Using it
Type a latitude and a longitude in decimal degrees, such as 41.71463, -72.72713, or with letters, such as 41.71463 N 72.72713 W. The output is the locator; set Characters to 4 for a square, as used on HF, or 8 for an extended square. Links open the point in a map app; nothing is looked up until you follow one.
Swap the boxes to turn a locator back into coordinates. You get the centre of the box, which is what contest distances are measured from, and its corners in the rows.
How it divides the world
Longitude and latitude are shifted to start from zero, at 180° west and 90° south. The first pair of letters, the field, splits the world into 18 × 18 boxes of 20° by 10°, lettered A to R. The two digits split each field into 10 × 10 squares of 2° by 1°. The second pair of letters splits each square into 24 × 24 subsquares, A to X, of 5′ by 2.5′. Two more digits make an extended square.
The sample is W1AW, the ARRL's station in Newington, Connecticut, in FN31PR. Any two points in one subsquare are under about 10 km apart, which is why six characters are enough for most VHF and UHF contacts.
Each pair
| Pair | Name | Characters | Box size |
|---|---|---|---|
| 1st | Field | A–R | 20° × 10° |
| 2nd | Square | 0–9 | 2° × 1° |
| 3rd | Subsquare | A–X | 5′ × 2.5′ |
| 4th | Extended square | 0–9 | 30″ × 15″ |
Questions
Upper or lower case?
The RSGB's VHF handbook asks for capitals throughout, FN31PR, but FN31pr is common and means the same.
Why is it called Maidenhead?
John Morris, G4ANB, designed it, and the IARU's VHF Working Group adopted it at a meeting in Maidenhead, England, in April 1980.
What datum does it use?
WGS 84, the same as GPS, since 1999.
Sources
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