SWG to mm² Converter

Convert British Standard Wire Gauge (SWG) to millimeters and square millimeters instantly. Compare with AWG, IEC standards, and calculate electrical properties.

About SWG: British Standard Wire Gauge (SWG) is the British standard for wire diameters. Unlike AWG, SWG sizes are based on specific diameter values rather than a mathematical formula. SWG was widely used in the UK and Commonwealth countries.

Select SWG size (from 7/0 to 50)
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Understanding SWG (British Standard Wire Gauge)

British Standard Wire Gauge (SWG) is a standardized wire gauge system used in the United Kingdom and some Commonwealth countries. Unlike AWG, SWG sizes are not based on a mathematical formula but on specific diameter values defined in the standard.

Historical Background: SWG was first defined by the British Board of Trade in 1884. It was widely used in the UK and British Empire for electrical wiring, mechanical applications, and sheet metal thickness. Although largely superseded by metric standards, SWG is still used in some traditional applications.

SWG vs AWG vs IEC: Key Differences

Feature SWG (British) AWG (American) IEC (Metric)
Origin United Kingdom (1884) United States (1857) International
Basis Specific diameter values Mathematical formula Cross-sectional area (mm²)
Size Range 7/0 to 50 0000 to 40 0.5 to 1000 mm²
Primary Use UK, Commonwealth North America International, Europe
Formula No formula (table-based) D = 0.127 × 92^(36-n)/39 A = π × (d/2)²
Typical Application Traditional wiring, mechanical uses Electrical wiring in USA Modern electrical installations

Common SWG Sizes and Applications

SWG Diameter (mm) Typical Applications Approx. Current (Copper)
7/0 12.70 Heavy power distribution, bus bars 300+ A
4/0 10.16 Main power feeders, large motors 200 A
0 8.23 Service entrance, large appliances 125 A
4 5.89 Electric range circuits, subpanels 85 A
10 3.25 General power circuits, machinery 40-55 A
14 2.03 Lighting circuits, outlets 20-25 A
18 1.22 Light-duty wiring, appliance cords 10-15 A
22 0.71 Low-voltage wiring, electronics 3-5 A
30 0.315 Fine instrument wiring, coils < 1 A
40 0.122 Very fine wire, specialized uses Minimal

Conversion Guidelines

1

SWG to Metric: SWG sizes are defined by specific diameters in inches or millimeters. Conversion to metric involves looking up the diameter in the SWG table and calculating the area using the standard formula: Area = π × (diameter/2)².

2

SWG to AWG Approximation: There's no direct formula to convert between SWG and AWG. The closest approximation is found by comparing diameters. For example, SWG 10 (3.25 mm) is approximately equivalent to AWG 8 (3.26 mm).

3

SWG to IEC: Convert SWG to metric area (mm²), then select the nearest standard IEC size. SWG 10 (8.30 mm²) is closest to IEC 10 mm², though the actual diameter differs slightly (3.25 mm vs 3.57 mm).

Important: When converting between gauge systems for electrical applications, always verify current ratings and safety standards. Different countries have different regulations and derating factors.

Material-Specific Electrical Properties

SWG Size Diameter (mm) Area (mm²) Copper Resistance (Ω/km) Approx. Current (A)

Note: Current ratings are approximate for copper wire with PVC insulation at 30°C ambient temperature. Always consult local electrical codes for specific applications.

Historical and Modern Usage

Historical Buildings

SWG wiring in older UK buildings, heritage electrical systems

Industrial Machinery

Older industrial equipment from UK manufacturers

Model Engineering

Traditional model making, miniature steam engines

Educational

Understanding imperial measurement systems

Frequently Asked Questions

SWG (British Standard Wire Gauge) and AWG (American Wire Gauge) are different wire gauge systems. SWG is based on specific diameter values defined in a table, while AWG uses a mathematical formula. For the same gauge number, SWG wires are generally larger than AWG wires. For example, SWG 10 is 3.25 mm in diameter, while AWG 10 is 2.59 mm.

SWG is largely obsolete for new electrical installations, having been replaced by metric (IEC) standards in most countries. However, it is still encountered in:
  • Older buildings in the UK and Commonwealth countries
  • Historical restoration projects
  • Some traditional industries and crafts
  • Model engineering and hobbyist applications
  • Documentation and specifications for older equipment

To convert SWG to metric:
  1. Look up the diameter in millimeters for your SWG size using the SWG table
  2. Calculate the cross-sectional area: Area = π × (diameter/2)²
  3. For electrical applications, select the nearest standard IEC size
  4. Consider current ratings based on the material and installation conditions
  5. If replacing SWG wiring, consult with a qualified electrician about local regulations

SWG was developed before standardized mathematical formulas for wire gauges were established. The sizes were originally based on practical manufacturing considerations and traditional wire drawing practices. The SWG table represents specific diameter values that evolved over time based on industrial practice rather than mathematical consistency. This is in contrast to AWG, which was developed later with a mathematical basis.

Generally not recommended. Mixing different wire gauge systems in the same installation can lead to:
  • Inconsistent current carrying capacity calculations
  • Difficulties with termination and connectors
  • Compliance issues with electrical codes
  • Safety concerns due to different insulation standards
If you need to work with existing SWG wiring, it's best to:
  1. Consult a qualified electrician familiar with both systems
  2. Use appropriate adapters or conversion methods
  3. Consider replacing the entire circuit with modern metric wiring
  4. Ensure all work complies with current local electrical regulations