Acreage calculator
A field measured 1,320 ft by 1,320 ft is 40 acres — 1,742,400 square feet, or 16.19 hectares. An acre is exactly 43,560 square feet, so any rectangular block is its two sides multiplied and divided by 43,560; the calculator above does that for five field shapes, prints the same plot in every unit at once, and saves it as a named field the rest of the site can use.
Your answer
Area of the field20 ac
Plot sheet
What this means for the field in front of you:
- This plot in every unit
- 80,937 m² · 809.4 a · 8.094 ha · 871,200 ft² · 20 ac
- As the register writes it (ha a ca)
- 8 ha 9 a 37 ca
- Distance around the plot
- 1,207 m
- Measured by
- measured sides
Three fields are kept; saving a fourth replaces the oldest.
How the answer is worked out
Acreage is a rectangle problem only when the field is a rectangle, and most are not. This calculator takes the shape you actually measured and turns it into an area, then prints that area in every unit a farm, a county office or a seed order might want.
Rectangle. Length × width in the unit you measured, converted to square metres, then to acres at 4,046.8564224 m² per acre.
Four-sided field. Four sides and one diagonal. The diagonal cuts the plot into two triangles and each triangle is closed with Heron’s formula, so the answer is fixed instead of hinged. If the sides and diagonal cannot actually form a plot — one length longer than the other two of its triangle together — the calculator says so rather than printing a number.
Triangle. Three measured sides, Heron’s formula, same guard.
Trapezoid. The two parallel sides averaged and multiplied by the perpendicular distance between them — the shape of a field cut by a road or a creek running at an angle.
Circle. A centre-pivot is described by its diameter, so that is what the form asks for: area is π × (diameter ÷ 2)².
Nothing is rounded on the way through. The area is carried in square metres at full precision and converted once, at the end, for display — so the acres, the square feet and the hectares on the sheet are the same number, not three separately rounded ones.
The plot that comes out of this page is the site’s unit of work. Save it under a name — “North 40”, “Home place” — and the seeding, fertiliser, sprayer and ration pages will offer it back, so a field is measured once and then used, instead of being typed into every calculator in turn.
A worked example
Every figure below is computed by the calculator above from this case — nothing here is typed in by hand:
- Shape of the field
- Rectangle
- Sides measured in
- ft
- Length
- 1320
- Width
- 1320
Area of the field:40 ac
- This plot in every unit
- 161,874 m² · 1,619 a · 16.19 ha · 1,742,400 ft² · 40 ac
- As the register writes it (ha a ca)
- 16 ha 18 a 74 ca
Worked by Agrimetra with the same engine the calculator above runs.
What this means in the United States
Most US farmland sits on the Public Land Survey grid, so acreage comes in familiar blocks: a section is 640 acres, a quarter 160, a 'forty' 40. An acre is exactly 43,560 square feet, which is why a 1,320 ft × 1,320 ft block is 40 acres on the nose.
Questions growers actually ask
- How many square feet are in an acre?
- Exactly 43,560. The acre is defined as 10 square chains, and a chain is 66 feet, so 66 × 660 = 43,560 square feet. In metric terms the international acre is exactly 4,046.8564224 square metres.
- Why does the four-sided shape ask for a diagonal?
- Because four side lengths alone do not fix an area — the same four sides hinge into shapes of very different size. One diagonal splits the plot into two triangles whose areas are fixed, which is the honest way to measure a field that is not square. A tool that answers from four sides alone is guessing.
- Are my acres survey feet or international feet?
- This calculator uses the international foot, which is what the US has used for length since the survey foot was retired at the end of 2022. The two feet differ by two parts per million, so an area differs by four: on a 40-acre block that is about seven square feet, well inside the error of a wheel or a tape.
- Can I measure a centre-pivot circle with it?
- Yes — pick the circle shape and give the diameter, which is the number a pivot is described by. A 1,320 ft diameter pivot covers about 31.4 acres inside a 40-acre square, which is why pivot corners are counted separately.
Last updated:
Sources
- NIST — the US survey foot is deprecated as of 1 January 2023 (checked September 22, 2026)
- Bureau of Land Management — the Public Land Survey System (checked September 22, 2026)