Calculate Geometry

[JeffreyStewart]

I am trying to calculate the area of a group of polygons, but to first test Calculate Geometry tool (which I've never used) I started with a 1/4 square mile box. Area is the field I created for the calculations. I set the Data Type to Double and the number format to Numeric and I am using the Coordinate System of the map. When calculating square miles I get 0.0625. I have changed Coordinate System, I have changed Projection, everything I can think of but the results are the same:

Oddly enough, when I change from square miles to US Survey Acres the answer is correct. 

Can anyone offer some insight into what I may be doing incorrectly? 

[BarryNorthey · Accepted solution]

The calculation in sq mi seems to be correct.

Shape_Length is the perimeter of your box (square) polygon. Each side is 5280/4 = 1320 ft

Area is 1320 x 1320 = 1,742,400 sq ft

1 sq mi = 27,878,400 sq ft

1,742,400 / 27,878,400 = 0.0625 sq mi