HomeCalculatorsWireSWG to mm Calculator
Wire calculator

SWG to mm Calculator

Convert Standard Wire Gauge (SWG) to millimetres and mm² for British/imperial wire sizing. Runs locally in your browser. Free online tool — no sign-up.

Instant result
Result
—

Enter values to calculate.

Inputs—
Mode—
Formula—
Trust summary CVP VERIFIED · CVP protocol 1.0.0-proposed · Engineering assurance
Input interpretation
Enter values to calculate.
Result
—
Assurance
Engineering
Declared partition coverage
PASS · 8/8 declared partitions (heavy-gauge, zero-alias, ordinary-gauge, endpoints, field-mm, field-mm2, invalid-type, invalid-range) · Matrix
Known limitations
  • Discrete gauges 7/0 (−6)…50 only
  • Core CVP does not include live graph, viewport, or pointer interaction.
Model
SWG diameter (mm) from the Imperial SWG inch table × 25.4, and optional round-wire area (mm²) from that millimetre.
Scope
Canonical size is the Imperial SWG inch table (7/0–50). millimetre = inch × 25.4 (exact).
Verification
Engine tested · Source checked · v1.3.1 · CVP VERIFIED · CVP protocol 1.0.0-proposed · Engineering assurance· View Manifest · CVP overview · Specification
Versions
Calculation 1.3.1 · CVP protocol 1.0.0-proposed · Evidence 2026-09-10.authoritative-provenance
Verification revision
2026-09-10.authoritative-provenance · 3/3 property · digest c2cc961fa949
Legacy regression
76/76 tests · Production surface contract 4/4
Reference
O1 model · O2 expected_values · O2 numerical_behavior
Interfaces
PASS · UI (SSR) / REST / MCP — ui-ssr is query-result HTML, not a live browser session. Error-path engine·REST·MCP 1/1 (status, code, calculation_version). SSR compared on URL-canonical requested calculations; empty query is idle (not an error) and JSON-typed object/array inputs are REST/MCP-only.
Supplemental domain review
Not performed
Named expert review
Not performed
CVP suite
62/62 golden · 7/7 CVP boundary · 13/13 invalid · 3/3 property · 4/4 cross-interface · 1/1 CVP contract · Manifest
Sources
  • British Standards Institution — BS 3737:1964 — Guide to series of basic sizes for metal sheet, strip and wire — Status: Withdrawn. Public catalogue confirms identity/status. Exact PDF table/clause/page is not asserted. The 57-row inch dataset was independently checked 2026-09-10 against Wikipedia, Engineering Toolbox, and Trojan SF (57/57 inch identity); 0.324 in is canonical SWG 0, with 1/0 as alias.
  • NIST Guide to the SI (SP 811) — Millimetre as the SI unit for length; 1 in = 25.4 mm exactly
Sources
Evidence
63 legacy golden · 8 legacy boundary · legacy regression suite · 62/62 oracle-backed golden · 13/13 invalid · Artifact integrity PASS
This calculator CURRENT · Public schema 1.3.1 matches · Semantic contract ✓ · Production attested · Public/cache ✓ · Origin ✓
Semantic contract
PASS
Full verification

Manifest identity, reference classes, interfaces, suite, and production records.

Formulas

Core equations used by this calculator.

Diameterdₙ (mm) = tabulated inch × 25.4
AreaAₙ (mm²) = (π/4) × dₙ² (exact mm)
Heavy gauges7/0, 6/0, … 2/0 (legacy −6…−1); 0 ≈ 1/0
iSWG sizes are discrete tabulated values (not the AWG logarithmic formula). Canonical size is the Imperial inch table; millimetres are inch × 25.4 (exact) and area uses that millimetre, not a rounded display value. Enter 14, 7/0, 2/0, or 0. Legacy −6…−1 still works. Non-integers and gauges outside 7/0–50 are invalid (not rounded, not 0 mm). For tube and sheet metal, SWG usually means wall thickness, not outer diameter.

How to use

1

Choose diameter or area

SWG → mm for diameter; SWG → mm² for round cross-section.

2

Enter the SWG number

Type 14, 7/0, 2/0, or 0. Ordinary gauges 1–50 are entered as written. Non-integers are rejected.

3

Read the result

Result updates as you type. Use the chart for a quick lookup or to confirm heavy gauges.

Example calculations

Common configurations with formula and result.

