Grid reference converter
Convert an Ordnance Survey grid reference to latitude and longitude — or the other way round — with canonical 6- and 10-figure forms, eastings and northings, and quick links to open the point on a map. Everything runs in your browser.
How the OS National Grid works
The National Grid divides Great Britain into 100 km squares, each named by a pair of letters: the first letter identifies a 500 km block, the second a 100 km square within it, and the letter I is never used. NN covers Lochaber and Ben Nevis, SH covers Eryri (Snowdonia), and SK takes in much of the Peak District.
The figures after the letters split evenly in two: the first half is the easting (how far east within the square), the second half the northing (how far north) — “along the corridor, then up the stairs”. More figures mean a smaller square:
| Figures | Example | Square size |
|---|---|---|
| 4 | NN 16 71 | 1 km |
| 6 | NN 166 712 | 100 m |
| 8 | NN 1667 7127 | 10 m |
| 10 | NN 16670 71279 | 1 m |
A grid reference names a square, not a point, so this converter reports the coordinates of the centre of the square you give it. Northern Ireland is mapped on the separate Irish Grid, which uses a single letter and different projection constants — this converter covers the British grid only.
OSGB36 and WGS84 — why the datum matters
Ordnance Survey maps, and every grid reference on them, are built on OSGB36 — a datum fixed by the 1936–1962 retriangulation and shaped to fit Britain closely. GPS satellites use the global WGS84 datum. The same summit has slightly different latitude and longitude in each system, and treating WGS84 coordinates as if they were OSGB36 can put the plotted point out by up to about 120 m on the ground.
This converter uses the standard seven-parameter Helmert transformation between the two datums, which is accurate to around 5 m anywhere in Great Britain. That is comparable to the error in a handheld GPS fix and fine for walking navigation; surveyors who need centimetre accuracy use Ordnance Survey’s OSTN15 correction grid instead. Dedicated GPS units set to the “British Grid” position format perform the same kind of conversion internally.
Six figures or ten?
A 6-figure reference pins down a 100 m square — about the limit of what you can plot by eye with a romer on a 1:25 000 map, and precise enough to identify a summit, a sheepfold or a path junction. It is the standard currency of route cards and guidebooks.
A 10-figure reference pins down a 1 m square. Phones and GPS receivers will happily display one, but the fix behind it is rarely accurate to better than a few metres, so the final figures carry more precision than the position really has. Ten figures are useful for recording a waypoint you want to revisit, or for passing on a machine-read position exactly as the device shows it.
If you need to give a position to mountain rescue, a 6-figure reference plus a short description (“NN 166 712 — on the path north-west of the summit”) is easier to read back accurately over a poor phone connection than a string of ten digits. If your phone shows a more precise fix, offer that as well and let the team decide what to use.
In an emergency in the hills, call 999 (or 112) and ask for the police, then mountain rescue. Give your grid reference if you can, describe the ground around you, and stay put if it is safe to do so. Read our safety notes before you go.
Plan the day itself with the walking-time calculator and the daylight safety planner.