An acre is a unit of land area commonly used in the United States and the United Kingdom. It is defined as exactly 43,560 square feet, which is about 4,047 square meters or roughly 0.405 hectares

. Traditionally, an acre is the area of land one chain (66 feet) by one furlong (660 feet), which equals 10 square chains or 1/640 of a square mile

. If shaped as a square, an acre measures about 208.7 feet (63.6 meters) on each side

. To help visualize the size, an acre is roughly 90% of an American football field, which is about 48,000 square feet. You could also fit about 16 tennis courts or 9 basketball courts within an acre

. In summary:

• 1 acre = 43,560 square feet = 4,047 square meters = 0.405 hectares
• Dimensions (rectangle): 66 feet by 660 feet (1 chain by 1 furlong)
• Dimensions (square): approximately 208.7 feet per side
• Roughly 90% the size of a football field
• Can fit 16 tennis courts or 9 basketball courts

This unit originated from the amount of land that could be plowed by a yoke of oxen in a day in medieval times

. Thus, an acre is a moderately large land area commonly used for measuring farms, residential lots, and other land parcels