Calculate Irregular Plot Area from Survey Inputs

A surveyor hands you a plat with five sides, two of them at odd angles. You can’t just multiply length by width—that only works for rectangles. If you try, you’ll either overcount the corners you don’t have or undercount the area you do. An irregular plot area calculator takes coordinates, side-and-angle measurements, or survey bearings and returns the actual enclosed area using the right formula for the shape you actually have—shoelace for coordinate polygons, Heron’s for triangles, or a bearing traverse for professional survey data.

The result is a planning-grade number. It tells you whether a listing’s claimed acreage holds up, how much sod or gravel you need for an L-shaped yard, or whether a pie-shaped cul-de-sac lot meets the minimum area your zoning code requires. For anything that goes on a deed or a permit, get a licensed surveyor to confirm.

• Vertices out of order. The shoelace formula needs points listed in sequence around the boundary—clockwise or counter-clockwise, your choice, but never zigzagging. If you paste them in a random order, the algorithm draws crossing lines and returns an area that’s either way too small or negative. Sort them by walking the boundary mentally before you enter anything.
• Self-intersecting polygon. Even one crossing edge turns a pentagon into a figure-eight, and the formula treats the overlapping sliver as negative area. The tool tries to warn you, but the safest fix is to plot the points on graph paper (or a quick sketch app) and confirm no lines cross.
• Mixing feet and meters in the same entry. One side measured from a survey in meters, the rest pulled from a county map in feet. The tool has no way to know you’ve mixed units—it just gives you a confidently wrong number. Convert everything to one unit before you paste.
• Angles that don’t close. In Sides & Angles mode, the interior angles of an n-sided polygon must sum to (n − 2) × 180°. A quadrilateral needs exactly 360°. If your field notes add up to 358°, either one angle was misread or the shape isn’t really four-sided. The calculator flags this, but many people ignore the warning and wonder why the area looks off.

Shoelace (coordinates): Multiplies each point’s x by the next point’s y, sums those products, does the reverse, and halves the difference. It’s the standard polygon-area method every GIS package uses under the hood. Works perfectly on flat (planar) coordinates; for lat/lon over large distances, the tool projects to a local plane first so curvature doesn’t creep in.

Heron’s (triangle, three sides): Computes the semi-perimeter s = (a+b+c)/2, then area = √[s(s−a)(s−b)(s−c)]. No angles needed, just three tape-measure lengths. If any side is longer than the sum of the other two, the triangle is impossible and the formula returns an error—a handy field check for bad measurements.

Bearing traverse: Converts each bearing-and-distance leg into x/y deltas, builds the coordinate list, then runs the shoelace. If the traverse doesn’t close perfectly (it rarely does in the field), an optional Bowditch adjustment spreads the gap proportionally across all legs before computing area.

How many points do I need for a polygon? Three minimum (a triangle). Most residential lots have four to six. Add more points wherever the boundary curves—every 10–20 ft along a road arc keeps the approximation tight.

Can I subtract a pond or driveway from the total? Yes. In Polygon mode, enter the hole’s coordinates in the “Holes” field. The calculator subtracts that interior area automatically.

What if my lot is L-shaped? Enter it as a single polygon with the concave corner included. Or split it into two rectangles, calculate each, and add them. Both approaches give the same answer if the vertices are right.

Are the results accurate enough for a mortgage appraisal? For a preliminary check, yes. For the actual appraisal document, your lender will require a licensed survey.