American Wire Gauge is a logarithmic scale: each step down in gauge number increases the cross-section by about 26%, and every three steps roughly doubles it. Metric cables are specified by cross-sectional area in mm², in a different set of preferred sizes, so a conversion is always approximate in one direction.
Formula
A = π / 4 × d²
n is the gauge number. For 1/0, 2/0, 3/0 and 4/0, use n = 0, −1, −2 and −3. The diameter is that of a solid round conductor. The definition comes from the standard that fixes 4/0 at 0.46 inch and 36 at 0.005 inch.
Worked example
AWG 10: d = 0.127 × 92(26/39) = 0.127 × 20.38 = 2.588 mm.
A = π / 4 × 2.588² = 5.26 mm².
AWG 4/0: d = 11.684 mm, A = 107.2 mm². Both agree with the widely published gauge tables.
Reading the result
The calculator returns the nearest AWG to a given mm² size. That is a match by area, not a statement that the cables are interchangeable. If your design requires at least 6 mm², AWG 9 (6.63 mm²) is the nearest, but AWG 10 (5.26 mm²) is smaller than the requirement. Round to the larger conductor when the area is a minimum.
Two further points matter when you compare cables:
- Stranding changes the numbers slightly. The formula gives the solid-conductor area. A stranded cable with the same AWG number has a slightly larger overall diameter and a copper area a little under the nominal value. Manufacturers’ data sheets are the authority.
- Area is not ampacity. Allowable current depends on insulation temperature rating, installation method, ambient temperature and the governing code, which differ between NEC and IEC regimes. This page converts sizes only. Do not select a cable from the area alone.
Common sizes
Solid-conductor values from the formula above:
- 14 AWG: 1.63 mm diameter, 2.08 mm²
- 12 AWG: 2.05 mm, 3.31 mm²
- 10 AWG: 2.59 mm, 5.26 mm²
- 8 AWG: 3.26 mm, 8.37 mm²
- 6 AWG: 4.12 mm, 13.30 mm²
- 4 AWG: 5.19 mm, 21.15 mm²
- 2 AWG: 6.54 mm, 33.63 mm²
- 1/0 AWG: 8.25 mm, 53.48 mm²
Metric preferred sizes
IEC 60228 preferred conductor sizes include 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185 and 240 mm². AWG sizes rarely land on one of these, which is why a design moved from one system to the other usually needs a fresh calculation and not just a conversion.
Large conductors in kcmil
Above 4/0, American practice quotes size in kcmil, thousands of circular mils. One kcmil is 0.5067 mm², so a 250 kcmil conductor has a nominal area of about 126.7 mm². This calculator covers AWG sizes from 4/0 up to 30 AWG.
Substituting one size for another
When a spec calls for a metric cable and only AWG is on hand, or the other way round, choose the size with the same or larger area. A smaller conductor has more resistance per metre, so for the same current it loses more voltage and runs hotter. The length also matters: a short run of a slightly undersized cable may be acceptable on paper, but the voltage drop and the code limits have to be checked in each case.
Questions
What is the metric equivalent of 12 AWG?
About 3.31 mm². The nearest common metric size is 4 mm², which is larger, while 2.5 mm² is smaller.
Is AWG 14 the same as 2.5 mm²?
No. AWG 14 is 2.08 mm² and 2.5 mm² is between AWG 13 and 14. In practice they are treated as roughly comparable for light circuits, but the allowable current differs by code.
Why do larger AWG numbers mean thinner wire?
The number counts the drawing steps through dies used in manufacture. More steps give thinner wire.
How do I read 4/0?
It is spoken “four-aught” and is the largest standard AWG size before sizes are quoted in kcmil. It is 107.2 mm².