How to use the acreage map calculator
Type an address, town or landmark and press Search to jump there, or pan and zoom by hand. Zoom in until you can see the property clearly, then click each corner of the land in order around the edge. The shape closes itself, and the area, perimeter and side lengths update as you go. Drag any corner to fine-tune it, use Undo last corner to step back, and Copy link to save or share the shape.
The map opens on a sample 5-acre rectangle so you can see how it works; press Clear shape to start your own. If you already have the lot's dimensions from a plat or deed, the acreage calculator works from measurements instead and doesn't need a map.
How the area is calculated
area = |Σ (λ₂ − λ₁)(2 + sin φ₁ + sin φ₂)| × R² ÷ 2
acres = square metres ÷ 4,046.86
φ is latitude and λ longitude (in radians) for each pair of neighbouring corners, and R is the Earth's authalic radius, the radius of a sphere with the same surface area as the WGS84 ellipsoid. Measuring on the globe rather than on the flat map matters because web maps stretch areas away from the equator: at 45° north a flat-map measurement would overstate area by about 100%.
Worked example: the sample field
- The sample rectangle near 38.5° N runs 0.000905° of latitude north-south (100.6 m, 330 ft) and 0.002312° of longitude east-west (201.2 m, 660 ft at that latitude).
- Area: 100.6 × 201.2 = 20,234 m² = 5.00 acres, or 217,800 ft².
- Perimeter: 2 × (330 + 660) = 1,980 ft.
Tips for accurate results
- Zoom in close before clicking corners; at street level each pixel is under a metre.
- Use what you can see: fences, hedges, road edges and field margins. Street maps don't show property lines, so compare with your county's parcel viewer or plat.
- More corners for curves: a curved boundary or road frontage needs several points to follow it.
- For legal purposes, such as a sale, a dispute or fencing exactly on the line, only a licensed survey counts.
Privacy
Map tiles load from OpenStreetMap's servers, and a search sends what you type to OpenStreetMap's Nominatim service. The corners you draw stay in your browser and in the page address if you copy the link; we don't receive them.
Questions people ask
How do I measure acreage on a map?
Search for the address or place, zoom in until you can see the boundary, then click each corner of the property in order. The calculator closes the shape and works out the area on the curved surface of the Earth, so it stays accurate for large tracts too.
How accurate is measuring land on a map?
The maths is accurate to well under a tenth of a percent on a sphere; the real limit is how precisely you click the corners. On a street map, boundaries aren't drawn, so treat the result as an estimate. For buying, selling or fencing to the line, rely on a licensed survey.
How many acres is my lot?
Draw its outline on the map. Most suburban lots are between about 0.1 and 0.5 acres. One acre is 43,560 square feet, roughly a square 209 ft on a side.
Can I measure a lot from its dimensions instead?
Yes. If you have the side lengths from a plat or deed, the acreage calculator works it out from the measurements, including four-sided lots with unequal sides.
How do I measure the perimeter for fencing?
The perimeter is shown under the area, with each side's length. Add gates and allow for corners when you order fencing; the fence calculator does the post and panel counts.
Sources and assumptions
- Map data and tiles © OpenStreetMap contributors (ODbL), used under the OSM tile usage policy
- Place search by Nominatim (OpenStreetMap), one request per search
- Area uses the spherical polygon formula with the WGS84 authalic radius (6,371,007 m); perimeter uses great-circle distances. 1 acre = 43,560 ft² = 4,046.86 m² (NIST Handbook 44).
Last reviewed 27 October 2026. Every formula on this page is checked against the worked example above in our automated tests. Found a problem? Tell us.