Which formula is used to find circular mil area?

The term circular mil is common in expressing the cross sectional area of a wire. Electrical wires have very minute diameters that are often measured a thousandth of an inch. Hence, when you find the area of the circular wire, for convenience, the unit used is circular mil which is equivalent to one-thousandth of an inch. 

For example, a wire has a diameter of 6×10⁻⁵ inches. To find its area:

A = πr² = π( 6×10⁻⁵ /2)² = 2.83×10⁻⁹ in²

Since 1 mil = 1/1000 (inch) or 0.001 inch

A =  (2.83×10⁻⁹ in²)* (1 mil/ 0.001 inch)²
A = 0.00283 circular mils