Military Grid Reference System (MGRS)
The Military Grid Reference System (MGRS) is a twodimensional grid that uniquely identifies a square meter anywhere on the earth. The MGRS attempts to represent the entire surface of the Earth on a worldwide grid. The grid is based on the UTM (between 80°S and 84°N latitudes) and UPS (Universal Polar Stereographic) systems.
Currently, the Geographic Calculator only supports MGRS points in the UTMbased latitude range (between 80°S and 84°N ).
MGRS coordinates are comprised of 7 parts, 3 of which correspond to the UTM/UPS zone that contains the coordinate. In addition to the Datum (which is not explicitly expressed in the MGRS coordinate), the UTM/UPS zone and latitude band (described below) are part of the MGRS point.

The UTM area is divided into 60 longitudinal strips, each 6° wide. The strips are numbered 160 beginning at the 180°174° W (Zone 1) and increase to the East (i.e. 174°168° W = Zone 2, etc.).

Each strip (i.e. Zone) is then divided (horizontally) into 8° latitude bands. The bands are lettered “C” (80° S) through “X” (80° N) omitting the letters “I” and “O”.
MGRS Coordinates
The MGRS coordinate consists of seven parameters:

Datum (as applied to MGRS)
Equivalent to corresponding UTM/UPS datum, and thus is not expressed explicitly in the coordinate.

Equivalent to corresponding UTM/UPS zone number.
Example:
19TDJ3858897366
“19” is equivalent to the corresponding UTM zone. 
Equivalent to UTM/UPS latitude band letter.
Example:
19TDJ3858897366
“T” is the corresponding UTM latitude band letter. 
This is the first of 2 letters used to define the 100 kilometer square within the UTM/UPS block. The columns AZ (excluding "I" and "O"), like UTM, start at 180° and increase toward the East. Every 3 columns, the pattern repeats.
Example:
19TDJ3858897366
“D” is the MGRS column letter 
MGRS Row Letters
This is the second of 2 letters used to define the 100 kilometer square within the UTM/UPS block. Row letters increase to the North. The letters cycle backwards through the alphabet in the southern hemisphere (J, H, G, ... going south).Zone 1, AH; Rows

MGRS Easting value consists of 5 digits. Represents the number of meters east of the origin (i.e. southwest corner) of the 100 kilometer square in which it is contained.
Example:
19TDJ3858897366
“38588” is the MGRS Easting value 
MGRS Northing value consists of 5digits. Represents the number of meters north of the origin (i.e. southwest corner) of the 100 kilometer square in which it is contained.
Example:
19TDJ3858897366
“97366” is the MGRS Easting value
Example MGRS Coordinate Conversion
44° 13’ 36.96” N, 069° 46’ 08.11” W (Geodetic/WGS84) is equivalent to 19TDJ3858897366MGRS(WGS84).
UTM Zone 19
Latitude Band T
MGRS column D
MGRS row J
MGRS Easting 38588
MGRS Northing 97366
Note: The DATUM is not explicitly stated in the MGRS coordinate.
Note: Conversions between geodetic and MGRS are ellipsoid dependent.
Note: This coordinate system is not supported for vector data conversions
Parker, W. and Bartholomew, R., Issues in Automating the Military Grid Reference System, IN: The Fifth International Geodetic Symposium on Satellite Positioning, Vol. II, 903913.
Parker, W.S., and Bartholomew, R.G., 1988, Mapping, Charting, and Geodesy in the DoD Standard GPS Receivers, IEEE Plans Record, 145152.