Question 142574
Volume of the slab is {{{Length * Width * Depth}}}
{{{ Volume = 15*15*0.5}}} cubic feet
112.1 cubic feet

Convert to yards using 1 cubic yard = 27 cubic feet
112.5 ft^3 * (1y^3/27ft^3) = 4.166 cubic yards. 

Rounded to nearest, 4 cubic yards. 
Thus the pad will be a little thinner than 6 inches deep.