GeoPunk – geographic tools for Great Britain

Map Area & Distance Calculator

Draw a polygon or line on the map to measure area, perimeter and distance in UK units, then copy or download the shape as GeoJSON or WKT.

Jump to a postcode

Use the toolbar on the left to draw a shape. Click each point in turn, then click the first point again or double-click to finish. While drawing, use Delete last point to undo a misplaced vertex. Once finished, use the Edit tool to move vertices or the Delete tool to remove the shape. Click any vertex afterwards to see its coordinates and OS grid reference.

Export coordinates in: WGS84 (EPSG:4326)
OSGB36 / British National Grid (EPSG:27700)
Area
Perimeter
Vertices
Total distance

Segments

Draw a line to see individual segment distances here.

How to use the Map Area & Distance Calculator

  1. Choose a mode - "Measure area" to draw a polygon, or "Measure distance" to draw a line.
  2. Find your area (optional) - enter a postcode in the "Jump to a postcode" box and click Go to pan and zoom the map there first.
  3. Draw on the map - click to place each vertex, then click the first point again (polygon) or double-click (line) to finish.
  4. Edit if needed - use the Edit tool to drag vertices, or Delete to clear the shape and start again.
  5. Read the results - area, perimeter, distance and segment lengths update instantly, and you can switch units at any time.
  6. Export - copy the GeoJSON or WKT, or download the GeoJSON file, for use in other GIS software.

Measuring area in hectares, acres and more

Rather than measuring area directly from latitude and longitude - which distorts distances differently depending on where you are in the world - this tool projects every vertex onto the British National Grid using the same OSGB36 datum transformation as our OS Grid Reference Finder. The area is then calculated from those planar metre coordinates using the shoelace formula, giving an accurate result for fields, plots and boundaries anywhere in Great Britain. Switch between square metres, hectares, acres, square kilometres and square feet using the dropdown next to the area figure.

Measuring distance and route length

Switch to "Measure distance" and draw a line to get the cumulative length of a route, boundary or path. Each segment between consecutive points is measured separately - useful for checking individual legs of a journey - alongside the running total. Distances are available in metres, kilometres, miles, yards or nautical miles.

Exporting as GeoJSON or WKT

Every shape you draw can be copied as GeoJSON - the format used by most web maps, APIs and modern GIS software - or as WKT (Well-Known Text), the plain-text format used by spatial databases like PostGIS and many desktop GIS packages. By default both are exported in standard WGS84 longitude/latitude coordinates (EPSG:4326), so they'll drop straight into whichever tool you're using next. Use the "Export coordinates in" switch above the results to output them in OSGB36 / British National Grid (EPSG:27700) easting/northing metres instead - useful if you're working in GIS software that expects National Grid coordinates. Polygons also include a bounding-box WKT in the same chosen datum, handy for quickly framing a search area or map extent.

Related tools on GeoPunk

Frequently Asked Questions

Each vertex of your polygon is projected from WGS84 latitude/longitude onto the British National Grid - the same OSGB36 projection used by Ordnance Survey maps - using a full Helmert datum transformation, then the area is calculated with the shoelace formula on those projected metre coordinates. Working in a planar metre-based grid rather than raw latitude/longitude gives an accurate result anywhere in Great Britain.

They describe the same geometry in different formats. GeoJSON is a JSON structure widely used by web maps, APIs and GIS software; WKT (Well-Known Text) is a plain-text format used by spatial databases such as PostGIS and many desktop GIS tools. This tool lets you copy either format, or download the GeoJSON as a file.

Accuracy depends mainly on how precisely you trace the boundary or route on the map - the underlying National Grid projection and datum transformation are accurate to within a few metres anywhere in Great Britain. For legal or survey-grade boundaries, always verify against official Ordnance Survey or Land Registry data.

You can draw a shape anywhere on the map, but the area and distance figures rely on the British National Grid projection, which is only defined for Great Britain. Shapes drawn outside that area may show inaccurate or missing measurements, though GeoJSON and WKT export will still work correctly anywhere.

Area can be shown in square metres, hectares, acres, square kilometres or square feet. Distance and perimeter can be shown in metres, kilometres, miles, yards or nautical miles. Change the unit at any time using the dropdown next to each result - the figures update instantly without needing to redraw the shape.

While actively placing points, use the Delete last point control to undo a misplaced vertex, or click the first point again (or double-click) to finish the shape. Once a shape is finished, use the Edit tool in the map toolbar to drag its vertices, or the Delete tool to remove it and start again.
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