ϟ

Common wire / mesh

SWG 14

table lookup
2.032 mm · 3.2429 mm²
ϟ

Finer wire

SWG 22

table lookup
0.7112 mm · 0.3973 mm²
ϟ

Heavy gauge

SWG 7/0

7/0
12.700 mm · 126.6769 mm²
ϟ

Tube wall example

SWG 16 wall

wall thickness
1.6256 mm (0.064 in)

Common SWG sizes

Common values at a glance.

SWGDiameter (mm)Diameter (in)Area (mm²)
7/012.70000.5000126.6769
0 (≈1/0)8.22960.324053.1921
103.25120.12808.3019
142.03200.08003.2429
161.62560.06402.0755
181.21920.04801.1675
200.91440.03600.6567
220.71120.02800.3973
250.50800.02000.2027
300.31500.01240.0779
i Full 7/0–50 chart is below. Diameter mm = inch × 25.4 (exact); area uses that millimetre. Area assumes a solid round conductor of that diameter.

SWG to mm calculator specification

Version 1.3.1 · Engine tested

Calculation status

Review policy · Evidence

Definition
Standard Wire Gauge (SWG), also called Imperial Standard Wire Gauge or British Standard Wire Gauge, is a British table of wire (and often sheet/tube wall) thicknesses. Unlike AWG, SWG is a defined lookup table rather than a single closed-form formula. This tool returns diameter (mm) and round-wire area (mm²) for SWG 7/0 through 50.
What it calculates
SWG diameter (mm) from the Imperial SWG inch table × 25.4, and optional round-wire area (mm²) from that millimetre.
Inputs
  • SWG gauge (14, 7/0, 2/0, 0 or 1/0, or legacy n = −6…50); required
  • field: mm (diameter) or mm2 (round area); default mm
  • Legacy UI/query alias a for swg
Outputs
  • Diameter in millimeters (inch × 25.4, exact)
  • Cross-sectional area in square millimeters (round solid of that diameter)
Formula
d_mm = d_in × 25.4; A = (π/4)×d_mm²
Assumptions
  • Canonical size is the Imperial SWG inch table (7/0–50). millimetre = inch × 25.4 (exact).
  • Area uses that millimetre, not a rounded display value.
  • Area assumes a solid circular cross-section of that diameter.
  • For tube/sheet use-cases, interpret d as wall/sheet thickness.
  • SWG 0 and 1/0 share the tabulated 0.324 in size used here; some catalogues list 1/0 separately.
Units
  • mm (diameter / wall thickness)
  • mm² (round area)
Boundary conditions
  • Missing/empty swg → MISSING_REQUIRED_INPUT (not 0 mm)
  • Unparseable swg → INVALID_NUMBER
  • Non-integer SWG → SWG_NOT_INTEGER (not Math.round)
  • SWG outside 7/0 (−6)…50 → SWG_OUT_OF_RANGE (not 0 mm)
  • field not mm|mm2 → INVALID_MODE
  • Lower SWG number → thicker diameter (within table)
  • field=mm → diameter; field=mm2 → πd²/4 from exact mm
  • UI/share mode=swg-mm|swg-mm2 maps to field; query a aliases swg
Example
SWG 14 → 2.032 mm, 3.2429 mm²
Validation cases

11 published on this page · 76/76 tests · Production surface contract 4/4 · View evidence

  • SWG 7/0 → 12.700 mm / 126.6769 mm²
  • SWG 14 → 2.032 mm / 3.2429 mm²
  • SWG 22 → 0.7112 mm / 0.3973 mm²
  • SWG 50 → 0.0254 mm / ≈0.000507 mm²
  • SWG 0 → 8.2296 mm / 53.1921 mm²
  • SWG 16 → 1.6256 mm
  • SWG 60 (out of table) → error SWG_OUT_OF_RANGE
  • SWG 14.4 (non-integer) → error SWG_NOT_INTEGER
  • SWG 10 diameter > SWG 16 → true
  • SWG 14 mm vs mm2 consistency → mm2 = πd²/4 from exact mm
  • Same SWG twice → deterministic identical diameter
Sources
  • British Standards Institution — BS 3737:1964 — Guide to series of basic sizes for metal sheet, strip and wire — Status: Withdrawn. Public catalogue confirms identity/status. Exact PDF table/clause/page is not asserted. The 57-row inch dataset was independently checked 2026-09-10 against Wikipedia, Engineering Toolbox, and Trojan SF (57/57 inch identity); 0.324 in is canonical SWG 0, with 1/0 as alias.
    Supports: Historical definition/reference for Imperial SWG diameter series (inch). Used as historical identity, not a current BSI product standard, and not a row-by-row BSI PDF transcription.
  • NIST Guide to the SI (SP 811) — Millimetre as the SI unit for length; 1 in = 25.4 mm exactly
    Supports: d_mm = d_in × 25.4; diameter output in mm and area in mm²
Calculation version
1.3.1

Background

Interpretation and common distinctions.

