What is the unit called an acre?

☙

acre

1. the statute acre
Frequently asked questions
Convert between acres and hectares; find acreage from the lengths of sides; estimate surface area on slopes
Origin of the acre
Abolishing the acre
Sources & resources (separate page)
2. acre as a unit of length (medieval)
3. acre as a tax unit (geld acre, ware acre, medieval)

The statute acre, in the English-speaking world, before 8ᵗʰ – 21^st century, the principal unit of land area. At one time many different acres existed in England (notice the see-also list above), and this acre was often called the “statute acre”, to indicate it was the one established by law, at least as early as the 14ᵗʰ century. In places like the United States, where the statute acre is overwhelmingly predominant, the word “statute” is usually omitted. The (statute) acre is:

A square plot of ground 208.7 feet on a side will cover an acre. An American football field, 360 feet by 160 feet, is about 1.3 acres; 12 high school basketball courts are a little more than 1 acre.

In the United States, because the acre is a land measure it is currently based on the U.S. survey foot and not on the international foot.

The acre is not a measure of surface area on the actual surface of the earth, but on an imaginary, hill-less, standardized ellipsoid. That result comes from using only strictly horizontal dimensions in calculating acreage. See the FAQ3.

Frequently asked questions about the acre

1. What are the dimensions of the acre?

“I just wanted to know what are the dimensions of an acre—how many feet wide and long?”

“If an acre is a square or rectangular why doesn't anybody on the web know what the measurements are in feet, not sq. feet. I have looked in about 1000 different places and no one has an answer that simple.”

“If I am to measure an acre as length x width what would that measure be? I have looked up many sites but cannot find this measurement.”

Answer: Since 1305 the acre hasn't had any fixed length or width, just a fixed area. Unlike the square foot, square yard, square meter, etc., the acre is purely a unit of area, = 43,560 square feet.

The two sides of a 1-acre rectangular lot can be any lengths as long as multiplying one by the other gives 43,560 (if the sides were measured in feet).

If a 1-acre lot is a square, its sides will be 208.7103256 feet long (208.7103256 × 208.7103256 = 43,560).

Imagine a sidewalk 5 feet wide. If it were (43560 ÷ 5 = ) 8712 feet long it would take up an acre, a long skinny acre.

2. What is the perimeter of an acre?

“Can you tell me [if the distance around] one acre of land is more than a mile? I want to purchase property, but I want the distance around the property to be at least 1 mile.”

Answer: Because the acre is a measure of area, it has no definite perimeter. For example, using the examples in question one, the perimeter of the long, skinny sidewalk acre would be 17,434 feet (more than 3 miles), but the perimeter of the square acre would be 835 feet. If you don't know the shape of the lot you can't determine the perimeter.

3. How are acres measured on hillsides?

“If I buy a hill, it being sold as 242 acres, how are the acres measured? Are they as if the hill were cut off and the land was level? Or up one side of the hill and down the other as if it were covered with a blanket and then the size of the blanket was measured?”

Answer: It's as if the hill were cut off and the land was level. To quote from an old authoritative surveying text, by two professors at MIT:

Horizontal Lines. -- In surveying, all measurements of lengths are horizontal or else are subsequently reduced to horizontal distances. As a matter of convenience, measurements are sometimes taken on slopes, but the horizontal projection is afterward computed. The distance between two points as shown on a map then is always this horizontal projection.

Charles B. Breed and George L. Hosmer.
The Principles and Practice of Surveying, Vol 1.
New York: John Wiley & Sons, 1908.

The modern method of observing with the circle level provided with a telescope having a vertical motion enables one to ascertain at once the horizontal angle between points of different elevations. Then we project all vertices upon the surface of an ellipsoid which differs the least possible from the mathematical shape of the earth and bind these points with lines upon this surface. After deducting equally from each angle the computed spherical excess, the computations are carried forward in accordance with the established principles of plane trigonometry.

Converting acres to other units

Download a printable, letter-size chart (as seen at the right) to convert between acres and hectares visually.

The calculator below gives the number of acres in a rectangular field. Or, enter the acreage and a width, and the length will be calculated.

