Web Mercator to OSGB36 National Grid Coordinate Converter
Free online tool to convert coordinates from Pseudo-Mercator / Spherical Mercator (EPSG:3857) to OSGB36 / British National Grid (EPSG:27700). Professional-grade accuracy using proj4, zero install, runs in your browser.
About Web Mercator
Pseudo-Mercator / Spherical Mercator (EPSG:3857). The projection used by Google Maps, OpenStreetMap and most web mapping. Not suitable for precise surveying due to scale distortion.
About OSGB36 National Grid
OSGB36 / British National Grid (EPSG:27700). Ordnance Survey Great Britain 1936 datum — the national grid system for England, Scotland and Wales. Universally used in UK survey and cadastre.
Frequently Asked Questions
What is the difference between Web Mercator and OSGB36 National Grid?
Web Mercator (EPSG:3857) is projected and based in Global, while OSGB36 National Grid (EPSG:27700) is national grid based in UK. The projection used by Google Maps, OpenStreetMap and most web mapping. Not suitable for precise surveying due to scale distortion. In contrast, ordnance survey great britain 1936 datum — the national grid system for england, scotland and wales. universally used in uk survey and cadastre.
Is this conversion accurate for surveying?
Yes. The conversion uses the industry-standard proj4 library with full datum shift parameters (). Typical accuracy is sub-meter for civilian applications. For cadastral precision, always verify against the official agency datum.
What coordinate format should I use?
For Web Mercator, enter coordinates as easting, northing in meters (e.g. 514500, 5033880). The tool auto-detects the format.
Can I convert many points at once?
Yes, use our batch CSV converter. Upload a CSV with your points, select the transformation, download the converted file. Free with a SurveyingPedia account.
Is there an API?
Yes — call /api/v1/transform?from=EPSG:3857&to=EPSG:27700&coords=lat,lng for programmatic access. It returns the transformed coordinates as JSON.