This can be a tricky question so as a disclaimer always round up so you don't run short with whatever type of material or materials your project calls for. This happens a lot because clients use the basic math formulas for figuring out how many yards they need and don't account for uneven grades or the material packing. 

The math formula is as followed: Just to be upfront, I'm in no way shape or form a school teacher so if this doesn't make sense to you or seems ridiculous give Google a try my friend lol - I'm the owner of a rock company, so bare with me. I try to be as personable as possible and rather not copy and paste from another person. 

Here we go! Length times width gives you the square footage of your area. L x W = SqFt - If you have multiple spots or a weird shaped area just break it down in a few different squares and then add each SqFt together. Remember you are estimating and it does not have to be exact. 

Okay, so you got your square footage by multiplying your length times width and just for example you came up with 1,200 Total SqFt that you need to cover with sand (or rocks).

Next thing you need to do is multiply the SqFt times the depth in inches you need to fill to get the total cubic inches. My area SqFt is 1,200 and I need to put 3 inches of sand. So you would multiply 1,200 times 3 inches. 1200 x 3 = 3,600 cubic inches 

Now we have to divide this number by 324, this is how many cubic inches are in a cubic yard. So, 3,600 divided by 324. This looks like this 3,600/324= 11.11 cubic yards. So basically the math says you need 11.11 cubic yards, I always round this number up by about 10%-20% just to be safe. So lets just say if I came up with 11.11 yards I'll order 12-13 yards of sand just to be safe. 

Hopefully this helps, if not please don't worry. Call us! We are here to help you and completely understand this isn't something you do everyday.