If you post the dimensions and how thick you want the mud I can quickly tell you the exact yardage.  Then you need to add in some extra for margin, like said above.

Examples:   L x W x H = Volume then you divide by 27 (cubic feet in a cubic yard (3x3x3)) =  Cubic Yards.             Depth of concrete is usually in inches so that must be converted to feet.  like 5" would be 5/12th's  which is .4167  

       10 ft x  10 ft x 1 ft (thickness) = 100cu.ft./ (27cu.ft/ft) =  3.7 yds.  + 10%  would be approx. 4 yards of mud

      10 ft x 10 ft x .4167 (5") =  41.67/27 =  1.543 yards round to 1.6 then add on another 10% therefore about 1.8 therefore you'd might as well go 2 yards and poor it just a little thicker.    

Hope this helps.     Goodluck.            The larger the pour and the farther from the batch plant you should allow a bigger margin for variance, this is also assuming your pad is really smooth and consistent.