Square feet to acres (sq ft to acre)

One square foot is exactly 0.0000229568 acres. That ratio belongs to the customary system itself and is much older than any link to metric: the 1959 international yard and pound agreement later fixed the yard and the pound against the meter and the kilogram, but it left the ratios between customary units (feet to the yard, ounces to the pound, cups to the gallon) exactly as they were.

Worked through: 7,500 sq ft x 0.0000229568 = 0.172176 acre.

Going the other way, one acre is 43,560 square feet.

For mental arithmetic: Divide by 43,560, or divide by 40,000 and take off 9% for a quick estimate.

An acre is exactly 43,560 square feet, and that number is not arbitrary: it is one chain by one furlong, 66 feet by 660, which is the area a yoke of oxen was reckoned to plow in a day. The reason it matters now is land listings: a lot quoted at 10,000 square feet is 0.23 acres, and the two units are used by different parties to the same transaction. A useful anchor is that an American football field including both end zones is about 1.32 acres.

Precision is worth matching to the input. A measurement given as 1,000 sq ft carries one or two significant figures, and reporting its conversion to six implies a precision the original never had.