Maidenhead grid squares
The Maidenhead Locator System, also called the QTH locator, packs a global
position into a short code such as IO91WM. Amateur radio operators use it to
exchange locations compactly and to compute distances between stations. Each
pair of characters refines the position: the first pair selects a large field,
the second a square within it, the third a subsquare, and the fourth an extended
square.
How it works
The grid divides the world from the south pole and the antimeridian. The first character pair (letters A to R) splits longitude into 18 bands of 20 degrees and latitude into 18 bands of 10 degrees. The second pair (digits 0 to 9) divides each band into ten, giving 2 degrees of longitude and 1 degree of latitude. The third pair (letters A to X) divides by 24, and the fourth pair (digits 0 to 9) divides by another ten.
IO91WM -> field IO + square 91 + subsquare WMAdding the running offsets and returning the center of the final cell yields the latitude and longitude.
Example and tips
The locator IO91WM resolves to roughly 51.46 N, 0.21 W near London. Longer
locators are more precise, so prefer a 6 or 8 character grid when accuracy
matters. Because a locator names an area, two nearby points can share the same
short code, which is expected behaviour rather than an error.
Precision at each character length
Each pair of characters refines the position by subdividing the previous level. The practical precision of each length:
| Locator length | Characters | Grid size (approx) | Typical use |
|---|---|---|---|
| 2 | Field only (e.g. IO) | 20° × 10° | Continent-level context |
| 4 | Field + Square (e.g. IO91) | 2° × 1° | Regional ham radio exchanges |
| 6 | + Subsquare (e.g. IO91WM) | 5’ × 2.5’ (~9 × 5 km) | Standard contest QTH exchange |
| 8 | + Extended (e.g. IO91WM12) | 30” × 15” (~900 × 500 m) | Precise antenna location |
For most amateur radio contests and log exchanges, a 6-character locator is the standard. The 8-character form is mainly used when calculating precise antenna azimuths and distances to the nearest kilometer.
Why Maidenhead was created
Before the Maidenhead system was adopted at a 1980 VHF conference in Maidenhead, UK, amateur radio operators used a variety of incompatible grid systems for their logs and contest exchanges. The new system gave a universal, compact code that could be exchanged verbally in a few seconds, written on a paper log without ambiguity, and decoded into a geographic position without specialized software. It encodes the entire Earth’s surface into 18 × 18 fields, each subdivided hierarchically, using alternating letter and digit pairs.
Amateur radio uses beyond contests
Maidenhead locators appear in several amateur radio contexts beyond contest exchanges:
- APRS (Automatic Packet Reporting System): station location beacons include a locator that mapping software decodes to plot the station on a map
- Weak signal digital modes (FT8, JT65, WSPR): the short exchange format typically includes a 4-character locator, enabling the calculation of distance and azimuth for every contact logged
- Repeater directories: repeater listings commonly include a 6-character locator so operators can compute which repeaters are within range of their current position
- Satellite and EME (Earth-Moon-Earth): precise 8-character locators are used to calculate antenna elevation and azimuth to a moving satellite or the moon
Distance and azimuth from locators
Once you have the coordinates for two Maidenhead locators, calculating the great-circle distance and bearing between them is a standard haversine formula calculation. This is how contest scoring systems verify claimed distances, how WSJT-X displays the distance and direction of an FT8 contact, and how operators determine which of their directional antennas to point toward a station they want to work.