Sometimes when sowing seed or applying fertilizer to hilly ground, it is useful to know the actual surface area, as if the land were flat. The calculator below makes a rough estimate based on the slope of the ground.

Origin of the acre: 4 by 40, sized for ploughing

The acre was originally the amount of land that could be plowed in a single day with oxen, or actually, what could be done by midday, since refueling took all afternoon (the oxen had to be put out to pasture). Similar units of land area are found wherever animals are used for plowing. The German Morgen and Roman jugerum had much the same meaning. The word “acre” is Northern European in origin. For a discussion of whether the word reached England by way of the Norse invasions of France, see Normandy acre.

Like many units of land area, the acre was first thought of as a piece of land having certain fixed dimensions. An acre was 40 perches long and 4 perches wide. The length of the acre, 40 perches, was roughly the distance a team of oxen could plow before needing a breather (this furrow-long became the furlong, 220 yards). Ploughmen prefer long furrows, because turning a team of oxen is a cumbersome process.

A strip 40 perches long and 1 perch wide was called a rood (not to be confused with the rod, a name from the Saxon gyrd used by the 13ᵗʰ century as a synonym for the perch.) So 1 acre was 4 roods. Not until much later (the 16ᵗʰ century, according to R. D. Connor) did most people begin to think of the acre as so many square feet or square rods.

In actual use in the Middle Ages the size of the acre varied greatly, generally being larger in poor land than in good. In some contexts it was almost synonymous with “small holding.”

Another complicating factor is that there were a variety of perches. As you can see, the area of the acre depends on the length of the perch.

The king's rod or perch, however, remained constant for eight centuries at 16½ feet, and that perch set the size of the statute acre.

Abolishing the acre

In 1979, Council Directive 80/181/EEC of the European Community¹, governing standardization on metric units in the European Union, included an exception that permitted Ireland and the United Kingdom to continue using the acre for a limited time. The Council was supposed to set an end date by 31 December 1989. In 1989, the directive was amended to leave the setting of the date to the U.K. and Ireland. Finally in 2007 the exception was allowed to expire², since Ireland had finished converting its land registration system to meters by the end of 1998, and the U.K. sometime afterwards. Beginning 1 January 2010 the acre could no longer be used in the U.K. for any economic, public health, public safety or adminstrative purpose.

In New Zealand, the Weights and Measures Act 1987 provided that "Weights and measures of the metric system shall, except as expressly provided in this Act, be the only weights and measures used for trade in New Zealand," and the hectare is listed in the schedule of units. The hectare is currently (2012) in use, but in real estate advertisements, for example, it is often supplemented by the size in acres.

In Canada, the Weights and Measures Act 1985 permits use of the acre and the hectare, as well as certain old French units for land in Quebec granted under seigneurial tenure.

In England, 11ᵗʰ – 19ᵗʰ century, a unit of length = 4 perches or rods, = 66 feet, the width of the original acre. In medieval documents it usually, if not always, appears as part of a qualifying phrase that indicates that the width is meant. For example:

In the 17ᵗʰ century this distance became the length of the surveyor's chain. This length is also the distance between wickets in cricket, and the width of the strip of land that could be acquired by eminent domain for a road in the less-developed parts of the British Commonwealth.

The acre survived as a 66-foot unit of length into the 19th century in Bedfordshire and Buckinghamshire. In Derbyshire it could be either 84 or 96 feet, while in Yorkshire as a unit of length the acre was 84 feet.¹

In England the acre was also a unit of tax assessment. As such, it was not strictly related to the actual dimensions of the property. The following terms are mostly used by modern scholars:

geld acre

In Anglo-Saxon and Norman England, the acre as a unit of assessment of the king's geld, the crown tax. So for example, a person down in the tax rolls as the holder of 40 geld acres might, but probably didn’t, hold 40 areal acres (6400 square rods) of land. If the tax were, say, 2 shillings per acre, he owed 80 shillings, regardless of the actual size of his property.

ware acre

In East Anglia, 1/120th of a carucate (also in this case not an areal unit), a unit of assessment of taxes due the king that were to be paid in something other than money (for example, in fodder for horses).