A mil is one thousandth of an inch. A circular mil is the area of a circle one mil in diameter.
That definition looks arbitrary until you see what it buys you.
Why the unit exists
Calculate the area of a round conductor in normal units and you need pi:
S = π × d² / 4Define your area unit as “the area of a circle one mil across” and the pi cancels out. Area in circular mils is simply the diameter in mils, squared:
cmil = d² d in mils12 AWG is 80.8 mils across. Square it and you get 6,530 circular mils. No pi, no division. That is the entire reason the unit was adopted — it made conductor arithmetic doable on paper.
Converting to mm²
One circular mil is a very small area. In metric terms:
1 mil = 0.0254 mm
1 cmil = π/4 × 0.0254² = 5.067 × 10⁻⁴ mm²So the conversion is:
mm² = cmil × 5.067 × 10⁻⁴That 5.067 factor appears in the ASTM specification itself. It is not an approximation someone chose — it is π/4 times the square of a millimetre-to-inch conversion, and it is exactly where the pi went.
Working from the diameter directly:
mm² = d² × 5.067 × 10⁻⁴ d in milskcmil and MCM
Above 4/0, gauge numbers stop and conductors are named by their circular mil area instead. Since the numbers get large, they are given in thousands: kcmil.
250 kcmil is 250,000 circular mils. Multiply by 0.5067 and you get 126.7 mm².
On older drawings you will see the same sizes written as MCM. M is the Roman numeral for a thousand, so MCM meant thousand circular mils. The term was replaced because M is also the SI prefix for mega — a factor of a million, not a thousand — and the collision caused enough confusion to be worth fixing.
250 MCM and 250 kcmil are the same conductor. Nothing changed but the label.
Quick reference
| Size | cmil | mm² |
|---|---|---|
| 14 AWG | 4,110 | 2.08 |
| 12 AWG | 6,530 | 3.31 |
| 10 AWG | 10,380 | 5.26 |
| 4 AWG | 41,740 | 21.1 |
| 4/0 AWG | 211,600 | 107.2 |
| 250 kcmil | 250,000 | 126.7 |
| 500 kcmil | 500,000 | 253.3 |
| 1000 kcmil | 1,000,000 | 506.7 |
Where you still meet circular mils
Mostly in North American practice and on equipment nameplates, lug markings, and cable schedules from US and Canadian designers. IEC product standards do not specify cables in kcmil at all — the equivalents published alongside metric sizes are reference only.
If you are converting a North American design to metric cable, the circular mil figure is your starting point and the kcmil converter will take it the rest of the way.