Question
i can not remember for the life of me how to figure out how may sq feet are in a cubic yard when you are using 4 in depth.....i think its 81 but i am not sure.....or how to determine how much i would caculate a 10x10 patio at 4 inch depth for cubic yards

Answer
The math is pretty straightforward:

1 foot = 12 inches, therefore 4 inches = 1/3 of a foot, or .33

So, a 10'x10'x.33' patio gives us 33 cubic FEET.
A yard is 3 feet long; a cubic yard is 3'x3'x3', or 27 cubic feet.  

For your patio, 33 cubic feet divided by 27 = 1.22 cubic yards of whatever you're doing to the deck (concrete, decomposed granite, etc.)

Just to check my math, or if you want to see how much 3" or 5" deep would be, go to http://www.concrete.com/calculatorsmaterials.htm#volume

I don't know why the inch/feet calculation on this web site would be slightly different, but as with any bulk item, the concrete truck hopper is not going to be the best measuring device!  

Adding a little extra is always a good idea (the rule-of-thumb is add 10% to the total, in case the forms are deeper, its a dry day, or your just plain messy it pouring)

The joys of math!