Square feet measures area and miles measures distance. Therefore, your question cannot be answered.
40*40 = 1600 square feet
The answer depends on the units of measurement used for the numbers in the question: feet, metres, centimetres, miles, etc.
40 square feet = 4.4 square yards.
40 square yards = 360 square feet.
30 x 40 = 1200 square feet.
40 square meters = 430.56 square feet.
40 square meters are 430.556417 square feet.
40 square feet = ~3.72 m2
40' x 40' (1,600 square feet) = 177.77 square yards.
14' x 40' = 560 square feet
40 square feet.
About 430.56 square feet.