To find the volume of sand needed to fill the sandbox, we need to calculate the volume of the sandbox. The sandbox is in the shape of a rectangular prism with a length of 5 feet, a width (or depth) unknown, and height 1 foot. Since the sandbox is square, we can assume that the width is also 5 feet. Therefore, the volume of the sandbox is:

Volume = length x width x height
Volume = 5 feet x 5 feet x 1 foot
Volume = 25 cubic feet

Since each bag of sand holds 1 cubic foot of sand, we need to divide the volume of the sandbox by the volume of each bag to find the number of bags needed:

Number of bags = Volume of sandbox / Volume of each bag
Number of bags = 25 cubic feet / 1 cubic foot/bag
Number of bags = 25 bags
Therefore, we need 25 bags of sand to fill the square sandbox measuring 5 feet long and 1 foot high.