How Online Area Calculators Actually Work
Most casual users think of area calculation as a single problem with one answer. It isn't. Two calculators given the same polygon can return different acreages — and both can be "right" depending on which kind of area they're computing.
The flat-plane assumption
The simplest area formula treats the Earth as flat. You take the latitude/longitude pairs of each polygon corner, pretend they're (x, y) coordinates on a piece of paper, and apply the shoelace formula — a high-school geometry trick that turns a list of vertices into an area number.
For small areas — backyards, parking lots, single buildings — this works fine. The Earth's curvature over a 100-meter polygon is too small to measure. But the assumption breaks down as polygons get larger or move farther from the equator.
Why latitude and longitude aren't equal
One degree of latitude is roughly 111 kilometers, anywhere on Earth. One degree of longitude, however, varies wildly. At the equator, it's also about 111 km. At 45° north or south, it's 78 km. At 60°, it's only 55 km. At the poles, longitude collapses to a single point.
A naïve area calculator that treats lat/lng as flat coordinates ignores this entirely. A square polygon drawn over Anchorage, Alaska might compute as several times its real size — because the longitude lines, which look straight on the map, are actually converging.
Spherical geometry, briefly
The fix is to compute area on a sphere. The formal name is "spherical excess" — there's a geometric trick where the area of a polygon on a sphere is proportional to the sum of its interior angles minus the sum a flat polygon would have. Google's geometry library implements a refined version of this called Girard's theorem, and it works correctly for polygons of any size, anywhere on Earth.
You don't need to understand the math to use it. The takeaway is: a calculator using spherical geometry will give the same area for a 1-acre lot in Florida and a 1-acre lot in Alaska, and the same area for a small lot and for a country-sized polygon. A flat-plane calculator won't.
How this calculator does it
Satellite Area Calculator uses Google Maps' built-in spherical geometry library — the same code that powers area calculation in Google Earth Pro and many commercial GIS tools. For polygons it calls computeArea, which applies the spherical excess formula. For circles, it uses the cap-of-sphere formula (area on a sphere bounded by a great-circle radius), not the flat πr² you'd use for a paper circle.
For most users, the difference between flat πr² and spherical cap area is tiny — even at a 10-mile radius the spherical correction is a fraction of a percent. We use the spherical version anyway, so a polygon and a circle of the same nominal area always agree.
Distance is the same problem
Distance between two points has the same flat-vs-spherical issue. The straight-line formula from grade school (√((x₂-x₁)² + (y₂-y₁)²)) is wrong on a sphere over any meaningful distance. The correct formula — the haversine — accounts for curvature.
The shortest path between two points on a sphere is a "great circle." That's why a flight from New York to Tokyo arcs over the Arctic instead of going due west: the great-circle route is much shorter than the constant-bearing line that looks straight on a Mercator map.
The pinning-it-down step: imagery accuracy
Spherical geometry handles math perfectly. The remaining error in any satellite measurement comes from where you click, which depends on the imagery itself: pixel resolution, orthorectification, and image age. We discuss those limits in the imagery accuracy guide.
Why some other calculators give different answers
You might compare measurements from different tools and get slightly different numbers. The common reasons:
- Flat shoelace vs spherical excess. The biggest source of disagreement, especially for large or non-equatorial polygons.
- Different unit roundings. 0.25 acre vs 0.247 acre is the same area; one is just rounded harder.
- Different polygon paths. If you click the corners slightly differently, you get a slightly different area. This isn't a bug; it's the resolution of the input.
- Different ellipsoid models. Earth isn't a perfect sphere. Some tools use a more sophisticated ellipsoidal model (WGS84) that's marginally more accurate over very large areas. The difference is microscopic for everyday use.
Bottom line
For lots, fields, roofs, and most everyday measurement: any reasonable calculator that uses spherical (not flat) geometry will give you the right answer to within a fraction of a percent. The remaining error is in the imagery and your clicks, not the math.