[Qgis-user] geometry attribute units

[john polo]

Hi,
I have a polygon that I am trying to measure the perimeter for. I used 
the Export/Add geometry columns function and a column was thus added. 
The value in the column has a value that I don't understand, mostly 
because I don't know what unit it used for the calculation. The 
perimeter would probably be between .7 and 1 mile or the equivalent in 
metric. The value given in the column is 0.0133.

How do I tell what the unit is supposed to be for these calculations? 
(And how would I set/change it?) I've looked in Settings>Options on the 
main QGIS window and the layer and haven't found anything. I did find 
where one sets the units for the measure tool and that is set to meters, 
but I am assuming the units selected for the measure tool aren't 
necessarily the same as for the calculation for the geometry.

John