How this instrument works
An acre is defined as exactly 43,560 square feet -- not a rounded figure but the product of two older survey units: a chain, 66 feet, run alongside a furlong, 660 feet, gives 43,560 square feet in a single strip, and that strip is where the acre's modern definition comes from. The chain and furlong were themselves working tools of English land surveying, sized so a furlong was roughly the distance a team could plow before resting. That history is why 43,560, an otherwise odd-looking number, is exact rather than approximate: 4,840 square yards, or equivalently 43,560 square feet, is simply how the acre has been fixed since long before any metric definition existed.
This calculator is for when you have two measured sides rather than an already-known acreage figure. A buyer standing on a lot with a frontage and a depth measurement from a plat map or survey stakes, a farmer laying out a rectangular field, or anyone fencing a yard from a tape-measured length and width can get straight to acres without first multiplying by hand and then looking up a square-feet-to-acres factor separately. That two-step arithmetic is easy to get wrong on a phone calculator, particularly with decimal feet, so this page does both steps -- the multiplication and the division by 43,560 -- in one place from the two numbers you actually have.
- Enter the lot's Length (ft); it opens at 200.
- Enter the lot's Width (ft); it opens at 100.
- Read the result from Area (acres) beneath both fields -- it recalculates the instant either Length or Width changes.
- The page opens already showing a 200 ft x 100 ft rectangle at 0.459137 acres, so you can see the arithmetic before entering your own measurements.
- Measure both sides in the same unit, feet, before entering them -- mixing feet and another unit will silently produce a wrong acreage.
Worked example -- a 200 ft x 100 ft lot
Enter 200 into Length (ft) and 100 into Width (ft), both field defaults. Area (acres) reads 0.459137 -- the rectangle covers 20,000 square feet, and 20,000 divided by 43,560 comes to just under half an acre, a typical suburban single-family lot size in much of the US.
Change Length to 660 and Width to 66 -- a surveyor's furlong run alongside a chain -- and Area (acres) reads exactly 1.000000. That is not a coincidence: it is the historical strip of land the acre's 43,560-square-foot definition was originally built from, so this particular rectangle was always going to land on exactly one acre.
Questions
How many square feet are in an acre?
Exactly 43,560 square feet. That figure equals 4,840 square yards and traces back to a historical surveying strip one chain (66 feet) wide by one furlong (660 feet) long -- 66 x 660 = 43,560 exactly, which is why the number looks odd but is not a rounded approximation.
Why does this calculator ask for length and width instead of just acres?
Because it is built for the moment you have two measured sides -- a frontage and a depth from a plat map, or a tape-measured yard -- rather than an already-known total acreage. Multiplying the two and dividing by 43,560 in one step avoids the manual arithmetic error that creeps in when doing that calculation by hand, especially with decimal feet.
Where does the 660 ft x 66 ft 'furlong x chain' figure come from?
It is the traditional English surveying strip the acre's 43,560-square-foot definition derives from: a chain (66 feet, a standard surveyor's measuring chain) run alongside a furlong (660 feet, roughly the distance a plow team would work before resting). Entering 660 and 66 into this calculator returns exactly 1.000000 acre for that reason.
What does a larger rectangular parcel look like in acres?
A 1,000 ft by 500 ft parcel covers 500,000 square feet, which comes to about 11.478421 acres -- roughly the size of a small agricultural field or a large commercial site, and large enough that the difference between measuring in feet versus another unit becomes easy to notice if a mistake is made.
Does this use the US survey acre or the international acre?
It uses the 43,560-square-foot definition common to both. The US survey acre and the international acre differ from each other by only about two parts per million -- a gap from the US survey foot versus the international foot -- which is far smaller than the precision of any real-world tape measurement, so it makes no practical difference for a rectangular lot, field, or yard calculation like this one.
Can I use this for a non-rectangular lot?
Not directly -- the formula here is strictly length times width, which only describes a rectangle (or, close enough, a lot with roughly square corners). An irregular or angled lot needs its area worked out shape by shape, or taken from a surveyor's plat, before converting that square-footage total to acres.