Pick a Standard Wire Gauge (SWG) size to see its diameter in mm and inches and its cross-sectional area.
How to Use the SWG to mm Calculator
Pick a Standard Wire Gauge size from the dropdown — sizes run from the oversized 7/0 down to the hair-thin 50 — and the diameter and cross-sectional area appear immediately.
Where the AWG to mm Calculator on this site runs a single formula, this one runs a lookup table, and that's not a design choice — it's a consequence of history. American Wire Gauge was defined mathematically from the outset, as a clean 92^(1/39) geometric ratio between steps. British Standard Wire Gauge wasn't. Each SWG size was fixed by legal definition, one gauge at a time, before anyone thought to make wire sizing follow a tidy formula.
7/0 SWG = 12.700 mm
1 SWG = 7.620 mm
20 SWG = 0.914 mm
50 SWG = 0.0254 mmThere's no equation that reproduces this sequence — plug in a gauge number and there's no calculation that spits out the right diameter, which is exactly why this page needs a table baked into it rather than a one-line formula like its AWG counterpart.
SWG originated in the UK, formalized as the Imperial Standard Wire Gauge in the 19th century, and it's now largely a historical artifact outside a few corners of the world. You'll still run into it on older British and Commonwealth electrical drawings, some UK manufacturing specs, and certain musical-instrument-string and jewelry-wire suppliers who never switched conventions. North America standardized on AWG decades ago and never looked back, so if you're converting a spec sheet and unsure which system it's using, the country of origin is usually the fastest tell.
Area follows the same circular formula regardless of which gauge system produced the diameter — π/4 × diameter², the geometry of a circle doesn't care how the diameter number was arrived at.