SWG to mm: Standard Wire Gauge Conversion Table

Standard Wire Gauge — SWG, sometimes called Imperial Wire Gauge — is the British gauge system. It is not AWG, and the same number means a different wire in each.

16 SWG is 1.626 mm. 16 AWG is 1.29 mm. Reading one as the other gives you a conductor 59% too large or 37% too small.

Full conversion table

Cross-sections are calculated from the diameter as π·d²/4 and are for solid round conductors.

SWG and AWG are not interchangeable

Both systems run backwards — a higher number means a thinner wire — and both use the 0-series for the largest sizes. That surface similarity is exactly what causes the mistakes.

Compare the same numbers side by side:

SWG is consistently the larger conductor at the same number, by 12 to 29% across the common range. In area terms that is a difference of up to 66%.

There is no simple offset to apply. The two systems are built differently — AWG is a pure geometric progression between two fixed diameters, while SWG was assembled from a table of values that does not follow a single ratio. You have to convert through diameter.

Where you still meet SWG

British and Commonwealth engineering drawings, older UK installations, sheet metal and fastener specifications, guitar and piano strings, knitting needles, and a good deal of legacy industrial equipment.

It is not used for modern cable specification. Cable in the UK and across the metric world is specified in mm², and has been for decades. If SWG appears on a cable drawing, it is almost certainly an old one.

Birmingham Wire Gauge is a third system

BWG, sometimes called Stubs Iron Wire Gauge, is different again and turns up mainly in tube and needle specifications. 16 BWG is 1.651 mm, close to 16 SWG's 1.626 mm but not identical, and nowhere near 16 AWG.

Three systems, same numbers, three different wires. When a drawing gives a gauge number without naming the system, that is worth resolving before anyone cuts anything.

FAQ

What is 16 SWG in mm?

1.626 mm, which is 0.064 inches. Its cross-section is 2.076 mm².

Is SWG the same as AWG?

No. At the same number, SWG is 12 to 29% larger in diameter across the common range. 16 SWG is 1.626 mm and 16 AWG is 1.29 mm.

How do I convert SWG to mm²?

Convert the gauge to a diameter in mm using the table, then apply π·d²/4. The table above gives both.

Is SWG still used?

Not for cable. It survives on older British drawings, in sheet metal and fastener work, and in some non-electrical trades. Modern cable is specified in mm².

What is the difference between SWG and BWG?

They are separate systems with different values. 16 SWG is 1.626 mm; 16 BWG is 1.651 mm. BWG appears mainly in tubing and needle specifications.

Does SWG have an equivalent metric cable size?

Not directly. Convert to mm², then round to the nearest standard metric size the same way you would from AWG — see why AWG doesn't match metric sizes, which applies equally here.

Reference

For American gauges, use the AWG to mm² converter. For conductors above 4/0 AWG, see kcmil to mm².