BYTETOOLS

Geodesic Polygon Area Calculator

Measure the true area of a polygon on the globe from GeoJSON, KML, WKT or coordinates, in square metres, hectares, km2, acres and square feet.

Areas use the spherical-excess line integral on the WGS-84 authalic sphere (radius 6 371 007.181 m), the same approach as Google's SphericalUtil and most GIS packages. Edges are treated as great circles.
Drop a .geojson, .kml or .txt polygon file here, or click to browseMeasured locally — the file is never uploaded

Paste or drop a polygon to measure its true area on the globe.

What is the Geodesic Polygon Area Calculator?

Measuring a polygon on a flat map underestimates or overestimates its real size, badly so away from the equator.

  • Spherical-excess area on the WGS-84 authalic sphere, not a flat-map approximation
  • Reads GeoJSON, KML, WKT and plain coordinate lists, including MultiPolygons
  • Subtracts interior rings so holes are excluded from the total
  • Square metres, hectares, square kilometres, acres, square feet and square miles at once
  • Great-circle perimeter alongside the area, plus a per-polygon breakdown
  • Files are read locally in your browser and never uploaded

How to use the Geodesic Polygon Area Calculator

  1. 1

    Drop a .geojson, .kml or .txt file, or paste the polygon into the box.

  2. 2

    Check the format badge on the results — it tells you how your input was read.

  3. 3

    Read the area in every unit, plus the perimeter, in the summary panel.

  4. 4

    Open the per-polygon table when your file contains more than one shape.

  5. 5

    Copy the summary line to paste into a report or an email.

About the Geodesic Polygon Area Calculator

Measuring a polygon on a flat map underestimates or overestimates its real size, badly so away from the equator. This calculator uses the spherical-excess line integral — the 2·atan2 accumulation over each edge that Google's SphericalUtil and most GIS packages use — on the WGS-84 authalic sphere, so the answer is the area on the actual globe rather than on a projection.

Feed it a GeoJSON Polygon or MultiPolygon, a KML file, a WKT geometry, or simply one "latitude, longitude" per line. Interior rings are subtracted automatically, so a field with a pond in it gives the right number. Results come back in square metres, hectares, square kilometres, acres, square feet and square miles, together with the great-circle perimeter.

Nothing is uploaded. The file you drop is read by your browser and measured locally, which matters when the polygon is a client's land parcel, a farm boundary or a site you are not free to paste into an online GIS.

Frequently asked questions

Why is geodesic area different from the area my drawing tool shows?

Most drawing tools measure area in the map's projection. Web Mercator, the projection behind almost every slippy map, stretches everything toward the poles, so a field in Scotland can look nearly twice its real size. A geodesic calculation measures the shape on the globe itself and is projection independent.

How accurate is this compared with a full ellipsoidal calculation?

It uses the authalic sphere, the sphere with exactly the same surface area as the WGS-84 ellipsoid, so global totals are right and ordinary field or parcel polygons land within roughly 0.1 to 0.3 percent of a true geodesic area. For a legal boundary you still want a surveyed figure, not a browser tool.

Does it handle polygons with holes?

Yes. In GeoJSON, KML and WKT the first ring is the outer boundary and every later ring is treated as a hole and subtracted. In a plain coordinate list, leave a blank line between rings and the first block becomes the outer boundary.

Does my polygon need to be closed?

No. If the last point does not repeat the first, the ring is closed for you before the area is computed, which is also how the perimeter is measured. GeoJSON does require closed rings, so a validator may still complain about the original file.

Can I measure a polygon that crosses the antimeridian?

The area formula itself handles it, because it accumulates a signed excess edge by edge rather than working in flat coordinates. Do check the vertex order, though: a polygon whose longitudes jump from 179 to -179 without intermediate points may not describe the shape you meant.

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