Area calculator

How to use it

1. 1

   Find the place

   Type an address or a town, or move the map to the spot. Switch to “Aerial” to see USGS imagery in the US, which makes it easier to follow fences, paths and property lines.

2. 2

   Tap along the edge

   Each tap adds a point, and three points are enough to get the area. Drag a point to move it, or drag the small dot in the middle of a side to add one. Tap the first point, or “Close area”, to finish.

3. 3

   Read the result and save it

   You’ll see the area in square feet, acres and square miles, and the perimeter. Draw more shapes, copy the link, or download them as KML (Google Earth), GeoJSON or GPX.

What it’s for

• A field or a lot

  Check the acreage of land before you buy, lease or plant it, and compare it with your deed or the county records.

• A yard, a patio or a project

  Work out how much sod, gravel or landscape fabric you need, and the perimeter for the fence.

• A roof or a barn

  Measure a roof’s footprint on the aerial image before asking for quotes. That’s the area seen from above: on a pitched roof, the real surface is somewhat bigger.

• School or curiosity

  Compare the size of a park, a lake or a neighborhood, or draw several shapes and add them up.

How it’s calculated

The area is calculated on the WGS84 ellipsoid, the shape of the Earth that GPS uses, with Charles Karney’s GeographicLib: the shape is measured as it sits on the Earth, without flattening it onto a map first. The math error is negligible; what really matters is where you put the points.

It’s the horizontal area, the shape seen from above, so slope isn’t counted. On a 20% grade the ground has 2% more surface than its footprint, and on a 50% grade, 12% more.

The perimeter is the sum of the sides, each one measured along the shortest path on the ellipsoid.

Why your number may differ from the deed or the county

Mapsfera measures what you draw. It doesn’t look up property records, so its number doesn’t replace the official one: use it to check that figure, to measure before you visit, or to measure a part of a property that isn’t recorded anywhere.

The acreage on a deed comes from a land survey, sometimes a very old one. The acreage in a county’s parcel map comes from its GIS layer, which is drawn for tax purposes and doesn’t always follow the real property lines. Neither has to match the other, and neither has to match what you see on the ground.

All of these are horizontal areas, like this one. County maps use State Plane projections, designed to distort distances by no more than 1 part in 10,000, so the projection changes the area by a few hundredths of a percent at most. If your number and the official one are further apart, it’s almost always where the line is drawn: trace it on the aerial image and zoom in close before each point.

In Spain, the official figures are in the Catastro (each parcel’s area) and in SIGPAC (the farm subsidy registry, which splits parcels by land use). The Spanish version of this page explains both.

Several shapes, holes and files

You can draw as many shapes as you like: they’re added up in the big number, and each one is listed separately. To leave something out of a field (a house, a pond), measure it as another shape and subtract it, or open the field from a file that already has the hole, which is taken out automatically.

You can open KML (Google Earth, Google My Maps), GeoJSON (QGIS and most mapping apps) and GPX files (watches and phones: walk around the field with GPS recording). Open lines are closed automatically, joining the last point to the first. The file never leaves your device.