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, , of a circle with a diameter of mils, is given by the formula: In Canada and the United States, the Canadian Electrical Code (CEC) and the National Electrical Code (NEC), respectively, use 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: 0.7854 square mils (1 square mil is about 1.273 circular mils) 7.854 × 10−7 square inches (1 square inch is about 1.273 million circular mils) 5.067 × 10−10 square metres 5.067 × 10−4 square millimetres 506.7 μm 1000 circular mils = 1 MCM or 1 kcmil, and is (approximately) equal to: 0.5067 mm, so 2 kcmil ≈ 1 mm (a 1.3% error) 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