An acre is equal to 43,560 square feet, so 100 acres equals 4,356,000 square feet. An acre is 1/640 of a square mile, so 100 acres also equals about 0.16 square miles. Because 1 acre equals about 4046.86 square meters, 100 acres equals 404686 square meters in the metric system.
Acre has been a unit of measurement for area since the Middle Ages. Then, an acre was defined as one day's plowing for an oxen team, which could vary greatly based on terrain and other factors. The measurement was later standardized, but for a while there were many variations on the size of an acre in different regions.