Assuming the last "8" is also in feet, just multiply 16 x 16 x 8. The answer will be in cubic feet. If the 8 is in inches, divide by 12 first, to convert to feet.
32 cubic ft.
540 feet2
15 * 5 * 0.5 = 37.5 cu ft. or 1.39 cu yards.
192
just about 32 cubic feet of sand
72 cubic feet
32 cubic ft.
4 x 6 x .75 = 18 cu ft of sand
18ft³
540 feet2
It will take 72 sq. ft. of sand. Or 2.66 sq. yds. That will fill the box even.
15 * 5 * 0.5 = 37.5 cu ft. or 1.39 cu yards.
192
just about 32 cubic feet of sand
Volume of sand required = 7 ft * 7 ft * 1/6 ft = 49/6 cubic feet = 8.166... cubic ft.
lots and lots
All you have to do is find the maximum volume of your sandbox. To do this, multiply the length (6 ft.) by the width (6 ft.) by the depth (10 ft.). 6 x 6 x 10 = 360 cubic feet This is if you want to fill the sandbox to the brim. If you want something about half the depth so that the sand isn't falling out everywhere, you just multiply: 6 x 6 x 5 = 180 cubic feet