kcmil to mm2 Converter: Exact Area and Metric Size
kcmil to mm2 for every standard size from 250 to 2000 kcmil, with the exact cross-section, the IEC 60228 size to specify and the equivalent conductor diameter.
Above 4/0, the American system stops using gauge numbers and switches to kcmil — thousands of circular mils. A circular mil is the area of a circle one mil in diameter, so 250 kcmil means 250,000 circular mils.
The conversion is one multiplication:
mm² = kcmil × 0.5067250 kcmil is **126.7 mm²**. 500 kcmil is **253.3 mm²**. 1000 kcmil is **506.7 mm²**.
Full conversion table
| kcmil | mm² | Diameter | IEC size to order | Increase |
|---|---|---|---|---|
| 250 | 126.7 | 12.70 mm | 150 mm² | +18% |
| 300 | 152.0 | 13.91 mm | 185 mm² | +22% |
| 350 | 177.3 | 15.03 mm | 185 mm² | +4% |
| 400 | 202.7 | 16.06 mm | 240 mm² | +18% |
| 450 | 228.0 | 17.04 mm | 240 mm² | +5% |
| 500 | 253.3 | 17.96 mm | 300 mm² | +18% |
| 550 | 278.7 | 18.84 mm | 300 mm² | +8% |
| 600 | 304.0 | 19.67 mm | 400 mm² | **+32%** |
| 650 | 329.4 | 20.48 mm | 400 mm² | +21% |
| 700 | 354.7 | 21.25 mm | 400 mm² | +13% |
| 750 | 380.0 | 22.00 mm | 400 mm² | +5% |
| 800 | 405.4 | 22.72 mm | 500 mm² | +23% |
| 900 | 456.0 | 24.10 mm | 500 mm² | +10% |
| 1000 | 506.7 | 25.40 mm | 630 mm² | +24% |
| 1200 | 608.0 | 27.82 mm | 630 mm² | +4% |
| 1250 | 633.4 | 28.40 mm | 800 mm² | +26% |
| 1500 | 760.1 | 31.11 mm | 800 mm² | +5% |
| 1750 | 886.7 | 33.60 mm | 1000 mm² | +13% |
| 2000 | 1013.4 | 35.92 mm | 1200 mm² | +18% |
Diameters are the equivalent solid conductor. Real kcmil conductors are always stranded and measure considerably wider.
The 600 kcmil problem
Look at the increase column. Most sizes cost you between 4% and 23% when you round up to the nearest metric size. 600 kcmil costs 32%.
The reason is arithmetic. 600 kcmil converts to 304.0 mm². The metric series has 300 mm² and 400 mm² and nothing between. At 304.0 you have just missed 300, so you go to 400 and buy a third more copper than the design called for.
1250 kcmil has the same problem at 26%, and 1000 kcmil at 24%.
If you are converting a North American design to metric cable and you have any freedom on the conductor size, these are the ones to look at twice. Dropping 600 kcmil to 300 mm² is a 1.3% reduction in cross-section — usually nothing — and it saves you a full size on the cable, the gland, and the lug. That is a real cost difference at this scale.
Check it against the ampacity and volt-drop calculation before you do it. But at 1.3%, it is a calculation worth running.
Two sizes that pass the resistance test
Metric cable is verified by conductor resistance, not by measuring copper. Most converted sizes fail the limit of the metric size they round up to, because the round-up leaves the conductor smaller than nominal.
Two kcmil sizes pass:
| kcmil | Rounds up to | Actual R | Limit | Result |
|---|---|---|---|---|
| 350 | 185 mm² | 0.0972 Ω/km | 0.0991 | **passes** |
| 750 | 400 mm² | 0.0454 Ω/km | 0.0470 | **passes** |
Both are cases where the step up is only 4 to 5%. Two others come within a fraction of a percent and still fail — 450 kcmil misses the 240 mm² limit by 0.03%, and 1200 kcmil misses 630 mm² by 0.4%. Close enough that the actual conductor from a specific manufacturer might go either way.
The same thing happens in the gauge range at 2 AWG and 2/0, and for the same reason. Four points in the whole system, and those are the only places a conversion is genuinely clean. See why AWG doesn't match metric sizes.
kcmil, MCM, and why the name changed
You will see the same sizes written as MCM on older drawings. M is the Roman numeral for a thousand, so MCM meant thousand circular mils. It was replaced by kcmil because MCM invites confusion with the SI prefix M, which means mega — a factor of a million, not a thousand.
250 MCM and 250 kcmil are the same conductor. If you are reading an old drawing, nothing has changed but the label.
A rounding difference worth knowing
At 2000 kcmil, the calculation gives 1013.4 mm². IEC 60228 lists the equivalent as 1010 mm², because that table is printed to three significant figures.
Every other size in the range matches exactly. If a document quotes 1010 and your calculation gives 1013, neither is wrong — you are looking at the same conductor described to different precision.
FAQ
What is 250 kcmil in mm²?
126.7 mm². The metric cable size that replaces it is 150 mm², which is 18% larger.
Is MCM the same as kcmil?
Yes. Both mean thousands of circular mils. kcmil is the current term; MCM appears on older drawings and in older catalogs.
What is 500 kcmil in mm²?
253.3 mm². IEC 60228 lists it as 253. The metric size to order is 300 mm².
How do I convert kcmil to mm² by hand?
Multiply by 0.5067. This comes from the rule that area in mm² equals the diameter in mils squared, times 5.067 × 10⁻⁴ — and for kcmil the circular mil area is already the diameter squared. More on the unit in circular mils explained.
Why does kcmil start after 4/0?
Because the gauge series runs out. AWG is defined from 4/0 down to 36 by a geometric progression, and sizes above 4/0 were never given gauge numbers. Above that point conductors are named by their actual circular mil area instead. See how the AWG system works.
Do IEC cables come in kcmil sizes?
No. IEC product standards specify metric cross-sections only. The kcmil equivalents published alongside them are for reference, not for ordering.
What diameter is 500 kcmil cable?
The equivalent solid conductor is 17.96 mm. Real 500 kcmil cable is stranded and wider — use the manufacturer's overall conductor diameter when sizing lugs and glands, not the solid figure. See conductor classes 1, 2, 5 and 6.
Reference
| Property | Value |
|---|---|
| Conversion factor | mm² = kcmil × 0.5067 |
| Range covered | 250 to 2000 kcmil |
| Equivalent metric range | 126.7 to 1013.4 mm² |
| Sizes that meet their metric limit | 350 and 750 kcmil |
| Worst round-up penalty | 600 kcmil, +32% |
For gauges below 4/0, use the AWG to mm² converter. To work from metric back to American sizes, see metric sizes.
Zakaria El Intissar
Electrical Engineer
Electrical engineer with 12+ years of experience in power systems. I create practical electrical calculators and standards-based technical guides that engineers can apply immediately in their daily work.
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