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Wire gauge systems: AWG, SWG and metric

Wire gauge numbers are not measurements. They are names from a drawing process, and the same number means different diameters in different countries. Here is how to read them without mixing them up.

Written and reviewed by SM Arijul Islam · Last reviewed 23 October 2026 · Editorial standards · Report an error

Why the numbers run backwards

Wire is made by drawing a metal rod through progressively smaller dies. Each pass makes the wire thinner and the gauge number larger. That is why 10 AWG is thicker than 24 AWG, and why a larger number is not a larger wire.

Three systems dominate: American Wire Gauge (AWG), the older British Standard Wire Gauge (SWG), and metric cross-sectional area in mm². They are not interchangeable. A 12 AWG wire is 2.05 mm in diameter; a 12 SWG wire is 2.64 mm. Always check the standard, not the number alone.

AWG in one formula

The AWG diameter in millimetres is d = 0.127 × 92^{(36 − n) / 39}, where n is the gauge number. For 0000 use n = −3; for 00 use −2; for 0 use −1. The area is A = π × (d ÷ 2)².

Each AWG step changes the diameter by a factor of 92^1/39 ≈ 1.1229, or about 12.3%. Area changes by about 26%, which is why three gauge steps roughly double the copper area.

Common AWG sizes for copper conductors
AWG	Diameter	Area	Chassis ampacity*
24	0.511 mm	0.205 mm²	3.5 A
22	0.644 mm	0.326 mm²	7 A
20	0.812 mm	0.518 mm²	11 A
18	1.024 mm	0.823 mm²	16 A
16	1.291 mm	1.31 mm²	22 A
14	1.628 mm	2.08 mm²	32 A
12	2.053 mm	3.31 mm²	41 A
10	2.588 mm	5.26 mm²	55 A
8	3.264 mm	8.37 mm²	73 A
6	4.115 mm	13.3 mm²	101 A
4	5.189 mm	21.2 mm²	135 A
2	6.544 mm	33.6 mm²	181 A

*Free-air chassis wiring at a 30 °C temperature rise, a common electronics reference. It is not a building-code ampacity and not a substitute for the manufacturer's table.

The ampacity number is not a property of the wire

Copper melts at the same temperature everywhere, but the current a wire can carry depends on insulation, ambient temperature, bundling, conduit, airflow and the allowable voltage drop. A 12 AWG conductor may be a 20 A branch circuit in a house, a much higher current in free air with high-temperature insulation, or something else again in an aircraft.

Use the manufacturer's table and your local electrical code. If you are unsure, use a larger conductor and consult a licensed electrician. A thicker wire is not automatically a safe substitute if the terminals, insulation or bending radius cannot take it.

SWG: the British system

SWG, also called Imperial Standard Wire Gauge, was used across the UK and Commonwealth. It is still found on older equipment, musical instrument wire, and some fencing and jewellery supplies. Its sizes do not line up with AWG: SWG 20 is 0.914 mm, close to AWG 19 at 0.912 mm, but SWG 12 is 2.642 mm against AWG 12 at 2.053 mm.

SWG sizes and their nearest AWG equivalent
SWG	Diameter	Nearest AWG	AWG diameter
10	3.251 mm	8	3.264 mm
12	2.642 mm	10	2.588 mm
14	2.032 mm	12	2.053 mm
16	1.626 mm	14	1.628 mm
18	1.219 mm	16 or 17	1.291 / 1.151 mm
20	0.914 mm	19	0.912 mm
22	0.711 mm	21	0.723 mm
24	0.559 mm	23	0.574 mm
26	0.457 mm	25	0.455 mm

“Nearest” means just that. Two wires either side of the same nominal size can differ by several percent in area, so check the actual diameter or area before substituting.

Metric cable is sold by area

Metric cable is described by cross-sectional area: 0.75 mm², 1.5 mm², 2.5 mm², 4 mm², 6 mm², 10 mm². That is the total copper area, not the diameter. A 2.5 mm² cable is not 2.5 mm across; a single solid conductor of that area is about 1.78 mm in diameter.

Metric conductor sizes and the nearest common AWG
Metric area	Nearest AWG	AWG area
0.5 mm²	20	0.518 mm²
0.75 mm²	18 or 19	0.823 / 0.653 mm²
1.0 mm²	17	1.04 mm²
1.5 mm²	15 or 16	1.65 / 1.31 mm²
2.5 mm²	13 or 14	2.62 / 2.08 mm²
4 mm²	11	4.17 mm²
6 mm²	9	6.63 mm²
10 mm²	7	10.5 mm²
16 mm²	5	16.8 mm²
25 mm²	3	26.7 mm²
35 mm²	2	33.6 mm²

When substituting, compare area and then check the installation rules. A 2.5 mm² cable is usually described as “14 AWG equivalent”, but its area is closer to 13 AWG, and the correct choice depends on the code, insulation and run length.

Stranded wire and the gauge of a bundle

Stranded conductors are often written as 7/0.2 mm or 19/0.25 mm: number of strands / strand diameter. To get area, multiply the area of one strand by the count. Nineteen strands of 0.25 mm is 19 × 0.0491 = 0.933 mm², roughly 18 AWG. Do not add diameters; add areas.

Welding cable and battery cable are often sold by AWG but with very fine strands for flexibility. A 4 AWG welding cable and a 4 AWG building wire have the same copper area but very different flexibility and insulation.

Voltage drop is the other limit

A conductor sized for current may still drop too much voltage over a long run. Voltage drop rises with current, length and resistance. For a 12 V circuit, even a 3% drop is only 0.36 V, so cable runs are kept short or the conductor is upsized. The resistance and voltage converters can help with the arithmetic; the final check should use the cable manufacturer's resistance per metre.

At mains voltage, voltage drop matters less for short runs but still matters for long feeders. At low voltage, it often governs the choice before ampacity does.

Quick checks before you buy

Match the standard: AWG, SWG or mm².
Compare area, not diameter, when substituting.
Check insulation temperature and installation method.
Check voltage drop on long or low-voltage runs.
For mains wiring, follow the local code and use a licensed electrician.

Frequently asked questions

Is a higher AWG number a thicker wire?

No. Higher AWG numbers are thinner. Each step up is about 12.3% smaller in diameter and about 26% smaller in area.

Is 12 AWG the same as 12 SWG?

No. 12 AWG is 2.053 mm in diameter; 12 SWG is 2.642 mm. The numbers are labels from different standards, not measurements.

What is 2.5 mm² cable in AWG?

It is between 13 AWG (2.62 mm²) and 14 AWG (2.08 mm²). Many suppliers call it 14 AWG equivalent, but compare the actual area and check the installation code.

Can I replace a wire with a thicker one?

Usually yes for current capacity and voltage drop, but check that the terminals accept the larger conductor, that the bending radius is workable and that the insulation rating is suitable. Never substitute a thinner wire.

Does silver plating increase ampacity?

Silver plating helps with corrosion and high-frequency performance, but the copper area still sets the current capacity. Do not rely on plating to make a smaller wire safe.

Sources

ASTM B258 — standard nominal diameters and cross-sectional areas of AWG sizes.
IEC 60228 — conductors of insulated cables.
NFPA 70 National Electrical Code — ampacity tables and installation rules.
Cable manufacturer datasheets for insulation ratings, resistance per metre and voltage drop.
Published SWG conversion tables used in UK trade catalogues.