What is SWG?

Standard Wire Gauge (SWG) — also called Imperial Standard Wire Gauge or British Standard Wire Gauge — is a British sizing system for wire diameter. The same numbers are commonly used in metalwork for sheet thickness and steel tube wall thickness.

Key points:

  • A smaller SWG number means a thicker size.
  • A larger SWG number means a thinner size.
  • Steps are not linear in millimetres — the gap between SWG 1 and 2 is not the same as between SWG 8 and 9.
  • SWG ≠ AWG ≠ BWG. Same number, different diameter.

Unlike AWG, SWG is a lookup table, not a single logarithmic formula. Canonical diameters are the Imperial inch table. This calculator converts dₘₘ = dᵢₙ × 25.4 (exact) and computes round area as A = (π/4) dₘₘ² from that millimetre — not from a rounded display value.

How to convert SWG to mm

Diameter

Look up the SWG number in the Imperial inch table (see chart below), then multiply by 25.4. Example:

  • SWG 14 → 0.080 in = 2.032 mm
  • SWG 22 → 0.028 in = 0.7112 mm
  • SWG 7/0 → 0.500 in = 12.700 mm

Cross-sectional area (round wire)

Aₙ (mm²) = (π)/4 dₙ²

where dₙ is the exact millimetre (inch × 25.4), not a 3-decimal display value. Area applies to a solid round conductor of that diameter. For tube walls or flat sheet, use the diameter column as thickness, not as a filled circular area.

Heavy gauges (7/0 … 2/0)

Label You can type Diameter (mm)
7/0 7/0 or −6 12.7000
6/0 6/0 or −5 11.7856
5/0 5/0 or −4 10.9728
4/0 4/0 or −3 10.1600
3/0 3/0 or −2 9.4488
2/0 2/0 or −1 8.8392
0 (≈1/0) 0 or 1/0 8.2296

Some catalogues list 1/0 separately (~8.236 mm). This tool uses the tabulated 0.324 in size for both 0 and 1/0.

SWG to mm conversion chart

Imperial Standard Wire Gauge. Inch is canonical. millimetre = inch × 25.4 (exact). Area uses that millimetre.

SWG # Diameter (mm) Diameter (inch) Area (mm²)
7/0 12.7000 0.5000 126.6769
6/0 11.7856 0.4640 109.0921
5/0 10.9728 0.4320 94.5638
4/0 10.1600 0.4000 81.0732
3/0 9.4488 0.3720 70.1202
2/0 8.8392 0.3480 61.3643
0 (≈1/0) 8.2296 0.3240 53.1921
1 7.6200 0.3000 45.6037
2 7.0104 0.2760 38.5989
3 6.4008 0.2520 32.1780
4 5.8928 0.2320 27.2730
5 5.3848 0.2120 22.7735
6 4.8768 0.1920 18.6793
7 4.4704 0.1760 15.6958
8 4.0640 0.1600 12.9717
9 3.6576 0.1440 10.5071
10 3.2512 0.1280 8.3019
11 2.9464 0.1160 6.8183
12 2.6416 0.1040 5.4805
13 2.3368 0.0920 4.2888
14 2.0320 0.0800 3.2429
15 1.8288 0.0720 2.6268
16 1.6256 0.0640 2.0755
17 1.4224 0.0560 1.5890
18 1.2192 0.0480 1.1675
19 1.0160 0.0400 0.8107
20 0.9144 0.0360 0.6567
21 0.8128 0.0320 0.5189
22 0.7112 0.0280 0.3973
23 0.6096 0.0240 0.2919
24 0.5588 0.0220 0.2452
25 0.5080 0.0200 0.2027
26 0.4572 0.0180 0.1642
27 0.4166 0.0164 0.1363
28 0.3759 0.0148 0.1110
29 0.3454 0.0136 0.0937
30 0.3150 0.0124 0.0779
31 0.2946 0.0116 0.0682
32 0.2743 0.0108 0.0591
33 0.2540 0.0100 0.0507
34 0.2337 0.0092 0.0429
35 0.2134 0.0084 0.0358
36 0.1930 0.0076 0.0293
37 0.1727 0.0068 0.0234
38 0.1524 0.0060 0.0182
39 0.1321 0.0052 0.0137
40 0.1219 0.0048 0.0117
41 0.1118 0.0044 0.0098
42 0.1016 0.0040 0.0081
43 0.0914 0.0036 0.0066
44 0.0813 0.0032 0.0052
45 0.0711 0.0028 0.0040
46 0.0610 0.0024 0.0029
47 0.0508 0.0020 0.0020
48 0.0406 0.0016 0.0013
49 0.0305 0.0012 0.0007
50 0.0254 0.0010 0.0005

