SWG to mm Calculator

About the SWG to mm Calculator

A SWG to mm calculator converts British Standard Wire Gauge (SWG, defined in BS 3737) numbers to metric diameter in millimetres and cross-sectional area in mm² — essential for UK, Australian, and Commonwealth engineers, metalworkers, and electricians working with legacy SWG specifications alongside modern metric standards. The Standard Wire Gauge (SWG), sometimes called the Imperial Wire Gauge, was the British standard wire sizing system and remains in use today for sheet metal (SWG sheet gauge), agricultural fencing wire, music wire, and legacy electrical applications across the UK, Australia, New Zealand, India, South Africa, and other Commonwealth countries. Modern UK electrical wiring under BS 7671 (IET Wiring Regulations) specifies conductors in mm² cross-section, not SWG, but SWG appears in older building plans, metalworking drawings, and some specialist applications. SWG and AWG are completely different standards despite both using numbers that decrease as wire increases in size: SWG 14 is 2.032 mm; AWG 14 is 1.628 mm — a significant difference that can cause specification errors if the standard is not clearly identified. Our complete SWG conversion table covers SWG 1 (7.62 mm) through SWG 40 (0.1219 mm) with metric equivalents.

How It Works

SWG sizes are defined in BS 3737 as exact decimal inch dimensions converted to mm. SWG 16 = 0.064 inches = 1.626 mm. SWG diameter formula approximation: d ≈ 0.324 × 0.8914^(SWG-1) inches. Converting to mm: × 25.4. Cross-sectional area: A = π × (d/2)² mm². Key SWG equivalents: SWG 1 = 7.620 mm (large structural wire); SWG 6 = 4.877 mm; SWG 10 = 3.251 mm; SWG 14 = 2.032 mm; SWG 16 = 1.626 mm (common electrical size); SWG 20 = 0.914 mm; SWG 24 = 0.559 mm; SWG 30 = 0.315 mm (fine wire). Sheet metal SWG: SWG 8 = 4.064 mm; SWG 10 = 3.251 mm; SWG 14 = 2.032 mm; SWG 16 = 1.626 mm; SWG 18 = 1.219 mm; SWG 20 = 0.914 mm. Comparison: SWG 16 = 1.626 mm; AWG 16 = 1.291 mm (SWG is significantly larger).

Tips & Best Practices

• ✓SWG vs AWG confusion: never assume SWG and AWG numbers are interchangeable. SWG 22 = 0.711 mm; AWG 22 = 0.644 mm. The difference (~10%) is enough to affect resistance, current capacity, and fit in connectors. Always mark drawings clearly as SWG or AWG and verify with the appropriate table.
• ✓UK metalworking: steel sheet and fence wire in the UK are commonly specified in SWG. Agricultural fencing: barbed wire is often 12.5 SWG (2.50 mm) or 14 SWG (2.032 mm). Steel straining wire: 8 SWG (4.064 mm). Chain link fencing: 9-11 SWG depending on grade. These specifications appear in BS and agricultural standards.
• ✓Legacy UK electrical: pre-1970s UK buildings may have wiring described in SWG in original installation records. SWG 7 (4.470 mm) was used for main feeds; SWG 14 (2.032 mm) for domestic circuits (now replaced by 2.5 mm² in modern installations). SWG 16 (1.626 mm) was the 15A lighting circuit standard (now 1.5 mm² per BS 7671).
• ✓Musical instrument strings: guitar and piano string gauges are specified in thousandths of an inch and sometimes in SWG. Guitar plain steel strings: 0.009" = SWG 36 (0.193 mm); 0.010" ≈ SWG 35; 0.012" ≈ SWG 34. Knowing the SWG equivalent helps when sourcing wire from UK or Commonwealth suppliers.

Who Uses This Calculator

UK and Australian engineers converting legacy SWG specifications in old drawings and standards to modern metric mm² equivalents. Metalworkers selecting the correct SWG from a metric measurement or specification. Electricians in Commonwealth countries interpreting historic installation records that specify wiring in SWG. Agricultural fencing contractors specifying or sourcing wire to SWG standards. Musical instrument technicians understanding string gauge in SWG versus decimal inch dimensions.