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. 

Just as everyone else has mentioned to order an extra amount, but you might want to ask the concrete plant what their "trip charge" is, too = this will help you make that decision to NOT short yourself on cement/concrete mix.

If the "trip charge minimum" is $150, and 1 yard of concrete is $110; you might as well get an extra yard, right from the beginning, BECAUSE if you are short, that last 1 yard could really cost you (trip charge plus concrete cost).

You can either have a place ready, for a little extra = like a 3x3 foot area, boxed out in front of a shed walk-thru door, or just have the last extra 1/2 yard poured into skidsteer bucket and when done and you are sure you don't need anymore concrete mix, then dump it on gravel road or driveway, and backdrag it, until all the small pieces are distributed evenly to dry.