1 mi² = 2.58998 km²
A kilometer is a measure of length, a squared kilometer is a square representing the area of a kilometer squared. Each side is a kilometer in length. You cant really compare the two, length and area...
Square kilometers represent an area, you can't convert to a linear dimension.
A kilometer is a length or distance measure. A square kilometer is an area measure. One cannot be converted to the other.
Multiply square km by 0.39 to get approximate square miles.