How is a Maidenhead locator built up?

Longitude is shifted by 180 degrees and latitude by 90 degrees so both are positive. The pair is then encoded as a field (two letters), a square (two digits) and a subsquare (two letters), each level subdividing the one above.

How large is each grid level?

A field is 20 degrees of longitude by 10 of latitude. A square is 2 by 1 degrees. A subsquare is 5 by 2.5 arc minutes. Each extra pair of characters refines the location further.

Why are the field letters only A to R?

There are 18 fields of 20 degrees across the 360 degrees of longitude and 18 fields of 10 degrees across the 180 degrees of latitude, so the field letters run from A to R, the first 18 letters of the alphabet.

What case should the locator be written in?

By convention the field letters are upper case and the subsquare letters are lower case, for example IO91wm. The system is case-insensitive when read, but this casing is the standard.

Lat/Lng to Maidenhead Locator

This tool converts decimal latitude and longitude to a Maidenhead Locator (also called a QTH locator or grid square), the compact grid-reference system used throughout amateur radio for contests, beacons and grid hunting. It outputs a 4, 6 or 8 character code such as IO91wm.

How it works

Both coordinates are first made positive by shifting:

adjLng = longitude + 180   (0 to 360)
adjLat = latitude  + 90    (0 to 180)

The result is then encoded in pairs, each level dividing the previous cell:

Field — two letters A to R: adjLng / 20, adjLat / 10
Square — two digits 0 to 9: each field split into 10 along longitude and latitude
Subsquare — two letters a to x: each square split into 24 along longitude and latitude

So a 6-character locator like IO91wm pins a point to within a few kilometres. By convention the field letters are upper case and the subsquare letters lower case.

Tips and notes

Four characters give a roughly 100 km cell, six characters about 5 km, and eight characters a few hundred metres. The locator encodes the south-west corner of the cell; this tool finds the cell that contains your point. Everything runs in your browser. Pair it with the DMS converters when working from chart coordinates.