ASTM B258 – 02IEC 60228:2004
Convert AWG to mm² and mm
Gives the conductor diameter in mm, the cross-section in mm², and the IEC 60228 size you can actually order. The converted figure and the size you buy are never the same number.
4 AWG solid round wire, 20 °C
- Diameter
- 5.19 mm
- Diameter
- 204.3 mils
- Diameter
- 0.2043 in
- Area
- 41,740 cmil
- IEC lists
- 21.2 mm²
21.1 mm² converted
25 mm² IEC nominal
4 AWG converts to 21.1 mm². No IEC 60228 cable is made in that cross-section. The smallest nominal size that is not smaller than the conductor is 25 mm², giving 18% more cross-section. The next size down, 16 mm², is 24% less.
| IEC 60228 conductor class | Max R at 20 °C | Min wires | Max dia. |
|---|---|---|---|
| Class 1 — solid | 0.727 | 1 | 5.7 mm |
| Class 2 — stranded | 0.727 | 7 | 6.6 mm |
| Class 5 — flexible | 0.780 | — | 7.8 mm |
| Class 2 — aluminium | 1.20 | 7 | — |
IEC 60228 notes that solid copper conductors of 25 mm² and above are meant for particular cable types, such as mineral insulated, rather than general-purpose use.
Full AWG reference table
Swipe the table sideways for the imperial columns. The AWG column stays put.
Tap a row to load that size
| AWG | Dia. milsmils | Dia. mmmm | Area cmilcmil | Area mm²mm² | IEC sizeIEC | R Ω/km cl.2Ω/km |
|---|---|---|---|---|---|---|
| 4/0 | 460.0 | 11.684 | 211,600 | 107.2 | 120 | 0.153 |
| 3/0 | 409.6 | 10.404 | 167,800 | 85.0 | 95 | 0.193 |
| 2/0 | 364.8 | 9.26 | 133,100 | 67.4 | 70 | 0.268 |
| 1/0 | 324.9 | 8.25 | 105,600 | 53.5 | 70 | 0.268 |
| 1 | 289.3 | 7.35 | 83,690 | 42.4 | 50 | 0.387 |
| 2 | 257.6 | 6.54 | 66,360 | 33.6 | 35 | 0.524 |
| 3 | 229.4 | 5.82 | 52,620 | 26.7 | 35 | 0.524 |
| 4 | 204.3 | 5.19 | 41,740 | 21.1 | 25 | 0.727 |
| 5 | 181.9 | 4.62 | 33,090 | 16.8 | 25 | 0.727 |
| 6 | 162.0 | 4.11 | 26,240 | 13.3 | 16 | 1.15 |
| 7 | 144.3 | 3.67 | 20,820 | 10.6 | 16 | 1.15 |
| 8 | 128.5 | 3.26 | 16,510 | 8.37 | 10 | 1.83 |
| 9 | 114.4 | 2.91 | 13,090 | 6.63 | 10 | 1.83 |
| 10 | 101.9 | 2.59 | 10,380 | 5.26 | 6 | 3.08 |
| 11 | 90.7 | 2.30 | 8,230 | 4.17 | 6 | 3.08 |
| 12 | 80.8 | 2.05 | 6,530 | 3.31 | 4 | 4.61 |
| 13 | 72.0 | 1.83 | 5,180 | 2.63 | 4 | 4.61 |
| 14 | 64.1 | 1.63 | 4,110 | 2.08 | 2.5 | 7.41 |
| 15 | 57.1 | 1.45 | 3,260 | 1.65 | 2.5 | 7.41 |
| 16 | 50.8 | 1.29 | 2,580 | 1.31 | 1.5 | 12.1 |
| 17 | 45.3 | 1.15 | 2,050 | 1.04 | 1.5 | 12.1 |
| 18 | 40.3 | 1.02 | 1,620 | 0.823 | 1 | 18.1 |
| 19 | 35.9 | 0.904 | 1,290 | 0.653 | 0.75 | 24.5 |
| 20 | 32.0 | 0.813 | 1,020 | 0.519 | 0.75 | 24.5 |
| 21 | 28.5 | 0.724 | 812 | 0.412 | 0.5 | 36.0 |
| 22 | 25.3 | 0.643 | 640 | 0.324 | 0.5 | 36.0 |
| 23 | 22.6 | 0.574 | 511 | 0.259 | 0.5 | 36.0 |
| 24 | 20.1 | 0.511 | 404 | 0.205 | 0.5 | 36.0 |
| 25 | 17.9 | 0.455 | 320 | 0.162 | 0.5 | 36.0 |
| 26 | 15.9 | 0.404 | 253 | 0.128 | 0.5 | 36.0 |
| 27 | 14.2 | 0.361 | 202 | 0.102 | 0.5 | 36.0 |
| 28 | 12.6 | 0.320 | 159 | 0.0804 | 0.5 | 36.0 |
| 29 | 11.3 | 0.287 | 128 | 0.0647 | 0.5 | 36.0 |
| 30 | 10.0 | 0.254 | 100 | 0.0507 | 0.5 | 36.0 |
| 31 | 8.9 | 0.226 | 79.2 | 0.0401 | 0.5 | 36.0 |
| 32 | 8.0 | 0.203 | 64.0 | 0.0324 | 0.5 | 36.0 |
| 33 | 7.1 | 0.180 | 50.4 | 0.0255 | 0.5 | 36.0 |
| 34 | 6.3 | 0.160 | 39.7 | 0.0201 | 0.5 | 36.0 |
| 35 | 5.6 | 0.142 | 31.4 | 0.0159 | 0.5 | 36.0 |
| 36 | 5.0 | 0.127 | 25.0 | 0.0127 | 0.5 | 36.0 |
| 37 | 4.5 | 0.114 | 20.2 | 0.0103 | 0.5 | 36.0 |
| 38 | 4.0 | 0.102 | 16.0 | 0.00811 | 0.5 | 36.0 |
| 39 | 3.5 | 0.0890 | 12.2 | 0.00621 | 0.5 | 36.0 |
| 40 | 3.1 | 0.0787 | 9.61 | 0.00487 | 0.5 | 36.0 |
| 41 | 2.8 | 0.0711 | 7.84 | 0.00397 | 0.5 | 36.0 |
| 42 | 2.5 | 0.0635 | 6.25 | 0.00317 | 0.5 | 36.0 |
| 43 | 2.2 | 0.0559 | 4.84 | 0.00245 | 0.5 | 36.0 |
| 44 | 2.0 | 0.0508 | 4.00 | 0.00203 | 0.5 | 36.0 |
How to read these numbers
Is AWG a diameter or a cross-section?