Supported and not supported

Supported

  • Integer SWG 7/0 through 50 (tokens 7/0…2/0, 0/1/0, or legacy −6…50)
  • Diameter (mm) and solid-round area (mm²)
  • Share URLs with a=14 or swg=14 and mode=swg-mm|swg-mm2

Not supported

  • Non-integer gauges (14.4 is not rounded to 14)
  • Gauges outside 7/0–50 (SWG 60 is not 0 mm)
  • AWG or BWG tables
  • Stranded equivalent area, insulated OD, or ampacity

Stable error codes: MISSING_REQUIRED_INPUT, INVALID_NUMBER, SWG_NOT_INTEGER, SWG_OUT_OF_RANGE, INVALID_MODE.

Wire, sheet, and tube

SWG started as a wire measure, but UK metal suppliers still use it for:

  • Wire and wire mesh — conductor or wire diameter
  • Sheet metal — sheet thickness
  • Steel tube — wall thickness only (not OD, not ID)

Wall thickness affects strength, weight, and flow capacity in conduit/plumbing. Always separate “gauge thickness” from “tube diameter” when ordering.

SWG vs AWG (and BWG)

System Region / use How size is defined
SWG UK / Imperial Fixed table
AWG North America Logarithmic formula
BWG Birmingham / Stubs Separate table

Example — gauge 14:

System ≈ Diameter
SWG 14 2.032 mm
AWG 14 1.628 mm

Never substitute one chart for the other.

Things to remember

  • Smaller SWG → thicker metal.
  • Use a chart or this calculator; mm steps between gauges are uneven.
  • For tube/sheet, SWG is thickness, not overall size.
  • Do not confuse SWG with AWG or BWG.
  • For electrical runs, diameter alone is not ampacity — check voltage drop and local code.

Other calculators in this family: AWG to mm, Wire gauge chart, Voltage drop .

AWG AWG to mm › AWG Wire gauge chart › V Voltage drop ›
}

Frequently asked questions

Key distinctions behind the calculation.

What is SWG?

Standard Wire Gauge (Imperial / British Standard Wire Gauge) is a British system that assigns fixed diameters to gauge numbers. It is widely used for wire, wire mesh, sheet metal, and steel tube wall thickness.

Does a larger SWG mean a thicker wire?

No — like AWG, a larger SWG number is thinner. SWG 10 (3.2512 mm) is thicker than SWG 22 (0.7112 mm).

How do I enter 7/0, 6/0, … 2/0?

Type 7/0, 6/0, 5/0, 4/0, 3/0, or 2/0. Legacy numeric codes −6…−1 still work. Enter 0 or 1/0 for SWG 0 (≈1/0).

Is there a simple formula like AWG?

No single closed-form formula is used here. Canonical diameters are the Imperial inch table; millimetres are inch × 25.4 (exact). Area is then A = (π/4)×d² for a round section, using that millimetre (not a rounded display value).

Is SWG the same as AWG?

No. Same gauge number usually means different diameters. Always convert with the correct system — use the AWG to mm tool for American Wire Gauge.

Does SWG mean tube OD or wall thickness?

For tube and sheet, SWG almost always means wall (or sheet) thickness, not the outer diameter of the tube. Confirm the manufacturer’s datasheet.

Why don’t SWG steps feel linear?

The difference between consecutive SWG numbers is not constant in mm. You need a chart or calculator rather than a fixed mm-per-gauge increment.

What about BWG?

Birmingham Wire Gauge (BWG / Stubs) is another table. Do not mix BWG, SWG, and AWG values.

What if I enter 14.4 or SWG 60?

Both are rejected. SWG is a discrete standard number: 14.4 is SWG_NOT_INTEGER (not rounded to 14). SWG 60 is SWG_OUT_OF_RANGE (not 0 mm).

What error codes does the API return?

Stable codes include MISSING_REQUIRED_INPUT (no swg), INVALID_NUMBER (unparseable swg), SWG_NOT_INTEGER (non-integer such as 14.4), SWG_OUT_OF_RANGE (outside 7/0–50), and INVALID_MODE (field not mm or mm2).