (Birmingham Wire Gauge), also known as Stubs Iron Wire Gauge, originates from the Birmingham steel industry and is most commonly encountered in two contexts:

• Tube wall thickness — BWG is the most common gauge convention for specifying tube wall thickness on imperial-dimensioned tube. If a seamless tube specification says "16 BWG wall," the wall is 1.651 mm. This convention persists widely in US-origin tube catalogues and is common on stainless, carbon steel, and copper tube specifications.
• Historical sheet steel — some older sheet steel, particularly for thicker gauges (1 through 14 BWG), was specified by BWG in the US and UK industries. Modern sheet specifications use metric thickness, but BWG references appear on legacy drawings.

Most published BWG tables end at gauge 35 (0.127 mm). Gauges 36 through 40 are defined in AWG and SWG but are not part of the standard BWG system. In the table above, BWG entries above gauge 35 are shown as “—” accordingly.

Practical Selection Guide

Always specify mm, note gauge as reference only

On any drawing or purchase order, give the actual millimetre thickness as the primary specification, with the gauge reference in brackets. For example: "1.6 mm (approx. 16 SWG)" or "1.024 mm (18 AWG)". This eliminates ambiguity and makes the intent clear regardless of which gauge system the supplier is familiar with.

Confirm the gauge system before cutting

When a drawing or quote says "18 gauge" without specifying AWG, SWG, or BWG, ask before ordering. The difference between AWG 18 (1.024 mm) and SWG 18 (1.219 mm) is nearly 0.2 mm — enough to affect fit-up in precision fabrication and to change structural load calculations.

US-origin materials

For electrical wire: AWG applies universally, standardised by ASTM B258. For US-origin sheet steel: confirm whether the supplier means AWG, US Sheet Metal Gauge (which is based on weight, not wire diameter, and differs again), or metric. Most contemporary US sheet steel catalogues now list metric thickness alongside the gauge reference.

UK and Australian heritage drawings

Assume SWG unless otherwise noted. This is the most likely intent for drawings produced in Australia or the UK prior to metrication (approximately 1970–1990 depending on industry).

Tube wall thickness

Assume BWG if a gauge system is not specified on a tube wall dimension. This is the most common convention for imperial-dimensioned tube in engineering catalogues. Verify against the supplier's specific table before cutting, as there are occasional variations between published BWG tables.

Measuring thickness on the job

A digital vernier calliper or micrometer gives the actual millimetre thickness directly. For sheet metal on the bench, a sheet metal gauge tool (pin gauge wheel) gives a quick gauge reading — but note that most Australian gauge wheels are marked in SWG. If you need to identify the material system on an unmarked sheet, measure in mm and cross-reference the table above.

Related Reference Guides and Supplies

• Material Density Chart: Steel, Aluminium, Brass and Metals — weight-per-area calculations for sheet metal
• Steel Grades Comparison: Carbon, Alloy and Tool Steels — AS/SAE/ISO cross-reference for material selection
• Drill Bit Size Chart: Metric, Imperial and Fractional — matching drill sizes to gauge sheet clearance holes
• Cable Management Guide: Trays, Conduit, Ties and Glands — AWG wire sizing in cable management applications
• Raw Materials at AIMS Industrial — tool steel bar, silver steel and bronze stock
• Measuring Tools at AIMS Industrial — digital calipers, micrometers and measuring equipment
• Wire Rope, Chain and Fittings — wire rope sizing in AWG and metric equivalents

People Also Ask — Wire Gauge and Sheet Metal Thickness

What does 18 gauge mean in sheet metal?

18 gauge refers to a specific thickness, but the actual dimension depends on the gauge system. In AWG (American Wire Gauge), 18 gauge is 1.024 mm. In SWG (British Standard Wire Gauge), 18 gauge is 1.219 mm. In BWG (Birmingham Wire Gauge), 18 gauge is 1.245 mm. When someone says "18 gauge sheet metal" without specifying a system, they most commonly mean SWG in Australian and UK contexts, or AWG in North American contexts. Always confirm the system and convert to millimetres before ordering.

What is the difference between AWG and SWG?

