Definitions

# Circular mil

A circular mil is a unit of area, equal to the area of a circle with a diameter of one mil. A mil is one thousandth of an inch. A circular mil is a convenient unit for referring to the area of the cross section of a wire or cable with a circular cross section, as the area in circular mils can be calculated without reference to pi.

Electricians in the US are familiar with the circular mil because the National Electrical Code uses the circular mil to define wire sizes larger than 4/0 AWG.

The area in circular mils, A, of a circle with a diameter of d mils, is given by:

$A=d^2$

In wire gauge (cable sizing), the acronym MCM or kcmil means a thousand circular mils, the area of a wire $scriptstyle\left\{sqrt\left\{1000\right\}\right\}$ ≈ 31.62 mils (which is 0.03162 inches) in diameter.

## Equivalence to other units of area

1 circular mil (1 cmil) is equal to π/4 square mils, or π/4,000,000 square inches. It is also approximately equal to:

• 0.7854 square mils (1 square mil is about 1.273 circular mils)
• equivalently, 0.7854 millionths of a square inch (1 square inch is about 1.273 million circular mils)
• 5.067×10-10 square metres, 506.7 μm²

1000 circular mil (1 MCM or 1 kcmil) is (approximately) equal to:

• 0.5067 mm²