How to work out acreage
Find the lot's area in square feet and divide by 43,560, the number of square feet in an acre. A lot 100 feet wide and 200 feet deep is 100 × 200 = 20,000 square feet, and 20,000 ÷ 43,560 = 0.459 acres — a little under half an acre. An acre has no fixed shape: it is any piece of land of 43,560 square feet, which as a square is about 208.7 feet on a side.
Most lots are not rectangles. A lot whose back line runs parallel to the street but is longer or shorter than the front is a trapezoid: add the front and the back, halve it, and multiply by the depth, measured square to the street. A lot 80 feet at the street, 120 at the back and 150 deep is (80 + 120) ÷ 2 × 150 = 15,000 square feet, 0.344 acres. Choose Trapezoid and type the three. A triangle with all three sides known — the way a survey or a plat often gives them — works from the sides alone: choose Triangle (three sides).
A lot with four sides and no right angle is two triangles. Measure one diagonal, corner to corner, and the four sides; each half is a triangle with three known sides. Add both as areas here and the acres are the total. That is more reliable than averaging opposite sides, which is close only on a lot that is nearly rectangular.
Acres for common lot sizes
| Lot | Square feet | Acres |
|---|---|---|
| 50 × 100 ft | 5,000 | 0.115 |
| 60 × 120 ft | 7,200 | 0.165 |
| 75 × 150 ft | 11,250 | 0.258 |
| 100 × 200 ft | 20,000 | 0.459 |
| 150 × 300 ft | 45,000 | 1.033 |
| 208.71 × 208.71 ft | 43,560 | 1.000 |
A quarter acre is 10,890 square feet and a half acre 21,780. One acre is also 4,840 square yards, 4,046.86 square meters, or 0.405 hectares; the square meters are on the page beside the acres.
The boundary, for a fence
The distance round the lot is shown with the area: 600 feet for the 100 by 200 lot. That is the length of fence that would enclose it, before you take out a gate or the side the house stands on. A triangle given by its base and height, or a trapezoid by its two parallel sides and its depth, has no fixed boundary — many shapes share those three numbers — so the page says so rather than print a number. When you need the fence, measure the slanted sides too and use the three-sided triangle, or a rectangle and a triangle for each slanted end.
What a measurement can and cannot tell you
A tape or a wheel along the lot lines gives a good estimate. The legal area of a lot is the one on its survey or deed, measured from monuments that may not be where a fence or a hedge suggests, and on a slope it is the area as if the land were flat, not the surface you walk. If the figure matters for a sale, a tax bill or a permit, use the survey.
Common questions
How many acres is 100 by 200 feet?
0.459 acres: 20,000 square feet ÷ 43,560.
How many feet is an acre?
An acre is an area, not a length: 43,560 square feet. As a square it is about 208.7 feet on each side; as a strip 66 feet wide — a surveyor's chain — it is 660 feet long.
How do I convert square feet to acres?
Divide by 43,560. Choose "Area I already know" and type the square feet, and the page shows the acres, square yards and square meters at once.
Can I add an odd corner to a rectangular lot?
Yes. Add each piece as its own area — the rectangle, then a triangle for the corner — and set any piece the lot does not include to subtract. The acres are the total.