An acre is a unit of land area equal to 43,560 square feet. It is commonly visualized as about 90% of a standard American football field (which is 48,000 square feet without end zones). An acre can take any shape, but the traditional measurement is one chain by one furlong, or precisely 66 feet by 660 feet. This comes out to be 10 square chains or 1/640 of a square mile. In other units, an acre is also equivalent to 4,840 square yards or approximately 4,047 square meters (0.405 hectares). It is a widely used unit for measuring land primarily in the United States and the UK, albeit with some historical variations in different regions such as Irish and Scottish acres, which are larger than the standard acre. To give more context, an acre can fit around 16 tennis courts or nine NBA basketball courts in terms of area.