Three systems, one wire

A conductor has one physical size. Whether it is written as AWG 12, SWG 14 or 3.3 mm² depends only on who is doing the writing and when they learned to do it. That is the whole problem: a panel built in India from an American datasheet and a British drawing will have all three on the table at once, and they do not line up.

The unhelpful part is that the two gauge systems number downwards — a bigger number is a thinner wire — while mm² numbers upwards. AWG 10 is fatter than AWG 14. 10 mm² is fatter than 4 mm². Anyone moving between them has to flip their intuition, and under time pressure that is where the mistake lives.

AWG is a formula. SWG is a list.

This is the single most useful thing to know about them, and almost no chart says it.

AWG is generated. Two sizes were fixed by definition — 36 AWG at 0.005 inch and 0000 AWG at 0.460 inch — and the 39 gauges between them are equal geometric steps. The ratio between the two is 92, so every gauge is the last one multiplied by the 39th root of 92:

d(n) = 0.127 mm × 92^((36 − n) / 39)

Everything about AWG falls out of that. Each step up in gauge number is 26 % less copper. Three steps halve the area — accurate to half a per cent, which is why the rule of thumb survives. Six steps quarter it. Ten steps are a factor of ten, near enough to do in your head. There is no table to memorise, only a function, and the chart below is a printout of it.

SWG is not generated. It was set by the British Board of Trade in 1884 as a schedule of diameters in inches, and there is no rule behind the numbers — the steps were chosen to match what the wire drawing trade was already making. You cannot compute SWG 19; you can only look it up. That is why the two systems drift apart in an irregular way rather than by a constant offset.

It also explains why SWG survives in India and Britain long after it stopped being a legal standard. It was never a calculation anyone did — it was a set of stock sizes, and stock outlives paperwork.

The trap: nearest is not the same as safe

Every converter will tell you the nearest metric size. That is the right answer to the question “what is this, roughly?” and the wrong answer to “what do I order instead?”

Thirteen of the forty-four AWG sizes round down to their nearest stocked metric size. The worst case is at the top of the range: AWG 4/0 is 107 mm², and the nearest metric size is 95 mm² — eleven per cent less copper. Take that substitution at face value on a long feeder and you have quietly given away current capacity and added volt drop, on the strength of a converter that was answering a different question.

So this tool shows both, and colours the nearest size red when it is the smaller one. If you are describing a conductor, take the nearest. If you are replacing one, take the next size up unless you have checked the smaller one against the load and the run length and know it passes.

What a gauge number does not tell you

Solid or stranded. The gauge describes the conductor area, so a stranded conductor of the same gauge has the same copper but a larger overall diameter — the strands do not pack perfectly. That matters for glands, ferrules and conduit fill, and not at all for current capacity.
Whether the insulation is included. It never is. A gauge is always the metal.
Current rating. That depends on insulation temperature class, installation method, ambient and grouping — none of which the gauge knows. A 2.5 mm² conductor is a different circuit in free air and in a bunched conduit at 50 °C.
Whether it is copper. Aluminium of the same gauge has about 61 % of the conductivity, so it carries less and drops more. The resistance figures above are given for both.

The resistance figures, and why yours differ

The Ω/km above are calculated for a solid conductor at 20 °C from the resistivity of the metal — 0.017241 Ω·mm²/m for copper. A 1 mm² conductor comes out at 17.24 Ω/km.

Your datasheet will say around 18.1 Ω/km for the same size, and it is not wrong. IEC 60228 quotes a guaranteed maximumthat absorbs manufacturing tolerance, and on a stranded conductor it also absorbs the fact that a helical strand is longer than the cable it runs in. Design to the standard's number; the one here is for understanding where it comes from.

AWG to metric

Down to AWG 24, with the nearest stocked metric size beside each. Red means that nearest size has less copper than the AWG one.

