A rectangular lot 100 by 120 feet is 12,000 square feet — 0.275 acre, or 1,115 square meters. Most lots are not that tidy: a pie-shaped lot on a cul-de-sac or a corner lot with a clipped corner needs every side. The calculator above starts from a shape, lets you drag the corners on the plan or type each side length from the survey, and gives the area in acres, square feet, square meters and hectares.
How the calculator works
Pick a shape to start from — rectangle, square, triangle, trapezoid, quadrilateral, pentagon, hexagon, L-shape — and enter its dimensions. The plan shows the lot; drag any corner to move it, click + on a side to add a corner, remove corners in the side list, or type a side's length and its end corner moves along it. A circle is exact and can be turned into an editable 24-sided contour.
The shoelace formula
Any lot with straight boundaries is a polygon, and its area follows from the coordinates of its corners: half the absolute value of Σ (xᵢ yᵢ₊₁ − xᵢ₊₁ yᵢ). The formula is exact; the only error is in the measurements. If the boundary crosses itself the calculator warns you — the formula then gives the difference of the two parts.
Units
| Lot | Square feet | Acres | m² |
|---|---|---|---|
| 50 × 100 ft (city lot) | 5,000 | 0.115 | 465 |
| Quarter acre (≈ 99 × 110 ft) | 10,890 | 0.25 | 1,012 |
| 100 × 120 ft | 12,000 | 0.275 | 1,115 |
| One acre (≈ 208.7 ft square) | 43,560 | 1 | 4,047 |
Triangles and trapezoids
A triangle by its three sides uses Heron's formula; a trapezoid by its two parallel sides and the distance between them is (a + b) ÷ 2 × h; a quadrilateral needs a diagonal as well as its four sides, because four sides alone don't fix its shape.
Common mistakes
- Multiplying the longest sides. Only works for a true rectangle.
- Four sides of a quadrilateral without a diagonal. The same sides can enclose very different areas.
- Trusting a sketch over the plat. The recorded survey governs the legal lot size.
For acres, hectares and square feet conversions, use the Acreage Calculator; for a fence around the lot, the Fence Calculator.