Circular mil

A circular mil is a unit of area, equal to the area of a circle with a diameter of one mil (one thousandth of an inch or ). It corresponds to approximately . It is a unit intended for referring to the area of a wire with a circular cross section. As the definition of the unit contains , it is easy to calculate area values in circular mils knowing the diameter in mils.
The area in circular mils, A, of a circle with a diameter of d mils, is given by the formula:
In Canada and the United States the Canadian Electrical Code (CEC) National Electrical Code (NEC), respectively, uses the circular mil to define wire sizes larger than 0000 AWG. In many NEC publications and uses, large wires may be expressed in thousands of circular mils, which is abbreviated in two different ways: kcmil or MCM. For example, one common wire size used in the NEC has a cross-section of 250,000 circular mils, written as 250 kcmil or 250 MCM, which is the first size larger than 0000 AWG used within the NEC.
1000 circular mil equals , so for many purposes, a ratio of 2 MCM ≈ 1 mm2 can be used with negligible (1.3%) error.
Equivalence to other units of area.
As a unit of area, the circular mil can be converted to other units such as square inches or square millimetres.
1 circular mil is approximately equal to:
1000 circular mils = 1 MCM or 1 kcmil, and is (approximately) equal to:
Therefore, for practical purposes such as wire choice, 2 kcmil ≈ 1 mm is a reasonable rule of thumb for many applications.
Square mils.
In square mils, the area of a circle with a diameter of 1 mil is:
By definition, this area is also equal to 1 circular mil, so:
The formula for the area of an arbitrary circle in "circular mils" can be derived by applying this conversion factor to the standard formula for the area of a circle (which gives its result in square mils).
Square inches.
To equate circular mils with square inches rather than square mils, the definition of a mil in inches can be substituted:
Square millimetres.
Likewise, since 1 inch is defined as exactly 25.4mm, 1mil is equal to exactly 0.0254mm, so a similar conversion can be done to convert circular mils to square millimetres:
Example calculations.
A 0000 AWG solid wire is defined to have a diameter of exactly . The cross-sectional area of this wire is:
Formula 1: circular mil.
Note: 1 inch = 1000 mils
Calculating diameter from area.
When large diameter wire sizes such as the widely used 250 kcmil and 350 kcmil wires are specified in kcmil, the diameter of the wire can be calculated from the area without using :
Note: We first convert from kcmil to circular mil
Thus, this wire would have a diameter of a half inch or 12.7 mm.
Metric equivalent.
Some tables give conversions to circular millimetres (cmm). The area in cmm is defined as the square of the wire diameter in mm. However, this unit is rarely used in practice. One of the few examples is in a patent for a bariatric weight loss device.
AWG circular mil formula.
The formula to calculate the circular mil for any given AWG (American Wire Gauge) size is as follows. formula_12 represents the circular mil area for the AWG size formula_13.
For example, a number 12 gauge wire would use formula_15: 
Sizes with multiple zeros are successively larger than 0AWG and can be denoted using "number of zeros/0"; for example "4/0" for 0000AWG. For an m/0AWG wire, use 
For example, 0000AWG (4/0AWG), would use formula_18; and the calculated result would be 211,600 circular mils.
Standard sizes.
Standard sizes are from 250 to 400 in increments of 50 kcmil, 400 to 1000 in increments of 100 kcmil, and from 1000 to 2000 in increments of 250 kcmil.
The diameter in the table below is that of a solid rod with the given conductor area in circular mils. Stranded wire is around 5% larger in diameter to allow for gaps between the strands, depending on the number and size of strands.
"Note: For smaller wires, consult ."