How to Read a Grid Reference
A grid reference names a square on the map, and the whole trick is remembering one rule: eastings come first — go along the corridor, then up the stairs. Read the numbered line to the LEFT of your point, estimate tenths across to it, then the numbered line BELOW it, and tenths up. Two letters (Britain) or one (Ireland) name the 100 km square, and six figures pin you to 100 m. The steps below walk through it; our grid tools do it live from your GPS if you'd rather tap than count.
Eastings first — the one rule that matters
Every grid reference is two half-numbers interleaved into one: how far east you are within the square, then how far north. The order never changes, and the classic mnemonic never fails: along the corridor, then up the stairs. Swapping them is the single most common error — and it puts you kilometres away, usually somewhere implausible, which is also how you catch it.
4, 6 or 8 figures — how precise is your reference?
The figure count sets the size of the square you're naming. Four figures (SU 38 14) is a 1 km square — fine for naming a hill. Six figures (SU 387 148) is 100 m — the everyday standard for meeting points, route cards and rescue. Eight figures (SU 3872 1481) is 10 m — about as fine as estimating by eye ever honestly gets on a 1:50,000 sheet. Whatever the count, split it evenly: first half easting, second half northing. And because the figures repeat every 100 km, the square letters aren't decoration — SU 387 148 and NN 387 148 are five hundred kilometres apart.
Taking a reference from a paper map: the romer trick
Estimating tenths by eye works, but a romer makes it honest: the corner scales printed on most baseplate compasses (and on many map cases) are graduated in tenths of a grid square for common map scales. Put the romer's corner on your point, read where the grid lines cut its scales, and the third and sixth figures stop being guesses. No romer? The edge of anything straight, marked against the grid line spacing, gets you close — or fold the map so a grid line lands next to your point and judge the fraction.
Print your own: our free grid reference card (PDF) has exact cut-out romer scales for 1:25,000 and 1:50,000 maps, the six-figure drill and the read-out script on one page — with a 100 mm check bar so you know your printer didn't rescale it. Print at 100% (“Actual size”), laminate if you like, share freely with your group.
Britain, Ireland and beyond — same skill, different letters
The reading skill transfers everywhere, only the labels change. Britain's OS National Grid uses two letters per 100 km square; the island of Ireland's Irish Grid uses one; New Zealand quotes a Topo50 sheet code in front of the figures; Australian maps carry MGA numbers with the same along-then-up logic. And if you're standing somewhere wondering what your own reference is, every one of those tools reads it straight from your phone's GPS — one tap, no counting tenths.
Step by step
- Find the square letters — printed in the map corner and on the grid itself: two letters in Britain (SU, NN), one in Ireland (J, O).
- Read the easting: take the number of the vertical grid line to the LEFT of your point — along the corridor first.
- Estimate tenths from that line east to your point — that's your third figure.
- Read the northing: the number of the horizontal grid line BELOW your point — now up the stairs.
- Estimate tenths from that line north to your point — the sixth figure.
- Put it together, letters first: SU 387 148 — square SU, easting 387, northing 148. Read the digits out one at a time.