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Wire size calculator / AWG to mm²

AWG to mm² chart

Exact cross-sections from NEC Chapter 9 Table 8, set next to the IEC 60228 size you would actually buy. Those are not the same number.

American Wire Gauge is a geometric series. Each step changes the diameter, and the area that goes with a gauge is published in circular mils and in square millimetres. A circular mil is the area of a circle one mil (0.001 inch) across. The square-millimetre column in Table 8 is that area converted, not a suggestion to order “3.31 mm²” cable.

IEC 60228 sizes are nominal: 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, and on up. Twelve AWG is 6,530 circular mils and 3.31 mm² exactly. The next IEC size at least that large is 4 mm². Four square millimetres is a different conductor, with its own maximum resistance. Using 4 mm² where a calculation called for 12 AWG is conservative on area. Using 2.5 mm² because it is “close” is not.

AWG and kcmil sizes from NEC Chapter 9 Table 8: circular mils, exact square millimetres, the next IEC 60228 size at least that large, overall stranded diameter, and DC resistance at 75°C in ohms per 1,000 feet for uncoated copper and aluminum.
Size Circular mils Exact mm² Next IEC mm² Diameter in Copper Ω/kft Aluminum Ω/kft
18 AWG 1,620 0.823 1.5 0.046 7.95 13.1
16 AWG 2,580 1.31 1.5 0.058 4.99 8.21
14 AWG 4,110 2.08 2.5 0.073 3.14 5.17
12 AWG 6,530 3.31 4 0.092 1.98 3.25
10 AWG 10,380 5.261 6 0.116 1.24 2.04
8 AWG 16,510 8.367 10 0.146 0.778 1.28
6 AWG 26,240 13.3 16 0.184 0.491 0.808
4 AWG 41,740 21.15 25 0.232 0.308 0.508
3 AWG 52,620 26.67 35 0.260 0.245 0.403
2 AWG 66,360 33.62 35 0.292 0.194 0.319
1 AWG 83,690 42.41 50 0.332 0.154 0.253
1/0 AWG 105,600 53.49 70 0.372 0.122 0.201
2/0 AWG 133,100 67.43 70 0.418 0.0967 0.159
3/0 AWG 167,800 85.01 95 0.470 0.0766 0.126
4/0 AWG 211,600 107.2 120 0.528 0.0608 0.1
250 kcmil 250,000 127 150 0.575 0.0515 0.0847
300 kcmil 300,000 152 185 0.630 0.0429 0.0707
350 kcmil 350,000 177 185 0.681 0.0367 0.0605
400 kcmil 400,000 203 240 0.728 0.0321 0.0529
500 kcmil 500,000 253 300 0.813 0.0258 0.0424
600 kcmil 600,000 304 400 0.893 0.0214 0.0353
700 kcmil 700,000 355 400 0.964 0.0184 0.0303
750 kcmil 750,000 380 400 0.998 0.0171 0.0282
800 kcmil 800,000 405 500 1.030 0.0161 0.0265
900 kcmil 900,000 456 500 1.094 0.0143 0.0235
1000 kcmil 1,000,000 507 630 1.152 0.0129 0.0212
1250 kcmil 1,250,000 633 800 1.289 0.0103 0.0169
1500 kcmil 1,500,000 760 800 1.412 0.00858 0.0141
1750 kcmil 1,750,000 887 1000 1.526 0.00735 0.0121
2000 kcmil 2,000,000 1013 — 1.632 0.00643 0.0106

How the calculator uses each system

In AWG mode the wire size calculator selects from Table 310.16 and uses the Table 8 resistance of that exact size. The result also shows the exact square millimetres and the next IEC size at least that large, labeled as a translation rather than as the thing you must order.

In mm² mode it selects an IEC 60228 cross-section. Voltage drop uses the IEC maximum DC resistance at 20°C, corrected to 75°C with a coefficient of 0.00393 per degree for copper and 0.00403 for aluminum. Those coefficients are the 20°C form of the Table 8 note, which quotes 0.00323 and 0.00330 at 75°C. Ampacity is taken from the largest AWG or kcmil whose Table 8 area does not exceed the IEC size. Sixteen mm² sits between 6 AWG (13.3 mm²) and 4 AWG (21.15 mm²), so its ampacity is the 6 AWG value. That mapping is conservative, and it is stated on the result because the NEC does not list metric building wire.

Resistance columns

The ohms per 1,000 feet are for stranded, uncoated conductors at 75°C. Coated (tinned) copper runs a few percent higher, and solid 14 through 8 AWG runs a little lower, because the strands of a Class B conductor take a longer path than the cable. Compact stranding changes the diameter, not the job of this chart. For a voltage-drop number on a size you already have, use the voltage drop calculator. For the current the same size may carry, use the ampacity chart.