It depends on whether you refer to 40 inches, 40 miles, 40 metres or whatever. A different answer in each case.
40*40 = 1600 square feet
40 foot = 40 feet
Feet and feet are the same measurement. Therefore, 40 feet is equal to 40 feet.
There are 1,219.2 centimeters in 40 feet.
40 yards = 120 feet
40' x 40' (1,600 square feet) = 177.77 square yards.
30 x 40 = 1200 square feet.
40 inches is 3.33 feet.
40 km = 131,234 feet.
40 yards (US) is 120 feet.
40*42=1680 sq. ft.
14' x 40' = 560 square feet