144 square inches (sq. in.) = 1 square foot (sq. ft.)
9 square feet = 1 square yard (sq. yd.)
30-1/4 square yards = 1 square rod (sq. rd.)
160 square rods = 1 acre (A.)
640 acres = 1 square mile (sq. mi.)
Sq. ′ and sq. ′′ are frequently used for square foot and square inch.
Thus, 15 sq.′ 6 sq.′′ means 15 square feet 6 square inches.
A square is 100 square feet. It is used in measuring roofing.