A diameter. That is the single biggest source of confusion, because both answers get called “12 AWG in mm.”
12 AWG is 2.05 mm across. Its cross-sectional area is 3.31 mm². Both figures are correct, they just describe different things. Use the AWG → mm tab for the diameter and AWG → mm² for the area.
Rule of thumb: if you are picking a gland, a lug, or a drill size, you want mm. If you are matching a cable specification or sizing a circuit, you want mm².
Where do the AWG numbers come from?
The gauge is a geometric series pinned at two points: 4/0 is 460 mils and 36 AWG is 5 mils. There are 39 steps between them, so each step multiplies the diameter by a fixed ratio.
r = (460 / 5) ^ (1/39) = 92 ^ (1/39) = 1.1229322
That gives a direct formula for any gauge number n, in millimetres:
d = 0.127 × 92 ^ ((36 − n) / 39)
For 12 AWG: 92^(24/39) = 16.16, times 0.127 = 2.05 mm. For the 0-series use n = 0 for 1/0, −1 for 2/0, and so on down to −3 for 4/0.
This is also why the scale runs backwards. A bigger number means a thinner wire, because it means more drawing steps.
Why does the tool give me a second, larger size?
Because you cannot buy the converted number. 12 AWG works out to 3.31 mm², and no cable is manufactured in that cross-section.
Metric cable comes in a fixed series: 1.5, 2.5, 4, 6, 10, 16, 25, 35 mm² and upward. The converted area almost always lands between two of them, so the tool rounds up to the smallest size that is not smaller than the conductor. That is the size to put on an order.
Rounding down is sometimes defensible, but it is a real reduction in copper, so the tool flags it when the gap is under 6% and leaves the decision to your ampacity and volt-drop calculation.
Can I write “12 AWG = 4 mm²” on a specification?
No. 4 mm² is what you order to replace 12 AWG, not an equivalent you can substitute in the other direction.
Cable is verified by DC resistance per kilometre, not by measuring the copper. A 12 AWG conductor runs about 5.21 Ω/km. The limit for 4 mm² is 4.61 Ω/km, so 12 AWG fails that test by roughly 13%. It clears the 2.5 mm² limit of 7.41 Ω/km comfortably.
This holds all the way up the range. An AWG conductor never satisfies the resistance limit of the metric size it rounds up to — only the size below it.
Why is my stranded conductor thicker than the table says?
Because the diameters here are for solid wire. Stranding leaves air between the strands, so the same nominal size measures wider.
| Construction | Max diameter |
|---|---|
| Class 1, solid | 2.4 mm |
| Class 2, stranded | 2.7 mm |
| Class 5, flexible | 3.0 mm |
That is a 25% spread at one nominal size. Size glands, ferrules and lugs off the construction you are actually pulling, not off the solid figure.
How do I estimate a gauge in my head?
Three gauge numbers roughly halve or double the area. 12 AWG is about half of 9 AWG and about double 15 AWG.
Six gauge numbers halve or double the diameter, near enough for field work. And every 10 numbers is about a factor of 10 in area.
Going the other way from metric, these pairs are close enough to recognise on a drawing:
| Metric | Closest AWG |
|---|---|
| 1.5 mm² | 15 AWG (14 is the safe substitute) |
| 2.5 mm² | 13 AWG (12 is the safe substitute) |
| 4 mm² | 11 AWG (10 is the safe substitute) |
| 6 mm² | 9 AWG (8 is the safe substitute) |
The safe substitute is always the next gauge down, because a lower AWG number is a thicker conductor.
Where the numbers come from
Diameters and cross-sectional areas are the standard nominal values of ASTM B258 – 02 Table 1, at a reference temperature of 20 °C. Areas in square millimetres follow the specification’s own rule, area = d² × 5.067 × 10⁻⁴ with d in mils, which is also how kcmil sizes are converted here.
Nominal cross-sections, maximum conductor resistances, minimum wire counts and maximum conductor diameters come from IEC 60228:2004, Tables 1, 2, 3 and Annex C. Resistance figures are maximum values at 20 °C for plain annealed copper, except the aluminium row, which is for aluminium and aluminium alloy. Annex C diameters are informative guidance for connector compatibility, not a requirement — binding diameters live in the product standard for the cable type.
The 1 400 and 1 800 mm² sizes are marked non-preferred in IEC 60228 and are skipped when a size is recommended. IEC 60228 product standards do not specify cables with AWG or kcmil conductors; the AWG equivalences printed in its foreword are for reference only.