BYTETOOLS

OS Grid Reference Converter

Convert WGS-84 latitude and longitude to a British National Grid reference and back, from 2 to 10 figures, with the OSGB36 position shown.

Latitude and longitude here are WGS-84 (what your phone reports). The grid itself is OSGB36 on the Airy 1830 ellipsoid, reached by the published Helmert 7-parameter shift — good to roughly 5 m, not the centimetre-level OSTN15 transformation.
538,876
Easting (m)
177,320
Northing (m)
TQ
100 km square
10
Figures

British National Grid reference

TQ 38876 77320

No spaces: TQ3887677320

Full easting / northing
538876.213 m E, 177320.118 m N
Same point on OSGB36
51.4772842, 0.0000196
OSGB36 in DMS
51°28'38.22"N 0°0'0.07"E
Datum shift applied
-1.857″ lat, 5.831″ lon

The same point at every precision

  • 10 figures — 1 m squareTQ 38876 77320
  • 8 figures — 10 m squareTQ 3887 7732
  • 6 figures — 100 m squareTQ 388 773
  • 4 figures — 1 km squareTQ 38 77
  • 2 figures — 10 km squareTQ 3 7

What is the OS Grid Reference Converter?

Ordnance Survey maps use the British National Grid: a reference like TG 51409 13177 names a square on the ground, with two letters for the 100 km square and an even number of digits splitting the easting from the northing.

  • Two-way conversion between WGS-84 degrees and British National Grid references
  • Helmert 7-parameter datum shift to OSGB36 on the Airy 1830 ellipsoid
  • Precision from a 10 km square down to a 1 m square, with correct truncation
  • Accepts lettered references with or without spaces, and plain numeric easting/northing pairs
  • Shows the OSGB36 latitude and longitude alongside the WGS-84 values so the datum shift is visible
  • Runs entirely in your browser — no coordinates are uploaded

How to use the OS Grid Reference Converter

  1. 1

    Choose a direction: WGS-84 latitude/longitude to a grid reference, or a grid reference back to coordinates.

  2. 2

    Enter the coordinates in decimal degrees or DMS, or paste a reference like TG 51409 13177.

  3. 3

    Pick how many figures you want, from 2 (a 10 km square) to 10 (a 1 m square).

  4. 4

    Read the letters, the full easting and northing, and the matching OSGB36 position.

  5. 5

    Copy the reference, or check the table showing the same point at every precision.

About the OS Grid Reference Converter

Ordnance Survey maps use the British National Grid: a reference like TG 51409 13177 names a square on the ground, with two letters for the 100 km square and an even number of digits splitting the easting from the northing. This converter turns the WGS-84 latitude and longitude your phone reports into that reference, and decodes any grid reference back into coordinates.

The route runs through the published Helmert 7-parameter shift from WGS-84 to OSGB36 on the Airy 1830 ellipsoid, then the standard Transverse Mercator series with the National Grid constants. That is accurate to roughly 5 metres — good enough for walking, cycling, land access and site notes, but not the centimetre-level OSTN15 transformation used for survey work.

All the maths happens in your browser. Nothing is uploaded, so you can convert grid references for a private site or a rescue callout without anything leaving the device, and the page still works with no signal.

Frequently asked questions

How do you read an OS grid reference?

Read it in three parts. The two letters name a 100 km square, then the digits split evenly: the first half is the easting and the second half the northing, both measured from that square's south-west corner. So TG 514 131 is 51.4 km east and 13.1 km north of the corner of square TG.

Why is the grid reference different from my GPS latitude and longitude?

Because they use different datums. Your GPS reports WGS-84, while the National Grid is built on OSGB36, and the two disagree by roughly 70 to 120 metres across Britain. This tool applies the datum shift for you, which is why the OSGB36 latitude and longitude shown differ slightly from what you typed.

How accurate is this converter?

The Helmert transformation it uses is accurate to about 5 metres anywhere in Great Britain. Ordnance Survey's own OSTN15 grid-shift file gets to a centimetre, but it is a large lookup table rather than a formula. For hiking, planning and general mapping, 5 metres is well inside the width of the square you are quoting.

How many figures should a grid reference have?

Six figures is the walking standard and names a 100 metre square, which is what most guidebooks and mountain rescue teams use. Eight figures gets you to 10 metres and ten figures to 1 metre. An odd number of digits is always a typo, since the digits must split evenly.

Does this work in Northern Ireland?

No. Northern Ireland uses the Irish Grid, which is a separate projection with its own letters and origin. This converter covers the British National Grid only, and it reports an error rather than guessing when a point falls outside it.

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