SQL Server 2008 Geography .STBuffer() distance measurement units

STBuffer is in meters. More info here.

To convert, miles to meters, divide the number of miles by 0.0006213712

(i.e. 5 miles / 0.0006213712 = 8,046.72 meters)

The unit of measurement depends on the spatial reference system in use. See this system view for details:

SELECT * FROM sys.spatial_reference_systems;