SWG to Millimeter Wire Size Converter & British Standard Wire Gauge (BS 3737

British Standard Wire Gauge (SWG), BS 3737 Standards & Magnet Wire Metrology

The British Standard Wire Gauge (SWG)—also known as the Imperial Wire Gauge or Legal Standard Wire Gauge—was established in 1883 by the British Board of Trade and standardized as BS 3737:

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• SWG vs AWG Structural Difference**:

• AWG (American Wire Gauge) follows an exact geometric progression formula: d_n = 0.127 × 92^(36-n)/39 mm.

• SWG (British Standard Gauge) is defined by a discrete legal lookup table of historical wire drawing dies spanning from 7/0 SWG down to 50 SWG .

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• Cross-Sectional Area & Circular Mils**:

A_mm^2 = (π) / (4) d_n^2

A_cmil = (d_mils)^2 = (d_inches × 1000)^2

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• Common SWG Sizes in Motor & Transformer Manufacturing**:

• 10 SWG: d = 3.251 mm, A = 8.30 mm^2 Heavy transformer secondary bus winding

• 14 SWG: d = 2.032 mm, A = 3.24 mm^2 Industrial welder / power inverter coil

• 18 SWG: d = 1.219 mm, A = 1.17 mm^2 1 HP to 5 HP AC motor stator winding

• 22 SWG: d = 0.711 mm, A = 0.397 mm^2 Ceiling fan and pump motor winding

• 30 SWG: d = 0.315 mm, A = 0.0779 mm^2 Relay coils and acoustic pickups

• 36 SWG: d = 0.193 mm, A = 0.0293 mm^2 Miniature RF chokes & watch coils

How to Use This Calculator

Enter your British Standard Wire Gauge (SWG / Imperial) — choose from 7/0 SWG (0.500 in = 12.700 mm), 6/0 SWG (0.464 in = 11.786 mm), 5/0 SWG (0.432 in = 10.973 mm), Conductor Material & Resistivity — choose from Annealed Copper (ρ = 1.724 × 10⁻⁸ Ω·m - Magnet Wire), Electrical Grade Aluminum (ρ = 2.824 × 10⁻⁸ Ω·m), Nichrome 80/20 Heating Wire (ρ = 1.08 × 10⁻⁶ Ω·m), and Wire Winding / Run Length (Meters) (e.g. 100 in m). Results update instantly as you type — adjust any value to compare scenarios.

The Formula

Cross-sectional Area: A = pi/4 * d^2

DC Resistance R = (rho * L) / (A in m^2)

Worked Example

An electrical technician rewinding a 3-phase industrial motor replaces burnt stator coils with 18 SWG enamelled copper magnet wire, measuring its diameter as 1.22 mm to ensure the replacement wire fits precisely into the motor core slots.

Understanding Your Result

Unit conversion precision: round to the same number of significant figures as your original measurement. Converting 6 feet to meters gives 1.8288 m — but if your measurement was approximate (±1 inch), expressing more than 2 decimal places is false precision. For international trade and technical specifications, SI units (meters, kilograms, liters) are the standard. US customary units (feet, pounds, gallons) differ from Imperial UK units: 1 US gallon = 3.785 L vs. 1 UK gallon = 4.546 L.