An acre is a unit of area used in the United States, the United Kingdom, and other imperial-system countries. Most commonly used for land, from residential lots to farms and parks. Metric countries measure land in hectares instead; the metric conversion chart covers area units alongside length, volume and weight.
Where the 43,560 Comes From
The number traces back to the chain, an old surveying unit equal to 66 feet. An acre was defined as 10 square chains, and one chain squared equals 4,356 square feet. Ten of those gives 43,560. Historical artifact, fixed in law since. Measuring still mixes units today — if a tape measurement comes back in inches, convert inches into feet and inches before you square anything.
The Formula
Acres = Square Feet ÷ 43,560
Square Feet = Acres × 43,560
Examples: 2,500 sq ft ÷ 43,560 = 0.0574 acres. 0.25 acres × 43,560 = 10,890 sq ft.
One quirk worth knowing: area scales with the square of length, so doubling a plot's sides quadruples its acreage. That squared relationship is exactly what our scale factor tool works out for plans, maps and models.