AWG	Dia mm	Area mm²	Nearest metric	Ω/km Cu
4/0	11.68	107.22	95-11.4 %	0.16
3/0	10.40	85.03	95+11.7 %	0.20
2/0	9.27	67.43	70+3.8 %	0.26
1/0	8.25	53.48	50-6.5 %	0.32
1	7.35	42.41	35-17.5 %	0.41
2	6.54	33.63	35+4.1 %	0.51
3	5.83	26.67	25-6.3 %	0.65
4	5.19	21.15	25+18.2 %	0.82
5	4.62	16.77	16-4.6 %	1.03
6	4.12	13.30	16+20.3 %	1.30
7	3.66	10.55	10-5.2 %	1.63
8	3.26	8.37	10+19.5 %	2.06
9	2.91	6.63	6-9.6 %	2.60
10	2.59	5.26	6+14.0 %	3.28
11	2.30	4.17	4-4.1 %	4.13
12	2.05	3.31	4+20.9 %	5.21
13	1.83	2.62	2.5-4.7 %	6.57
14	1.63	2.08	2.5+20.1 %	8.29
15	1.45	1.65	1.5-9.1 %	10.45
16	1.29	1.31	1.5+14.6 %	13.17
17	1.15	1.04	1-3.6 %	16.61
18	1.02	0.82	0.75-8.9 %	20.95
19	0.91	0.65	0.75+14.9 %	26.41
20	0.81	0.52	0.5-3.4 %	33.31
21	0.72	0.41	0.5+21.8 %	42.00
22	0.64	0.33	0.5+53.6 %	52.96
23	0.57	0.26	0.5+93.7 %	66.78
24	0.51	0.20	0.5+144.2 %	84.21

SWG to metric

The 1884 schedule in its original inches, converted. Same colour rule.

SWG	Dia in	Dia mm	Area mm²	Nearest metric
1	0.3000	7.620	45.604	50+9.6 %
2	0.2760	7.010	38.599	35-9.3 %
3	0.2520	6.401	32.178	35+8.8 %
4	0.2320	5.893	27.273	25-8.3 %
5	0.2120	5.385	22.773	25+9.8 %
6	0.1920	4.877	18.679	16-14.3 %
7	0.1760	4.470	15.696	16+1.9 %
8	0.1600	4.064	12.972	10-22.9 %
9	0.1440	3.658	10.507	10-4.8 %
10	0.1280	3.251	8.302	10+20.5 %
11	0.1160	2.946	6.818	6-12.0 %
12	0.1040	2.642	5.481	6+9.5 %
13	0.0920	2.337	4.289	4-6.7 %
14	0.0800	2.032	3.243	2.5-22.9 %
15	0.0720	1.829	2.627	2.5-4.8 %
16	0.0640	1.626	2.075	2.5+20.5 %
17	0.0560	1.422	1.589	1.5-5.6 %
18	0.0480	1.219	1.167	1-14.3 %
19	0.0400	1.016	0.811	0.75-7.5 %
20	0.0360	0.914	0.657	0.75+14.2 %
21	0.0320	0.813	0.519	0.5-3.6 %
22	0.0280	0.711	0.397	0.5+25.9 %
23	0.0240	0.610	0.292	0.5+71.3 %
24	0.0220	0.559	0.245	0.5+103.9 %
25	0.0200	0.508	0.203	0.5+146.7 %
26	0.0180	0.457	0.164	0.5+204.6 %

Which one is my drawing in?

When a drawing just says “16” with no system named, the context usually settles it.

A number with mm² after it is not a gauge at all — it is the area, and it is the only one of the three that says what it means.
An American datasheet, or anything electronic — hook-up wire, transformer windings, PCB harnesses — is AWG.
An older Indian or British drawing, panel wiring, or a winding shop is SWG. Motor rewinders in particular still order by SWG.
Anything to an IS or IEC standard is mm², and has been for decades.

If it genuinely cannot be resolved, measure it. A micrometer on the bare conductor and the area is arithmetic — which is all any of these systems ever was.

Project		Circuit ref
Prepared by		Date
Checked by		Date

AWG to mm² Converter — Wire Gauge Chart

Set the size any way you like

Buying this in metric