AWG (American Wire Gauge) and SWG (British Standard Wire Gauge) are two different gauge systems that use the same numbering range but map to different physical dimensions. AWG diameters follow a strict mathematical progression defined by ASTM B258 (92:1 ratio over 39 steps). SWG diameters are tabulated in BS 3737:1964 and follow a different progression. The same gauge number gives a different thickness: AWG 12 is 2.052 mm, SWG 12 is 2.642 mm, a difference of nearly 0.6 mm from the same gauge marking.

How do I convert gauge to millimetres?

Use the cross-reference table above. First confirm which gauge system applies (AWG, SWG, or BWG), then find the gauge number in the table and read the corresponding millimetre value. For AWG, you can also use the ASTM B258 formula: diameter (inches) = 0.005 × 92^((36−n)/39), where n is the gauge number (with 4/0 = −3, 3/0 = −2, 2/0 = −1, 1/0 = 0). Multiply by 25.4 to convert to mm. For SWG and BWG, a table lookup is the only reliable method.

What gauge sheet metal is used for HVAC ductwork?

Standard HVAC ductwork in Australia is typically 0.6 mm to 1.0 mm thick sheet, which corresponds approximately to 18 to 20 SWG range. US-specified ductwork commonly references 18 to 22 AWG (1.024 mm to 0.644 mm). The Australian Sheet Metal Workers Association and AS 4254 specify duct wall thickness in millimetres. Always verify the applicable standard and convert gauge references to mm before fabricating.

What is the thickest gauge sheet metal?

In the AWG system, 4/0 (0000) is the heaviest at 11.684 mm diameter. In SWG, 4/0 is 10.973 mm. In BWG, 4/0 is 11.532 mm. However, at these dimensions the material is wire or rod rather than sheet. For practical sheet metal work, the thickest commonly available gauge sheet is around 8 to 10 gauge, which corresponds to roughly 3.2 to 4.0 mm. Above this thickness, material is generally classified as plate rather than sheet.

Why does a higher gauge number mean thinner material?

The numbering convention originates from the wire-drawing process. Starting from a thick rod, each pass through a draw plate reduces the diameter. Early gauge systems numbered each successive drawing pass, so more passes (higher number) produced finer wire. The convention of higher number = thinner material was retained when the systems were formalised, which is why it now seems counter-intuitive. Modern engineering practice avoids the confusion by specifying thickness in millimetres directly.

What gauge is car body sheet metal?

Automotive outer body panels are typically 0.7 mm to 0.9 mm (approximately 20 to 21 AWG or 21 to 23 SWG). Structural reinforcement panels and sills are generally 1.0 mm to 1.5 mm (approximately 17 to 19 AWG or 18 to 19 SWG). Older Australian-built vehicles (pre-1990s) commonly used SWG-referenced steel; modern vehicles use metric specifications from the OEM. Panel beaters typically order panel steel by mm thickness rather than gauge.

What is the difference between BWG and AWG?

BWG (Birmingham Wire Gauge) and AWG (American Wire Gauge) are both gauge systems, but they originate from different industrial traditions and map to different physical dimensions. BWG 16 is 1.651 mm; AWG 16 is 1.290 mm. BWG is most commonly used today for tube wall thickness specifications, while AWG is used for electrical wire and US-origin cable. BWG does not extend to the fine gauges that AWG covers (most BWG tables end at gauge 35).

How do I measure sheet metal thickness without a chart?

A digital vernier calliper gives a direct millimetre reading to 0.01 mm accuracy, which is sufficient for most gauge identification. A micrometer gives higher accuracy (0.001 mm) for thin sheet. A sheet metal gauge wheel (pin gauge) gives a quick gauge reading directly but note that most Australian gauge wheels are calibrated in SWG — confirm the system before assuming the gauge reading maps to another system. Once you have the millimetre measurement, use the table above to identify the corresponding gauge in each system.

What gauge is standard 1.6 mm sheet steel?

1.6 mm is approximately 16 SWG (1.626 mm) or 16 BWG (1.651 mm). The nearest AWG equivalent is AWG 16 (1.290 mm), though this is measurably thinner. In Australian metric specifications, 1.6 mm is one of the most common standard thicknesses for general structural sheet steel per AS/NZS 1594, and is widely stocked by sheet metal suppliers. It is a common choice for ductwork, enclosures, and light structural applications.

Need measuring equipment or raw material stock for your fabrication project? Contact the AIMS Industrial team on 1300 734 428 or email via our contact page. Our team can help match material specifications, recommend measuring tools, and advise on wire rope sizing for your application.