What is an acre equal to?

Answer 1

Oddly enough, an acre was not defined as a square, but as a rectangle -- for example a long, thin parcel of farmland that would be plowed in long furrows.

It is 1/640th of a square mile, or 4840 square yards, or 43560 square feet.

The simplest designation uses the obsolete units chain (66 feet) and furlong (660 feet or 10 chains or 1/8 mile). Thus an acre is 1 chain by 1 furlong, or 10 square chains, and may have been established as about a day's plowing for an ox.

The 1/640th of a mile can be seen in that there are 80 chains to a mile and 8 furlongs to a mile, a unit still seen in various horse race distances today.

The SI unit comparable to the acre is a hectare (10000 m2), which is about 2.47 acres.