Grid reference finder – OS National Grid references on a map

Ordnance Survey maps of Great Britain are covered by the National Grid: blue lines every kilometre, numbered eastwards and northwards inside lettered squares of 100 km. A grid reference such as NN 166 712 names a spot by its square (NN, around Fort William) and the figures read from the lines: 166 east, 712 north, which together give a 100 m square on the summit of Ben Nevis. Walkers, mountain rescue, farmers, surveyors and planning applications all use these references, and they are what the emergency services ask for when a phone cannot give an address. Type or paste a reference with or without spaces, or a plain easting and northing, and the map jumps to the square it stands for, with the centre in decimal degrees and degrees, minutes and seconds. In the other direction, click the map, search for a town or press "Use my location" and read the reference with 2 to 10 figures, plus the full easting and northing in metres, each ready to copy. The National Grid is drawn on the map: the 100 km squares with their letters, and the 10 km, 1 km and 100 m lines as you zoom in, so you can read a reference the way you would on a paper Explorer or Landranger map. A whole list of references or coordinates can be converted at once and downloaded as CSV, and the square can be saved as KML or GeoJSON.

How to use it

1. Type a grid reference such as NN 166 712, or click the map, search a town or press "Use my location".
2. Choose the precision, from 100 km (letters only) to 1 m (10 figures); 6 figures (100 m) is the usual choice for walkers.
3. Copy the reference, the easting and northing or the latitude and longitude.
4. Open "Convert a list" to convert many references or coordinates at once and download them as CSV.

How accurate is it?

How the conversion works

GPS and web maps use WGS84 latitude and longitude, while the National Grid is drawn on the older OSGB36 datum and the Airy 1830 ellipsoid. The tool converts between them in your browser with the 7-parameter Helmert transformation Ordnance Survey publishes for this purpose, then projects with the National Grid's Transverse Mercator formulas, which are exact to the millimetre. We checked the result against Ordnance Survey's official transformation OSTN15 at 757 towns across Great Britain: on average the two differ by 1.7 m, 95 % of towns are within 2.4 m, and the largest difference, in west Cornwall, is 4.0 m.

What that means for a grid reference

A 6-figure reference names a 100 m square and an 8-figure reference a 10 m square, so the few metres from the Helmert transformation only matter at the edge of a square, where the last figure can be one out. For a 10-figure (1 m) reference, or for survey work, use OSTN15 itself, for example in Ordnance Survey's free coordinate transformation tool. The phone's own position is usually good to 5–10 m outdoors, which is often a bigger error than the transformation.

Truncated, not rounded

Grid references are cut off, not rounded: NN 166 712 is the square whose south-west corner is 216,600 m east and 771,200 m north, and every point inside it has that reference. The tool therefore draws the whole square on the map and gives its centre, and a typed reference is never mistaken for a single point.

Good to know

Easting first

The figures are read easting first, then northing ("along the corridor, then up the stairs"). Swapping them gives a valid reference somewhere else entirely. If a reference ends up in the sea or in the wrong place, check the order and the two letters.

Northern Ireland and the Channel Islands

The National Grid covers Great Britain and the Isle of Man. Northern Ireland and Ireland use the Irish Grid, with single letters, and the Channel Islands use UTM. The tool still converts points there, because the grid's numbers extend over them, but it warns that the reference is not the one used locally.

Lost leading zeros

Each half of the figures must have the same number of digits. SD 372 036 is not the same as SD 372 36, which is invalid; spreadsheets often drop leading zeros from eastings and northings, so check the figures before pasting a list.