OneTaskBy Tim King · Updated
Area Calculator Map
Click Corners to Get Square Metres, Hectares and Acres
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How the Map Area Calculator works
This area calculator map measures any shape you trace. Zoom in until the boundary you want is clear, then click each corner in order around the shape. After three clicks the polygon closes and the panel shows the area in every common unit along with the perimeter; every further click adds a corner. Undo point removes the last one, Clear starts again, and Download GeoJSON saves the outline for a mapping program. The corners stay in the page — only the map tiles come from the network.
Area calculator map: tracing a boundary
The area is computed on a sphere rather than a flat plane, using the standard spherical polygon formula on an Earth of mean radius 6,371,008.8 m. For a paddock, a roof or a suburb that agrees with a survey to a fraction of a percent, which is closer than anyone can click. What it does not do is follow curves: a round pond needs a dozen corners to approximate it, and the more you place the closer the figure gets.
The units. A hectare is 10,000 m² and an acre is 4,046.856 m², so a hectare is 2.471 acres. A square kilometre is 100 hectares or 247.1 acres, and a square mile is 640 acres or 259 hectares. Square feet use the international foot (0.3048 m), which is what floor plans and roofing quotes use.
Perimeter. The boundary length is the sum of the great-circle distances between consecutive corners, back to the first, and is the figure you want for fencing, guttering or edging. The fence calculator turns it into posts and rails.
Clicking in order. Corners must go around the shape, clockwise or anticlockwise; a corner placed across the middle makes a bow-tie and the area of a self-crossing polygon is not what you expect. If the shape looks wrong, Undo point and place the corner again.
Roofs and slopes. The map shows a plan view, so a pitched roof measures at its footprint. For the true surface area divide by the cosine of the pitch: a 30° roof is the footprint × 1.155, a 45° roof × 1.414.
GeoJSON. The download is a single Polygon feature in WGS 84 with the area in square metres in its properties, which QGIS, Google Earth, Mapbox and most GIS software open directly. The GeoJSON editor on this site shows it again.
FAQ
How accurate is the area?
The maths is exact for the corners you click; the error is in the clicking. At a zoom where individual buildings are visible each corner is good to a metre or two, which for a half-hectare plot is well under one percent.
Why is my area wildly wrong?
The corners were probably not clicked in order around the boundary, so the polygon crosses itself. Undo back to the stray corner and place it again going clockwise or anticlockwise around the shape.
Can I measure a roof?
The map gives the footprint. For the sloped surface, divide by the cosine of the pitch — multiply by 1.155 for 30° and 1.414 for 45°.
Can I measure more than one shape at once?
No, one polygon at a time. Note the figure, download the GeoJSON if you want to keep the outline, then Clear and start the next.
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