Wire Gauge Converter — AWG and SWG

AWG, SWG, millimetres of diameter, square millimetres of area and circular mils, in both directions — and the thing a single-system calculator cannot tell you: a 16 AWG wire and a 16 SWG wire are not the same wire. The two scales coincide at exactly one gauge and disagree everywhere else, by up to 53% in diameter.

Last updated 24 Sep 2026

Wire gauges

AWG ⇄ SWG ⇄ mm ⇄ mm² ⇄ circular mils

What do you have?

AWG gauge

SWG gauge

The measured sizeRead in whatever the box below says.

The measured size is in

Not a circuit: the solid conductor in cross-section, the same gauge NUMBER in both systems, drawn to a single scale with a 5 mm bar for reference. The two circles are the point of the page — 16 AWG and 16 SWG are not the same wire, and neither are most other numbers. Very fine gauges shrink to a dot at this scale. A stranded conductor of the same gauge is slightly larger overall than the solid one drawn here.

1.2908mmExample

16 AWG solid copper

One formula, one table

AWG: d = 0.127 mm × 92^{(36 − n) / 39}, so n = 36 − 39 × log₉₂(d ÷ 0.127 mm) · SWG: read from the 1884 Board of Trade table · A = π d² ÷ 4 · circular mils = (d in thousandths of an inch)²

AWG: American Wire Gauge, ASTM B258: 36 AWG is exactly 0.005 in and 4/0 is 0.460 in, with 39 geometric steps between
SWG: British Standard Wire Gauge, legal from 1 March 1884, last published in BS 3737:1964 — a table of 57 values with no formula behind it
n: the gauge number; 1/0 = 0, 2/0 = −1, and so on downwards in both systems
circular mil: the area of a circle one mil across; 1,000 of them are a kcmil, the American cable unit
mm²: the metric conductor area, the basis of the IEC 60228 series (0.5, 0.75, 1, 1.5, 2.5, 4, 6, 10, 16, 25 …)

Worked example

16 AWG solid copper
d = 0.127 × 92^{(36 − 16)/39} = 0.127 × 10.163 = 1.2908 mm (0.05082 in)
A = π × 1.2908² ÷ 4 = 1.3087 mm², or 2,582.7 circular mils
16 SWG, from the table, is 0.064 in = 1.6256 mm — 25.93% larger in diameter
That is 2.0739 mm² against 1.3087 mm²: the 16 SWG wire carries 58.59% more metal than the 16 AWG wire
Nearest metric size at or above 1.3087 mm²: 1.5 mm²

The same gauge number in both systems

Computed by this page: the AWG column from the ASTM B258 formula, the SWG column from the published 1884 table. The systems agree within two per cent at exactly one gauge, 1/0. Everywhere from 1 to 40 the SWG wire is the thicker of the two, and at the fine end it carries well over twice the metal.
Gauge	AWG diameter (mm)	SWG diameter (mm)	AWG area (mm²)	SWG area (mm²)	SWG extra metal
4/0 (0000)	11.6840	10.1600	107.2193	81.0732	-24.4
2/0 (00)	9.2658	8.8392	67.4309	61.3643	-9.0
1/0 (0)	8.2515	8.2296	53.4751	53.1921	-0.5
1	7.3481	7.6200	42.4077	45.6037	7.5
2	6.5437	7.0104	33.6308	38.5989	14.8
4	5.1894	5.8928	21.1506	27.2730	28.9
6	4.1154	4.8768	13.3018	18.6793	40.4
8	3.2636	4.0640	8.3656	12.9717	55.1
10	2.5882	3.2512	5.2612	8.3019	57.8
12	2.0525	2.6416	3.3088	5.4805	65.6
14	1.6277	2.0320	2.0809	3.2429	55.8
16	1.2908	1.6256	1.3087	2.0755	58.6
18	1.0237	1.2192	0.8230	1.1675	41.8
20	0.8118	0.9144	0.5176	0.6567	26.9
22	0.6438	0.7112	0.3255	0.3973	22.0
24	0.5106	0.5588	0.2047	0.2452	19.8
26	0.4049	0.4572	0.1288	0.1642	27.5
30	0.2546	0.3150	0.0509	0.0779	53.0
36	0.1270	0.1930	0.0127	0.0293	131.0
40	0.0799	0.1219	0.0050	0.0117	133.0

What each system is

Three different scales attach three different sizes to the number 16. Naming the system is not pedantry; it is the difference between a wire that fits the terminal and one that does not.
System	Defined by	Range	Where you meet it
AWG	A formula: 0.005 in at 36, 0.460 in at 4/0, 39 geometric steps between (ASTM B258)	4/0 to 40 in normal use	North American wiring, datasheets, hook-up wire, magnet wire
SWG	A table, legal from 1884, last published in BS 3737:1964 — now withdrawn	7/0 to 50	Older British and Commonwealth practice, wire mesh, guitar strings, Indian and Pakistani winding wire
mm² (IEC 60228)	The conductor’s cross-sectional area, in a preferred series	0.5 mm² upwards	Everywhere outside North America; the unit a cable is actually sold in
Circular mils / kcmil	The square of the diameter in thousandths of an inch	Used above 4/0	North American heavy cable and busbar
Birmingham gauge (BWG)	A separate table again, and a third set of values for the same numbers	Tube and needle sizes	Tubing, hypodermic needles — not converted here, and not the same as either scale above

A 16 AWG wire and a 16 SWG wire are not the same wire

Wire gauge numbers count backwards because they count drawing operations: the wire was pulled through a succession of smaller dies, and the gauge is how many times. Two countries industrialised with two different sets of dies, and the two scales that resulted have no useful relationship to one another. The American Wire Gauge is a clean geometric progression, fixed by ASTM B258: 36 AWG is exactly five thousandths of an inch, 4/0 is 0.460 in, and the thirty-nine steps between are equal ratios, so a diameter follows from a formula. The British Standard Wire Gauge is not a progression at all. It is a table of fifty-seven diameters that the Board of Trade made legal in 1884, chosen by committee from the sizes British mills were already drawing, and there is no equation that generates it.

