GeoPunk – geographic tools for Great Britain

Ordnance Survey Grid Reference Finder

Find OS grid references, eastings & northings, lat/lng, and nearest postcode for any location in Great Britain.

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Postcode lookup

Latitude & longitude

Easting & northing

Click anywhere on the map to get the grid reference and coordinates for that point. Use the geocoder search box (top-right of map) to jump to a place name.

Latitude / Longitude
Degrees, Minutes, Seconds
OS Grid Reference
1km Quadrant Reference
Easting / Northing
Map Square
Nearest Postcode

How to use the OS Grid Reference Finder

This free tool works in four ways — use whichever suits you:

  1. Click the map — click anywhere in Great Britain to instantly get the OS grid reference, easting/northing, lat/long, DMS coordinates, and nearest postcode for that point.
  2. Postcode lookup — type any GB postcode (spaces optional) in the Postcode Lookup box and click Find.
  3. Lat/long entry — enter decimal latitude and longitude values and click Convert.
  4. Easting/northing entry — enter OSGB36 easting and northing values in metres and click Go.

You can also use the geocoder search box in the top-right corner of the map to jump to any place name in Great Britain, and toggle the British National Grid overlay using the layer control.

Convert a Postcode to an OS Grid Reference

The postcode lookup uses a database of Code-Point Open postcode centroids to find the latitude and longitude for any GB postcode, then converts those WGS84 coordinates to an OSGB36 six-figure grid reference using a full Helmert datum transformation. The tool also searches the same database to return the nearest postcode to any point you click on the map — useful for identifying addresses from a map location rather than the other way around.

Looking for broader postcode geography? See our Postcode Areas guide or explore Council Areas across Great Britain.

Convert Latitude and Longitude to an OS Grid Reference

GPS devices and online maps use the WGS84 datum. Ordnance Survey maps use the older OSGB36 datum. The two disagree by up to 140 metres across Great Britain, so a simple ellipsoid swap is not enough — a proper seven-parameter Helmert transformation is required. This tool performs that full transform (using the official OS parameters from A Guide to coordinate systems in Great Britain) before applying the Transverse Mercator projection, giving results accurate to within 2–5 metres anywhere in GB. Enter your WGS84 decimal lat/long values in the boxes above and click Convert.

Understanding OS Grid References

The British National Grid divides Great Britain into progressively smaller squares:

  • 500 km squares — identified by a single letter (S, T, N, O, H).
  • 100 km squares — each 500 km square is divided into 25 squares, labelled with a second letter (e.g. SJ, TQ, NY).
  • Six-figure reference — three-digit easting + three-digit northing within the 100 km square, accurate to 100 metres.
  • Eight-figure reference — four digits each, accurate to 10 metres.
  • Ten-figure reference — five digits each, accurate to 1 metre.

The 1 km quadrant reference shown in the results table refines a four-figure reference with a compass quadrant (NE, NW, SE, SW), which is the format used in many mountain rescue and emergency services contexts.

British National Grid map showing the lettered 100 km Ordnance Survey grid squares covering Great Britain
The British National Grid — 100 km squares labelled with two-letter prefixes used in every OS grid reference.

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Frequently Asked Questions

An Ordnance Survey (OS) grid reference is a code that uniquely identifies any location in Great Britain on the British National Grid. A standard six-figure reference (e.g. SJ 381 419) locates a point within a 100-metre square. It begins with two letters identifying the 100 km grid square, followed by three-digit easting and northing values within that square.

A six-figure reference is accurate to 100 metres — it places a point within a 100 m × 100 m square. An eight-figure reference is accurate to 10 metres, and a ten-figure reference to 1 metre. This tool shows the full ten-digit easting and northing in the Easting/Northing row.

Enter any GB postcode (spaces are optional) in the Postcode Lookup box at the top of the tool and click Find. The map will jump to that location and all grid reference values will populate automatically. Note that postcode lookup returns the centroid of the postcode unit, not a specific building.

Easting is the distance in metres measured eastward from the National Grid origin (a point south-west of the Isles of Scilly). Northing is the distance measured northward from the same origin. Together they give a precise Cartesian position within the British National Grid.

The British National Grid covers England, Scotland, and Wales, plus many surrounding islands. It does not cover Northern Ireland (which uses the Irish National Grid) or the Channel Islands. The grid spans roughly 700 km east–west and 1300 km north–south.

GPS and online maps use the WGS84 datum, while Ordnance Survey maps use the older OSGB36 datum. These two datums differ by up to 140 metres across Great Britain because they model the shape of the Earth differently. This tool applies a full seven-parameter Helmert datum transformation (using official OS parameters) before projecting coordinates onto the National Grid, giving conversions accurate to within 2–5 metres.
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