Geo Polygon Area Calculator

The Geo Polygon Area Calculator finds the area and perimeter of a plot of land, a mapped region, or any shape traced on the Earth's surface — from a list of latitude/longitude vertices. Property boundaries, farm fields, survey parcels, or a shape you traced on a map: enter the corner coordinates in order and get the area back in square meters, square kilometers, hectares and acres, plus the perimeter distance around the boundary.

How to use it

1. Enter your vertices one per line, as comma-separated latitude, longitude pairs — for example 40.800,-73.958, 40.800,-73.949, 40.764,-73.949, 40.764,-73.958.
2. List the points in order as you walk around the boundary, either clockwise or counter-clockwise. The calculator connects each vertex to the next, and the last vertex back to the first.
3. Read the results — vertex count, area in four units, and perimeter — updated automatically as you type.

Each coordinate accepts decimal degrees (40.7128) or DMS/DDM notation (40°42'46"N, 40 42 46 N) — the format is auto-detected per value, so you can mix styles across vertices if you need to. You need at least three vertices to form a polygon.

Why this isn't the Shoelace formula

A flat-plane polygon-area formula (the Shoelace formula) treats latitude and longitude as if they were plain x/y coordinates on a grid — which breaks down at real-world scale, because a degree of longitude is roughly 111 km wide at the equator but shrinks toward zero near the poles, while a degree of latitude stays a constant ~111 km everywhere. Run coordinates through a flat formula and the area comes out wrong by an amount that grows with both the polygon's size and its distance from the equator.

This calculator instead treats each edge as a great-circle arc on a sphere and computes the exact enclosed area with the geodesic spherical-excess formula (the same identity behind mapping libraries like Turf.js and Google Maps' area tools):

excess = 2 · atan2( tan(Δλ/2) · (tan(φ₁/2) + tan(φ₂/2)), 1 + tan(φ₁/2) · tan(φ₂/2) )

summed over every edge (φ = latitude, λ = longitude, Δλ = the edge's longitude difference), then multiplied by the Earth's mean radius squared. This is exact for polygons whose edges are great circles — not an approximation that only holds for small areas.

The perimeter is the sum of the great-circle (Haversine) distance between each pair of consecutive vertices, including the closing edge back to the first point.

Worked example

The four corners 40.800,-73.958, 40.800,-73.949, 40.764,-73.949, 40.764,-73.958 trace a roughly rectangular block of Central Park in New York City. The calculator reports an area of about 3.03 km² (≈ 303 hectares, ≈ 750 acres) and a perimeter of about 9.5 km.

Frequently asked questions

How accurate is this compared to a GIS tool like Turf.js?

The area formula is mathematically exact for great-circle edges, using the Earth's mean radius (6,371,008.8 m) as a sphere — the same radius Turf.js uses. Real property boundaries are usually surveyed as straight lines on the ground rather than true great circles, so for very large parcels a full ellipsoidal (WGS-84) geodesic calculation would differ slightly; for typical land-parcel and mapped-region sizes, the difference is negligible.

Does vertex order (clockwise vs counter-clockwise) matter?

No — the calculator takes the absolute value of the enclosed area, so the result is the same either way. Order does matter for connecting the right vertices to each other: list them consecutively around the boundary.

Can I use this for very large regions, like a whole country?

The formula holds for any simple (non-self-intersecting) polygon on the sphere, including large ones, as long as the edges don't cross each other.

Is my data sent anywhere?

No. All calculations run locally in your browser. Your coordinates never leave your device.