Wire gauge explained: how AWG and metric cable sizes work

Wire size is described in two main ways. North America uses the American Wire Gauge (AWG), where a higher number means a thinner wire: 10 AWG is about 2.59 mm across, while 22 AWG, common in electronics, is about 0.64 mm. Most of the rest of the world sizes cable by its cross-sectional area in square millimetres, such as 1.5 mm² or 2.5 mm². The wire gauge converter translates between the two; this guide explains how the system works and what it does and does not tell you.

Why the numbers run backwards

AWG has been used since 1857, and its numbers come from wire drawing, where metal rod is pulled through a series of progressively smaller dies. The gauge originally reflected the number of drawing operations: a thin wire needed more passes than a thick one, so it ended up with a bigger number. Sizes thicker than 1 AWG continue as 0, 00, 000 and 0000 (also written 1/0 to 4/0), and anything larger again is specified by area in thousands of circular mils (kcmil). Stranded wire is given the gauge of a solid wire with the same total copper area, so it is slightly fatter overall because of the gaps between strands.

The formula and rules of thumb

The modern definition, in the ASTM B258 standard, fixes two end points: 36 AWG is exactly 0.005 inch (0.127 mm) in diameter and 0000 AWG is 0.46 inch (11.684 mm). There are 39 steps between them, and the diameter changes by the same ratio at every step, so the diameter of gauge n is:

d = 0.127 mm × 92^((36 − n) ÷ 39)

Each step changes the diameter by a factor of 92^(1/39) ≈ 1.1229, which produces some handy rules of thumb:

• every 6 gauges halves or doubles the diameter, because 1.1229⁶ ≈ 2.005;
• every 3 gauges halves or doubles the cross-sectional area, and so roughly doubles or halves the resistance per metre;
• every 10 gauges changes the area, and the resistance, by a factor of about 10.

Worked through, 12 AWG is 2.053 mm in diameter with an area of 3.31 mm², and 10 AWG is 2.588 mm and 5.26 mm².

AWG and metric sizes

Because metric cable is sold by area, conversions rarely land exactly on a gauge. A 2.5 mm² cable, a common size for household socket circuits in many countries, sits between 14 AWG (2.08 mm²) and 13 AWG (2.62 mm²). A 1.5 mm² cable falls between 16 AWG (1.31 mm²) and 15 AWG (1.65 mm²), and 4 mm² between 12 AWG (3.31 mm²) and 11 AWG (4.17 mm²). When substituting one system for the other, always choose the size that is at least as large in area, never the nearest smaller one; the converter shows both the nearest gauge and the smallest gauge that is not undersized. Metric vs imperial: how we got two systems has more on why the two systems live side by side.

Resistance and voltage drop

Size matters because thinner wire has more resistance. Copper's resistivity at 20 °C is about 0.01724 Ω·mm² per metre, so its resistance per kilometre is 17.24 divided by the area in mm²: about 5.2 Ω/km for 12 AWG and 53 Ω/km for 22 AWG. Over a long run that adds up. A 30 m cable of 12 AWG has 60 m of conductor in the loop out and back, about 0.31 Ω, so a 15 A load loses about 4.7 V in the cable. That is 3.9% of a 120 V supply but only 2% of a 230 V one, one reason lower-voltage systems need thicker wire for the same power. The lost voltage becomes heat in the cable, worked out with the same rules as in Ohm’s law explained: voltage, current, resistance and power. The voltage drop calculator does this sum for copper or aluminium and for single- or three-phase circuits, and the PCB trace width calculator applies the same idea to copper tracks on circuit boards.

How much current a wire can carry

The question everyone wants answered, how many amps a wire can safely carry, has no single answer, which is why the converter deliberately does not show one. The limit, called ampacity, is set by how hot the conductor may get, and that depends on the insulation's temperature rating, how the cable is installed (in free air, in a wall, buried or in conduit), how many cables are bundled together and the surrounding temperature. Even the US National Electrical Code's main table gives three different figures for the same copper wire at 60, 75 and 90 °C insulation ratings, and other countries use their own rules, such as BS 7671 in the UK and the international IEC 60364.

For anything connected to mains, the wire size has to come from your local code's tables and the circuit's protective device, and the work belongs with a qualified electrician. For low-voltage projects, such as 12 V lighting, solar and vehicle wiring, use the cable maker's current rating and check the voltage drop as well, because a long thin run can waste a surprising share of the supply. The watts to amps calculator turns a load's wattage into the current the cable has to carry.