How to use it
- Type the latitude and the longitude of the place in decimal degrees, for example 51.5 and -0.1 for central London.
- Choose Characters: 4 for the field and the square, 6 for the smaller subsquare as well.
- Read Maidenhead locator for the square that holds the point, and Centre of the square for its middle.
- To check a locator, type it in Locator to check; the check line says whether the point is inside that square.
How the Maidenhead locator is built
The locator system of IARU Region 1, also called the Maidenhead locator, divides the globe into fields, squares and subsquares. The first pair of letters is the field, 20 degrees of longitude by 10 degrees of latitude. The pair of digits is the square inside that field, 2 by 1 degrees. The last pair of letters is the subsquare, 5 by 2.5 minutes.
Every pair counts from the same origin, 180 degrees west and 90 degrees south, towards the east and the north.
field: letters A to R
longitude index = (longitude + 180) ÷ 20
latitude index = (latitude + 90) ÷ 10
square: digits 0 to 9, 2 degrees of longitude and 1 degree of latitude each
subsquare: letters A to X, 5 minutes of longitude and 2.5 minutes of latitude each- The index is the whole part of the division, so a point exactly on a line belongs to the square that starts at that line.
- A point at the far east or north edge of the globe goes into the last field, square or subsquare.
What the centre and the check mean
The centre is the middle of the square that the locator names. It is the position to use when a single point is needed for a square.
The check uses the point's own coordinates. A locator that names a neighbouring square fails the check, even when that square is close to the point.
What a locator does not show
- A locator names an area, not an exact place. A 6-character square is 5 by 2.5 minutes, and the point can be anywhere inside it.
- The page does not check that a locator belongs to a station, a licence or a callsign.
- The IARU Region 1 handbook, section 4.3, describes the 4-character and 6-character forms. This page does not handle longer codes.
Worked example: 51.5, -0.1 at 6 characters
Longitude -0.1 plus 180 is 179.9. Divided by 20 that is 8, the field letter I. Latitude 51.5 plus 90 is 141.5. Divided by 10 that is 14, the field letter O.
Inside the field, 179.9 − 160 = 19.9 gives square digit 9 (19.9 ÷ 2 = 9.95), and 141.5 − 140 = 1.5 gives square digit 1. The subsquare offsets are 1.9 ÷ (5 ÷ 60) = 22.8, letter W, and 0.5 ÷ (2.5 ÷ 60) = 12, letter M. The locator is IO91WM, and the centre of its square is 51.520833, -0.125000.
| Step | Result |
|---|---|
| Longitude + 180 | 179.9 |
| Field (longitude, latitude) | I, O |
| Square (longitude, latitude) | 9, 1 |
| Subsquare (longitude, latitude) | W, M |
| Locator | IO91WM |
| Centre (latitude, longitude) | 51.520833, -0.125000 |
Frequently asked questions
What is a Maidenhead locator?
It is a short code for a square on the Earth, used by radio amateurs for distances, contests and antenna directions. The IARU Region 1 handbook calls it the locator system and gives its four-character square and six-character subsquare.
Why does the locator mix letters and digits?
The first pair of letters gives the field, of 20 by 10 degrees. The digits give the square inside that field, and the last letters give the subsquare. Every pair counts from 180 degrees west and 90 degrees south.
Should I use 4 or 6 characters?
Use 4 characters for a square of 2 by 1 degrees, which is large. Use 6 characters for the subsquare of 5 by 2.5 minutes, the finer form that the handbook describes.
Why are the letters in capitals?
The handbook writes the six-character form with two capital letters, two digits and two capital letters. The page writes capitals and accepts small letters when you type a locator.
Does a correct locator mean a station is there?
No. A locator names an area. The check says only whether the point is inside that square. The page does not look up stations, licences or callsigns.
Does the page keep the coordinates I type?
No. The calculation runs in your browser, and nothing you type is stored or sent to us.
Why is my point in a different square from the one I expected?
The squares are fixed by the origin at 180 degrees west and 90 degrees south. A point exactly on a line between two squares goes into the square that starts on that line, the one to the east or north.
Sources
Figures and text last checked on Thursday, November 12, 2026.