Maidenhead Grid Square Calculator
A six-character locator is a 6 km box, not a point — which makes it useless for pointing below 50 km and fine beyond 250.
What each locator length is worth
Bearing uncertainty against range
How a grid square changes shape with latitude
A six-character locator is a box, not a point
The subsquare is 5 minutes of longitude by 2.5 of latitude — about 9.3 × 4.6 km at the equator and 6.0 × 4.6 km at 50° north. The worst-case distance error between two such centres is the sum of their half-diagonals, roughly 7.6 km. So quoting a grid-square distance to better than a few kilometres is invented precision, which matters when an award or a contest category turns on a distance threshold.
The aspect ratio inverts at exactly 60°, which is a pleasing consequence of the grid being defined in degrees rather than distance. A square is 2:1 wide at the equator, exactly square at 60° — where the cosine is one half — and taller than it is wide above that: 3.2 × 4.6 km at 70°. The familiar wide-rectangle picture of a grid square is only true in the tropics. And because every level keeps the same 2:1 degree ratio, that flip happens at 60° for all of them.
Bearing uncertainty is the half-diagonal over the distance, which makes a six-character locator useless for pointing close in and perfectly good far out: ±20.7° at 10 km, ±4.3° at 50, ±2.2° at 100, and ±0.4° at 500. Below about 50 km the locator simply cannot aim an antenna; above 250 km it's inside any practical beamwidth.
That's the whole reason microwave and moonbounce operators exchange eight or ten characters. At 50 km, six characters gives ±4.3°, eight gives ±0.43° — which a one-degree dish can actually use — and ten brings it to ±0.018°, effectively a point. The extra characters aren't pedantry; they're the difference between hearing the station and not.
- The reverse bearing isn't the forward one plus 180° except on a meridian. Great circles curve, and over a transatlantic path the difference is tens of degrees — which catches people rotating a beam for a sked.
- The system was designed in 1980 at Maidenhead, Kent, to replace a patchwork of national schemes. The geometry was chosen for easy mental arithmetic rather than equal area, which is why cells shrink toward the poles and degenerate entirely at the top of the world.
- A locator says where an antenna is, not what it can hear. Two stations in the same subsquare can have completely different paths if one is on a hilltop and the other in a valley — terrain does more for a VHF path than a few kilometres of position ever will.
- Height never varies with latitude — only width does. A subsquare is 4.64 km tall from the equator to the pole, because latitude degrees are the same length everywhere.
How to use
- Enter two locators for distance and great-circle bearing.
- Check the bearing uncertainty before rotating a beam on a short path.
- Use eight or ten characters for microwave and moonbounce work.
- Remember the return bearing is not the outbound plus 180.
Frequently asked questions
How accurate is a six-character Maidenhead locator?
It is a box rather than a point — five minutes of longitude by two and a half of latitude, which comes to about 9.3 by 4.6 km at the equator and 6.0 by 4.6 at fifty degrees north. The half-diagonal is around 3.8 km, and that is the irreducible uncertainty in any position taken from it. Quoting a distance from one to better than a few kilometres is invented precision.
Why do microwave operators use eight or ten character locators?
Because bearing uncertainty is the half-diagonal divided by the distance, and at short range six characters is nowhere near enough. At fifty kilometres a six-character locator gives about four and a third degrees of uncertainty, an eight-character one gives 0.43 — which a one-degree dish can actually use — and ten characters brings it to 0.018. The extra characters are the difference between hearing the station and not.
Can I point a beam using a grid square?
It depends entirely on the distance. A six-character locator gives roughly twenty degrees of bearing uncertainty at ten kilometres, four at fifty, two at a hundred and less than half a degree at five hundred. So below about fifty kilometres it cannot aim an antenna at all, and above two hundred and fifty it sits comfortably inside any practical beamwidth.
Is a grid square the same shape everywhere?
No, and this catches people out. The grid is defined in degrees rather than in distance, so the width shrinks with the cosine of the latitude while the height never changes at all. A subsquare is 2:1 wide at the equator, exactly square at sixty degrees where the cosine is one half, and taller than it is wide above that — 3.2 by 4.6 km at seventy.
At what latitude is a grid square exactly square?
Sixty degrees, north or south. A grid cell is always twice as many degrees wide as it is tall, and a degree of longitude shrinks by the cosine of the latitude — so the width equals the height precisely where that cosine is one half. Because every level of the system keeps the same two-to-one degree ratio, the flip happens at sixty degrees for four, six, eight and ten character locators alike.
Why is the return bearing not just my bearing plus 180 degrees?
Because great circles curve on a sphere. Only on a meridian, or on the equator, does the reverse path leave at exactly the opposite heading. On a transatlantic path the difference between the outbound bearing and the reciprocal of the return can be tens of degrees, which matters when two stations are rotating beams for a scheduled contact.
How do I convert latitude and longitude to a grid locator?
Shift the coordinates so they start from the south pole and the antimeridian, then divide successively. The first pair of letters covers twenty degrees of longitude by ten of latitude, the digits divide that by ten each way, the second letter pair by twenty-four, and so on. Each pair adds one more level of subdivision, alternating letters and digits.
What do the parts of a Maidenhead locator mean?
The first two letters are the field, twenty by ten degrees. The two digits are the square, two by one degrees, which is what people usually mean by a grid square. The next two letters are the subsquare, five by two and a half minutes. Beyond that the extended square and the ten-character level subdivide again, each pair adding roughly an order of magnitude of precision.
How much error does a grid square add to a distance calculation?
The worst case is the sum of the two half-diagonals, which for two six-character locators is about 7.6 kilometres. On a fifty kilometre path that is fifteen per cent; on a five hundred kilometre path 1.5; on a transatlantic path under one. It is worth remembering when a contest score or an award claim turns on a distance threshold, because the figure is not as sharp as the digits look.
Where does the Maidenhead system come from?
It was agreed in 1980 at a conference in Maidenhead, Kent, to replace a patchwork of incompatible national systems. The geometry was chosen for easy mental arithmetic rather than for equal-area cells, which is why every field is twenty by ten degrees regardless of where it sits — and why the cells shrink toward the poles and the scheme degenerates entirely at the top and bottom of the world.
Does a grid square tell me anything about propagation?
Only where the antenna is, not what it can hear. Two stations in the same subsquare can have completely different paths available if one sits on a hilltop and the other in a valley, and on VHF and above terrain does far more for a path than a few kilometres of position ever will. The locator is a coordinate rather than a prediction.
Why is the height of a grid square the same at every latitude?
Because degrees of latitude are the same length everywhere — about 111 kilometres — while degrees of longitude converge toward the poles. A subsquare is therefore 4.64 kilometres tall whether it is on the equator or in Svalbard, and only its width changes. That asymmetry is what makes the cells flip from wide to tall as you go north.
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