Area calculator – measure land area on a map

The area calculator works like drawing on a printed map with a pen. Zoom to the place, click the first corner, then the next ones around the edge, and the shape closes itself after the third corner. The area and the perimeter update after every click and while you drag a corner, so you can trace a field boundary, a garden, a roof, a forest stand or a lake and adjust it until it fits the background map. Change the area unit to see the same shape in hectares or acres; the panel always shows a second unit next to the first, metric beside imperial. A shape can have up to 500 corners. The link in the address bar holds the corners, so you can send the measurement to a neighbour, a surveyor or a client, and the KML and GeoJSON downloads open in Google Earth and in GIS software with the area and perimeter stored as attributes.

How to use it

1. Search for the town or zoom the map to the area you want to measure.
2. Click on each corner of the shape in turn; after three corners the area is shown.
3. Drag a numbered corner to adjust it, remove one with its × button, or remove the last with "Remove last corner".
4. Pick the area and perimeter units, then copy the share link or download the shape as KML or GeoJSON.

How accurate is it?

Measured on the ellipsoid, not on the flat map

The web map uses the Mercator projection, which enlarges areas by the square of 1 divided by the cosine of the latitude: twice at 45° and four times at 60°. Measuring "on the screen" therefore gives very wrong areas away from the equator. This calculator ignores the projection and computes the area of the shape on the WGS84 ellipsoid with GeographicLib, the method used in professional GIS. As a check, a square of one degree by one degree at the equator comes out at 12,308.78 km², the published GeographicLib value.

Corners matter more than the maths

The calculation itself is exact to a fraction of a square metre, so the uncertainty comes from the corners you click. With the map zoomed in so that a field fills the screen, each click is within well under a metre. A corner that is 1 m off on a 100 m by 100 m plot changes the area by up to about 70 m², or 0.7 %. Aerial photos and survey plans are more precise than the background map, which shows roads and buildings but not property lines.

Straight edges are geodesics

Each edge between two corners is the shortest line on the ellipsoid. For plots and towns this is the same as a straight line. For very long edges along a parallel, such as the borders of Colorado or Wyoming, the geodesic bends towards the pole, so the official area can differ a little: Colorado drawn from its four corners gives about 60 km² less than the same box with edges along the parallels.

Good to know

Area on the map is not area on the slope

Like a land register, the calculator measures the horizontal area projected onto the ellipsoid. A steep hillside has more surface than its map area: a slope of 30 % adds about 4 %. Roofing and turf are sold by the sloping area, land by the map area.

Acres and hectares

One hectare is 10,000 m², about 2.47 acres. The acre here is the international acre of 4,046.856 m²; the old US survey acre is four parts per million larger, which only matters for very large tracts.

Crossed shapes

If the corners are clicked in the wrong order, two edges can cross and form a figure eight. The area of such a shape is not meaningful, so the tool warns you. Drag a corner or remove it and click again in order around the edge.