How to read a grid reference

A grid reference is a short code that pins down a spot anywhere in England, Scotland or Wales. Once you know the pattern you can read one off any British map in under a minute.

1. The two letters: which 100 km square

The National Grid splits Great Britain into squares 100 km across, and each square has a two-letter code. SH covers most of north-west Wales, NN covers the area around Ben Nevis and TQ covers London. On a printed map the letters are shown in the margin or in the corners where grid squares meet. They matter: the same digits in a different square are a completely different place, hundreds of kilometres away.

2. The digits: across, then up

The digits after the letters are always split into two equal halves:

  • The first half is the easting, how far east (to the right) you are from the square’s left edge.
  • The second half is the northing, how far north (up) you are from the square’s bottom edge.

Across first, then up. The usual memory aid is “along the corridor, then up the stairs”.

3. Worked example: Snowdon summit

The summit of Snowdon (Yr Wyddfa) is at SH 609 543.

  1. SH puts it in the north-west Wales square.
  2. Find the vertical grid line labelled 60 along the top or bottom edge of the map. The third digit, 9, means nine-tenths of the way from line 60 to line 61.
  3. Find the horizontal grid line labelled 54 along the side of the map. The last digit, 3, means three-tenths of the way from line 54 to line 55.
  4. Where those two positions meet is the summit, to within 100 metres.

4. Giving your own grid reference

To work out your own reference from a map, do the same steps in reverse. Find the grid line to your left and read its number. Estimate how many tenths of a square you are past it. Then do the same with the grid line below you. Write the letters, then the easting digits, then the northing digits.

The quickest way is to press Where am I? on this site. It reads your phone’s GPS and shows your grid reference at every precision.

Common mistakes

  • Swapping the halves. Reading up before across puts you somewhere else entirely. Always do eastings first.
  • Dropping the letters. 609 543 exists in every 100 km square in the country, so always include the letters.
  • Rounding up. Grid references are truncated, not rounded. If you are 98% of the way across a square, the digit is still 9, not 10.
  • Uneven digits. SH 6098 543 can’t be right because the two halves must be the same length. Recheck it.