UK Coordinates Explained: Lat/Long, Grid References, and What Format to Use

What Are Latitude and Longitude?

Latitude and longitude are the global standard for pinpointing any location on Earth. Latitude measures how far north or south you are from the equator, and longitude measures how far east or west you are from the Prime Meridian at Greenwich.

The UK sits roughly between 50°N and 60°N latitude, and between 8°W and 2°E longitude. London is at about 51.5°N, 0.1°W - just west of the meridian. Edinburgh is at about 55.95°N, 3.19°W. The further north and west you go in the UK, the higher the latitude and the more negative the longitude.

Every mapping app, GPS device, and location API uses latitude and longitude in some form. The differences are in how those numbers are written down.

Decimal Degrees vs DMS: Which Format Should I Use?

There are two standard ways to write latitude and longitude:

• Decimal degrees - e.g. 51.5010, -0.1416. A single number with a decimal point. Negative values mean south (latitude) or west (longitude).
• DMS (degrees, minutes, seconds) - e.g. 51° 30' 3.6" N, 0° 8' 29.7" W. The traditional format using degrees (°), minutes ('), and seconds ("). Direction is indicated by N/S/E/W letters instead of positive/negative signs.

Use decimal degrees when you're working with software, APIs, spreadsheets, databases, or any digital system. It's what Google Maps, Leaflet, and virtually every programming library expects. It's also easier to do maths with.

Use DMS when you're working with printed maps, surveying data, or communicating coordinates verbally. It's the traditional format you'll see on Ordnance Survey maps and in navigation contexts.

The two formats represent exactly the same information - they're just different ways of writing the same number. To convert, remember that 1 degree = 60 minutes, and 1 minute = 60 seconds.

OS Grid References: The UK's Own Coordinate System

The Ordnance Survey National Grid is a coordinate system designed specifically for Great Britain. Instead of lat/long, it uses a flat grid laid over the country, with positions written as a two-letter prefix followed by digits - for example, TQ 3080 7995 (the Shard in London).

The two letters identify a 100km square. The digits then narrow it down within that square. The more digits, the more precise:

• 6-figure (e.g. TQ 308 800) - accurate to 100 metres. Good enough for general navigation.
• 8-figure (e.g. TQ 3080 7995) - accurate to 10 metres. Suitable for most practical purposes.
• 10-figure (e.g. TQ 30800 79950) - accurate to 1 metre. Survey-grade precision.

You'll encounter OS Grid References on Ordnance Survey paper maps, in planning applications, land registry documents, rural addresses without postcodes, and anywhere that uses official British mapping data. They don't work outside Great Britain - Northern Ireland uses the Irish Grid, and the rest of the world uses lat/long.

WGS84 vs OSGB36: Why Coordinates Can Be ~100m Off

Here's where people get caught out. There are two different datums (mathematical models of the Earth's shape) commonly used in the UK:

• WGS84 - used by GPS, Google Maps, Apple Maps, OpenStreetMap, and most online tools. This is the global standard.
• OSGB36 - used by Ordnance Survey maps and the National Grid. This is optimised for accuracy across Great Britain specifically.

The same physical location has slightly different coordinates in each datum. The difference is typically around 100 metres - enough to put you on the wrong side of a field or in a different building.

This catches people out when they mix data from different sources. If you take a grid reference from an OS map, convert it to lat/long using simple maths (without a datum transformation), and plot it on Google Maps, it'll be about 100m off. You need a proper transformation to convert accurately between the two systems.

Most online tools, including our postcode converter, use WGS84. If you're working with OS Grid References, make sure you know which datum your data is in before converting.

Which Coordinate Format Does Each App Use?

Quick reference for the most common tools and platforms:

• Google Maps - decimal degrees, WGS84. Right-click any point to see coordinates.
• Apple Maps - decimal degrees, WGS84.
• Ordnance Survey maps - OS Grid References, OSGB36 datum.
• what3words - its own proprietary system. Three words map to a 3m square. Not compatible with standard coordinates without conversion.
• Sat navs (TomTom, Garmin etc.) - usually decimal degrees or DMS, WGS84. Check your device settings.
• Excel / Google Sheets - expects decimal degrees in separate lat and lng columns.
• GIS software (QGIS, ArcGIS) - configurable. Always check your project's coordinate reference system (CRS). The most common are EPSG:4326 (WGS84 lat/long) and EPSG:27700 (OS National Grid).

Common Mistakes When Working with UK Coordinates

These trip up beginners and experienced users alike:

• Swapping latitude and longitude. Latitude comes first in most systems (lat, lng), but some GIS tools and formats like GeoJSON use longitude first (lng, lat). Google Maps uses lat first. PostGIS uses lng first. Always check.
• Forgetting the minus sign on longitude. Most of the UK is west of the Greenwich Meridian, so longitude is negative in decimal degrees. London is about -0.1, Manchester is about -2.2, Cardiff is about -3.2. Drop the minus sign and you end up somewhere in continental Europe.
• Mixing WGS84 and OSGB36 data. As described above, this gives you a ~100m offset. If your points are consistently shifted from where they should be, a datum mismatch is the likely cause.
• Truncating decimal places too aggressively. Each decimal place matters:
  • 6 decimal places = ~11cm accuracy
  • 5 decimal places = ~1.1m accuracy
  • 4 decimal places = ~11m accuracy
  • 3 decimal places = ~110m accuracy
  • 2 decimal places = ~1.1km accuracy
  For most purposes, 6 decimal places is plenty. Rounding to 2 decimal places puts you in the right neighbourhood but not the right building.

How to Convert Between Formats

The most common conversions and how to handle them:

• Postcode to lat/long - use our Postcode to Lat Long Converter. It looks up coordinates from Ordnance Survey data for any of 1.7 million UK postcodes.
• Decimal degrees to DMS - take the whole number as degrees. Multiply the decimal part by 60 to get minutes. Multiply the remaining decimal by 60 to get seconds. For example: 51.5010 → 51° + (0.5010 × 60) = 51° 30.06' → 51° 30' 3.6".
• DMS to decimal degrees - degrees + (minutes ÷ 60) + (seconds ÷ 3600). For example: 51° 30' 3.6" = 51 + 30/60 + 3.6/3600 = 51.5010°.
• Grid reference to lat/long - this is more complex because it requires a datum transformation (OSGB36 to WGS84), not just simple maths. Use a proper conversion tool or library rather than trying to do it by hand.

Look Up Coordinates for Any UK Postcode

Our Postcode to Lat Long Converter lets you instantly look up the coordinates of any UK postcode in both decimal degrees and DMS format. You can convert a single postcode, batch convert up to 500 at once, or click the map to reverse-lookup the nearest postcode to any point. All lookups use local Ordnance Survey data in your browser - nothing is sent to a server.