Find the area of a lot in acres, square feet and hectares from its length and width, from four sides plus a diagonal for irregular lots, or from survey corner coordinates.
One acre is exactly 43,560 square feet (the area of a 66 × 660 ft strip, one chain by one furlong). A square acre is about 208.7 ft on each side. One hectare is 10,000 m², or about 2.471 acres, and one square foot is exactly 0.09290304 m². A typical suburban lot of 100 × 150 ft is 15,000 sq ft, which is 0.3444 acres or 0.1394 hectares.
To convert by hand: divide square feet by 43,560 for acres, and multiply acres by 0.404686 for hectares.
Four side lengths alone do not fix the area of a four-sided lot. The same four sides can be pushed into a thinner or wider shape, like a hinged frame, and the area changes. One diagonal locks the shape. It splits the lot into two triangles, and each triangle's area comes from Heron's formula: with sides a, b, c and s = (a + b + c) ÷ 2, area = √(s(s − a)(s − b)(s − c)).
Example: sides of 120, 210, 130 and 190 ft with a 240 ft diagonal give triangles of about 12,598 and 12,296 sq ft, a total of 24,894 sq ft (0.5715 acres). If the calculator says the measurements can't form a lot, two sides of a triangle add up to less than the diagonal, which usually means one measurement is wrong.
Do not average opposite sides (the "average length × average width" shortcut). It overstates the area of any lot whose corners aren't square.
If you have corner coordinates, from a survey plat, a site plan grid or a mapping tool, enter them in order around the boundary. The calculator uses the shoelace (surveyor's) formula: area = ½ |Σ(xᵢ·yᵢ₊₁ − xᵢ₊₁·yᵢ)|. It works for any simple polygon with any number of corners, as long as the boundary lines don't cross. The default five-corner example (0,0), (150,0), (170,120), (40,140), (−10,70) encloses 20,600 sq ft, or 0.4729 acres.
Coordinates in U.S. state plane systems are now published in international feet (the U.S. survey foot was retired at the end of 2022), and the difference is only 2 parts per million, which is negligible for lot area.
This calculator gives a planning estimate from the measurements you enter.
Tape measurements along fences, curved frontages, and lots on slopes can all differ from the recorded boundary, which is measured horizontally by a licensed surveyor.
For a purchase, a zoning variance, a subdivision or a fence on the property line, rely on the deed, the recorded plat or a new survey, and check your county assessor's record, which may list a different acreage for tax purposes.
Common questions about the Lot Size Calculator (Acreage)
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