Grid Square Locator (Maidenhead)

What a grid square is

A grid square is a compact code for a rectangular patch of the Earth's surface. The Maidenhead system divides the whole planet into a nested set of rectangles and gives each level a pair of characters. The more characters you use, the smaller and more precise the box.

The most common form hams exchange is six characters, like FN31pr or EM12 (four characters). Four characters describe a box roughly 1 degree of latitude by 2 degrees of longitude, about 70 by 100 miles in the continental US. Six characters shrink that to roughly 2.5 by 3.5 miles, which is close enough for almost every ham purpose.

The system is deliberately coarse. It is not meant to pinpoint your house, it is meant to be short enough to say over the air, log quickly, and compute distance from. That trade-off is exactly why it has survived so long.

How the locator is calculated: field, square, subsquare

A six-character locator is built from three pairs, each a step finer than the last. The system starts at the South Pole and the 180-degree meridian, so every value is a positive offset and there are no negative signs to juggle.

Notice that the first character of every pair encodes longitude and the second encodes latitude. In FN31pr: F and N are the field, 3 and 1 are the square, and pr is the subsquare. The field letters run A through R because 18 letters times 20 degrees covers the full 360 degrees of longitude, and 18 letters times 10 degrees covers 180 degrees of latitude.

The math the tool runs is straightforward. Add 180 to your longitude and 90 to your latitude to make both positive, then repeatedly divide and take the remainder at each level: divide by the cell size, the whole number picks the character, and the leftover feeds the next finer pair.

A worked example

Say you are at latitude 41.7167 N, longitude -72.7167 W (the ARRL headquarters area in Newington, Connecticut, the classic FN31pr example).

1. Shift to positive: longitude becomes 180 + (-72.7167) = 107.2833; latitude becomes 90 + 41.7167 = 131.7167.
2. Field: 107.2833 / 20 = 5 (integer part), so the 6th letter (A=0) is F. 131.7167 / 10 = 13, so the 14th letter is N. Field is FN.
3. Square: take the leftovers. Longitude remainder is 107.2833 - 100 = 7.2833; divide by 2 to get 3 (integer part), the digit 3. Latitude remainder is 131.7167 - 130 = 1.7167; divide by 1 to get the digit 1. Square is 31.
4. Subsquare: longitude remainder 7.2833 - 6 = 1.2833, divided by the 0.0833-degree subsquare width, gives 15, the 16th letter (a=0) p. Latitude remainder 1.7167 - 1 = 0.7167, divided by the 0.0417-degree height, gives 17, the 18th letter r. Subsquare is pr.

The result is FN31pr. You do not need to do this by hand: the calculator above handles the offsets and rounding. The point of walking through it is so the output is not a black box. If a locator ever looks wrong, it is almost always a sign flip on longitude (west should be negative before you enter it) or latitude and longitude entered in the wrong fields.

How to find your grid square

You have several easy routes, from most to least automatic:

• Use this tool: enter your latitude and longitude (decimal degrees, west longitude negative) and it returns your six-character locator.
• From your phone GPS: most map apps show your coordinates if you long-press your location. Copy them into the fields here.
• From your radio: many modern HTs and mobiles with built-in GPS display your grid directly. APRS-capable radios often show it on the position screen.
• From a callsign lookup: QRZ and similar databases list the grid an operator has on file, which is handy for logging a contact when you know their call but not their exact location.

For a fixed station, look your grid up once and write it down. It does not change unless you move more than a couple of miles, which is why subsquares are stable enough to print on a QSL card.

Where you will use your grid square

Grid squares are not just trivia. They are load-bearing in several corners of the hobby:

• Contests: VHF and up contests are usually scored on how many unique grids you work. Each new grid is a multiplier, so operators chase them deliberately, sometimes driving to a grid line to activate two at once.
• Satellites: when you make a contact through an amateur satellite, the standard exchange is grid squares. It is the quickest way for two stations passing overhead to confirm where they each are.
• 6 meters and weak-signal VHF: on the magic band, digital modes like FT8 send a four-character grid as part of the automated exchange, and software maps who you are hearing.
• Mapping and APRS: grids feed logging software, award tracking (like VUCC, which counts grids worked), and position maps.

Because a grid instantly gives distance and bearing, it is also a practical way to answer 'how far did that signal go?' without swapping full coordinates. If you are still getting your station on the air, our first ham radio setup guide covers where a grid fits into logging and antenna aiming.

Four characters, six, or eight?

How precise should your locator be? It depends on the use:

For nearly everything, six characters is the right answer. Four is enough when a digital mode limits the field length. Eight is a niche tool for rovers sitting exactly on a boundary who need to prove which side they are on. When in doubt, give six.

Common mistakes

• Forgetting the sign on longitude. In the US you are west of the prime meridian, so longitude is negative. Entering a positive number lands you in Asia.
• Swapping latitude and longitude. Latitude is the smaller number in the continental US (around 25-49), longitude the larger negative (around -67 to -125). If your grid comes out looking coastal-Pacific when you are in Ohio, you swapped them.
• Mixing degrees-minutes-seconds with decimal. This tool expects decimal degrees. Convert 41 deg 43' to 41.7167 first, do not type 41.43.
• Assuming the subsquare pinpoints your address. A subsquare is still a couple of miles across. It is not a privacy leak, and it is not survey-grade.
• Uppercasing the subsquare. By convention the field is uppercase and the subsquare lowercase, as in FN31pr. Most software does not care, but it is the tidy form.

Grid squares and getting on the air

You do not need a license to look up a grid square, but you do need one to transmit and start using it in contacts. A US Technician license opens VHF and UHF, where grid-based contesting, satellites, and repeater operation all live, and it is the natural first step. Our Technician license walkthrough and study planner lay out the path.

Once licensed, knowing which bands your grid exchanges happen on matters. The Technician band plan shows where 6 meters, 2 meters, and 70 centimeters sit, and the radio range calculator pairs nicely with a grid to sanity-check whether a distant contact is even line-of-sight.