MGRS converter

Type an MGRS reference, with or without spaces: 4Q FJ 1234 6789 or 4QFJ12346789. The result is the latitude and longitude of the centre of the square it names, with the south-west corner, the full 10-digit reference and the UTM position in the rows.

Type a latitude and longitude to go the other way. MGRS precision sets how many digits the reference gets, from 10 (a 1 m square) down to 2 (10 km).

UTM, the Universal Transverse Mercator system, cuts the world into 60 zones, each 6° of longitude wide, numbered eastward from 180°. Each zone gets its own flat map, a transverse Mercator projection centred on the zone's middle meridian, so within a zone a position is just metres east and metres north.

Eastings count from 500,000 m at the central meridian, so they're never negative. Northings count from the equator, which is 0 in the northern hemisphere and 10,000,000 m in the southern, again to keep them positive. The scale is set 0.9996 on the central meridian, which keeps distortion within about 1 part in 1,000 across the zone.

MGRS, the Military Grid Reference System NATO uses, writes the same UTM position in three parts. 18T is the grid zone: zone 18 and latitude band T, one of the 8° bands lettered C to X from 80° S (I and O are skipped, and X is 12° tall). WL names the 100 km square inside it, a column letter and a row letter. The digits are the metres east and north inside that square, as two equal halves.

Fewer digits name a bigger square: 10 digits pin a 1 m square, 8 a 10 m square, 6 a 100 m square. The digits are cut short, never rounded, so a shorter reference always names the square the point is in. Read back, a reference gives the centre of its square.

Wikipedia's article on MGRS uses Honolulu: 21°24′35.43″ N, 157°54′57.89″ W. That's zone 4, since 157.9° W is in the 4th 6° strip east of 180°, and band Q, which covers 16° N to 24° N. Projected onto zone 4 it's 612,344 m east and 2,367,894 m north. The 100 km square is FJ, so to 10 m the reference is 4Q FJ 1234 6789, the article's own example. Typed back in, that reference gives the centre of the 10 m square, the same point.

Two places break the 6° rule. Off south-west Norway, zone 32 is widened 3° to the west between 56° N and 64° N, and zone 31 shrinks to open sea. Around Svalbard, from 72° N to 84° N, zones 32, 34 and 36 aren't used, and 31, 33, 35 and 37 are widened to cover them. The converter applies both.

UTM stops at 84° N and 80° S. Closer to the poles, maps use the Universal Polar Stereographic grid, which this page doesn't convert.