Grid reference

*** Ordnance survey maps (not road maps) should be used to calculate grid references.

The United Kingdom National Grid is first divided up into 100km squares. Each of these is given code letters, e.g. TL.

Each 100km grid square is divided into 10km grid squares, and each of these is then divided into 1km squares.

On 1: 50 000 OS maps, the light blue lines mark 1 km squares. The grid reference of a 1 km square is comprised of the 100 km square code (e.g. NN) followed by four numbers that identify the bottom left corner of the square. The first two numbers refer to the horizontal scale and the second two the vertical square.

Grid reference illustration © CEH Monks Wood

 UK National Grid image

A six-figure reference can be derived by dividing the 1 km square into 10 from the bottom left corner.

The following diagram shows how the coloured 10km square is referred to as "TL63".
Remember to read eastings (across), and then northings (up).

If you aren't sure about how to work out a grid reference for a site, the Ordnance Survey provide a good, quick tutorial on how to do it. You can also find out more about the UK national grid through the online Wikipedia.

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