OS grid reference converter.

How OS grid references work

The British National Grid is a Transverse Mercator map projection with scale factor 0.9996012717 on the central meridian, true origin at 49° N, 2° W, and map coordinates of E 400 000 m, N −100 000 m at that origin, on the Airy 1830 ellipsoid. That puts the grid’s zero point (the false origin) 400 km west and 100 km north of the true origin, just south-west of the Scilly Isles, so every coordinate in Britain is positive. To shorten references the grid is cut into 100 km squares with two-letter codes (first letter the 500 km square, second the 100 km square, letters A to Z without I, laid out as in Figure 8 of the guide). Digits after the letters are the easting and northing inside that square, measured from its south-west corner.

Which point you get: a grid reference names a square, and this tool converts the south-west corner of that square, which is the convention for the digits. A 6 figure reference is therefore up to 100 m south-west of the middle of its square.

Method and accuracy

Going from a grid reference to WGS84 latitude and longitude takes two steps. First the OS projection formulae (Annex C.2) turn easting and northing into latitude and longitude on the Airy 1830 ellipsoid, which is OSGB36. Then a datum transformation moves that to WGS84: the position is converted to 3D Cartesian coordinates (Annex B), a seven-parameter Helmert transformation is applied (translations −446.448, +125.157, −542.060 m; scale +20.4894 ppm; rotations −0.1502, −0.2470, −0.8421 arc seconds), and the result is converted back (position-vector convention, equation 3 of the guide; reversed by changing the sign of every parameter, as the guide describes). Going from latitude and longitude to a grid reference runs the same steps in the other direction.

Accuracy limit: the guide says this Helmert transformation is only approximate, with an error of up to 3.5 m (95%) horizontally and should not be used for work needing better than that, or outside Britain. Ordnance Survey’s accurate method is the OSTN15 grid transformation, which is distributed under its own licence and is not included here. Use the official OS tools or OSTN15 for surveying, legal boundaries and anything where a few metres matter.

Height: this tool ignores height (it assumes zero ellipsoid height), which moves the result by far less than the transformation error. Great Britain only: latitude and longitude outside about 49° to 61.5° N and 9.5° W to 3.5° E are refused, and grid references must lie inside the 700 km by 1300 km grid in Figure 8. Northern Ireland uses a different grid (the Irish Grid) and is not covered.

Checked against OS worked examples: the code is tested against the guide’s worked examples for Annex B (Cartesian), Annex C (projection both ways) and Annex D (Helmert, which turns 53° 36′ 43.1653″ N, 1° 39′ 51.9920″ W, 299.800 m into 422297.792 E, 412878.741 N).

Sources

• Ordnance Survey, A Guide to Coordinate Systems in Great Britain, v3.6 (2020): section 7.1 and Figure 8 (false origin, letter squares), Annex A (ellipsoids, National Grid constants), section 6.2 and 6.6 (Helmert equation and parameters, 3.5 m accuracy), Annexes B, C and D (formulae and worked examples)