Grid reference converter

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 SKtakes 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:

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 onOSGB36 — 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.

Postcodes and grid references

A postcode names a group of delivery addresses, not a point. The position shown here is the ONS’s centre point for those addresses: in a town that is close to every door in the postcode, but a rural one can spread across a wide area of lanes and scattered farms, and the open hill has no postcodes at all — so “no postcode within 2 km” is the honest answer there, not an error. A postcode is fine for pointing a satnav at a car park; it is not a navigation fix, and should never be given as a position on the hill — use a grid reference. Postcode positions come from the ONS Postcode Directory via the freepostcodes.io API, published under the Open Government Licence (contains OS data © Crown copyright and database right, and Royal Mail data © Royal Mail copyright and database right).

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