So the numbers do not line up. Across the whole range from 4/0 to 40, the two systems agree to within two per cent at exactly one place: gauge 1/0, where SWG is 0.324 in and AWG 0.3249 in. Everywhere else they differ, and not by a rounding error. At 16 the SWG wire is 26% larger in diameter and carries 59% more metal. At the fine end it is worse: 37 SWG is 53% larger in diameter than 37 AWG, which is well over twice the copper. And the sense reverses in the /0 sizes, where SWG 4/0 is 13% smaller than AWG 4/0. There is no rule of thumb to remember here, which is exactly why a table is needed. The chart below plots that gap at every gauge from 1 to 40: it never comes close to zero, and the wobbles in it are the SWG table’s own irregularity showing through, because a hand-picked list of diameters does not follow a smooth curve.

What this means when you order something. If a drawing says “16 gauge” and does not say which system, it is ambiguous, and the ambiguity is worth a third of a millimetre. Indian and Pakistani winding-wire suppliers quote SWG; American datasheets quote AWG; European and Indian cable is sold in square millimetres and cares about neither. Mesh and perforated sheet are usually SWG in Britain. Guitar strings are SWG-derived. Hypodermic needles and tubing use the Birmingham gauge, which is a third table again and gives a third answer for 16. Always write the system next to the number.

Why metric wins in the end. The only quantity that matters electrically is the cross-sectional area of the conductor, because that is what sets its resistance. The metric series in IEC 60228 names that area directly — 0.5, 0.75, 1, 1.5, 2.5, 4, 6, 10, 16, 25 mm² and upwards — so there is nothing to look up and nothing to get wrong. Circular mils do the same job in American practice above 4/0: a circular mil is the area of a circle one thousandth of an inch across, so the area in circular mils is just the diameter in mils squared, and a thousand of them make a kcmil.

What this page does not do. It converts sizes, not capacities. How much current a wire may carry depends on its insulation, its grouping, the ambient temperature and the installation method, and no gauge table can answer it; that is an ampacity question and it belongs to the wiring standard that applies where the cable is installed. Measurements here are for the bare solid conductor: measure over the insulation and you will read several gauges too large, and a stranded conductor of the same nominal area is larger overall than the solid one. This page does the arithmetic. Where the answer carries a consequence — a dose, a structural load, a legal area, a medical measure — confirm it against the authority that governs it before relying on it.

Frequently asked questions

Is 16 AWG the same as 16 SWG?

No. 16 AWG is 1.2908 mm across and 16 SWG is 1.6256 mm — 26% larger in diameter and 59% more metal. They are different scales that happen to use the same numbers.

Do AWG and SWG ever agree?

Essentially once. At gauge 1/0, SWG is 0.324 in and AWG 0.3249 in, a quarter of one per cent apart. At every other gauge from 4/0 to 40 they differ by more than two per cent.

What is 16 AWG in mm²?

1.3087 mm². The nearest standard metric conductor size at or above it is 1.5 mm².

What is the AWG formula?

d = 0.127 mm × 92 raised to the power (36 − n) ÷ 39, from ASTM B258. Turned round, n = 36 − 39 × log base 92 of (d ÷ 0.127 mm). SWG has no formula; it is a table.

What is a circular mil?

The area of a circle one thousandth of an inch in diameter. The convenient part is that the area in circular mils is simply the diameter in mils squared. A thousand circular mils are a kcmil.

Which gauge system does India use?

Both, in different trades. Winding wire and mesh are usually quoted in SWG; cable is sold in square millimetres to IEC 60228; imported electronics wire comes in AWG. Write the system beside the number.

Can I use this to choose a wire for a given current?

No. Current capacity depends on insulation, grouping, ambient temperature and installation method, and is set by the wiring standard that applies where the cable is installed. This page converts sizes.

Related calculators

References

ASTM B258, Standard Specification for Standard Nominal Diameters and Cross-Sectional Areas of AWG Sizes of Solid Round Wires Used as Electrical Conductors. ASTM International. AWG is a formula, not a table: 36 AWG is 0.005 in and 4/0 is 0.460 in with 39 geometric steps between them.
BS 3737:1964, Specification for wire gauges, British Standards Institution — withdrawn, and copyrighted. Cited by number only. The Standard Wire Gauge was made a legal standard by Order in Council of 23 August 1883, effective 1 March 1884; the diameters used here are taken from published tables, not from the standard.
Wikipedia, Standard wire gauge (read 26 September 2026), citing LewCoS Wire Tables (1962) and Rowlett R, American and British Wire Gauges, University of North Carolina at Chapel Hill, 2001.
The Engineering ToolBox, SWG — Standard Wire Gauge, engineeringtoolbox.com (read 26 September 2026), citing BS 3737:1964. A second, independent publisher: it agrees with the table above at every one of the 57 gauges from 7/0 to 50.
CalcEngines Electronics, awg-wire-size-calculator — the record’s own expression is d = 0.127 × 92^((36 − n)/39) in millimetres. That implementation was read out of the shipped record and reused here rather than re-derived, and this page asserts agreement with it at every gauge from 4/0 to 40.