Maidenhead Grid Locator
Encode and decode amateur-radio Maidenhead grid locators.
Overview
The Maidenhead Grid Locator tool converts between latitude/longitude coordinates and Maidenhead grid squares - the alphanumeric squares that amateur radio operators use to describe their location during contests, propagation reports and DXCC awards. Type a coordinate and you get back the standard 6-character grid (e.g. IO91wm); type a grid and you get the centre coordinate of that square.
Maidenhead is the de-facto language of amateur radio geography. Logging programs, satellite-tracking software, propagation maps and contest scoring engines all assume Maidenhead. The standard goes back to a 1980 meeting in Maidenhead, England, and its design is deliberately compact: a 6-character grid identifies a 2.5 by 5 minute box anywhere on Earth, and adding two more characters tightens the box to about 240 metres. The tool supports up to 10-character precision (sub-30-metre accuracy) for niche applications.
How it works
A Maidenhead locator is read in pairs. The first pair (letters A-R) splits the globe into 18 x 18 fields of 10 degrees latitude by 20 degrees longitude. The second pair (digits 0-9) subdivides each field into a 10 x 10 grid of squares (1 degree latitude by 2 degrees longitude). The third pair (letters a-x) further subdivides each square into a 24 x 24 grid of subsquares (2.5 minutes latitude by 5 minutes longitude). Further pairs (digits, then letters again) extend the precision.
To encode, the tool shifts the input so latitude runs 0-180 and longitude runs 0-360, then iteratively divides by 18, 10, 24, 10 and 24. To decode, it walks the characters in pairs, accumulating the south-west corner of the cell, and adds half a cell width to return the centre.
Examples
- (51.5074, -0.1278) in central London encodes to
IO91wmat 6 characters - the standard amateur radio grid for that area. - (35.6762, 139.6503) in Tokyo encodes to
PM95vq. FN30asdecodes to roughly (40.0625, -74.0833) - central New Jersey.- (0, 0) encodes to
JJ00aaat 6 characters, sitting at the conventional starting corner of the grid.
FAQ
Why are amateur radio contacts logged in grids?
A 6-character grid is short, unambiguous and globally unique - perfect for voice or Morse exchanges where dictating a latitude and longitude would take far too long.
How big is a 6-character grid?
About 2.5 minutes of latitude by 5 minutes of longitude. At mid-latitudes that is roughly 4.6 km north-south by 6.5 km east-west.
Is 4-character precision useful?
Yes. A 4-character grid (e.g. IO91) identifies a 1 by 2 degree square and is the standard granularity for VHF contests in Europe.
Why are some letter ranges different (A-R vs A-X)?
Different levels of the grid divide into different numbers of buckets - 18 for the field, 24 for the subsquare. Each level uses the smallest letter range that fits its divisor.