I've never gotten a satisfactory answer to this, but no one I've talked to in the biz can tell me if "a yard" is a square yard (3'x3'x1') or a cubic yard (3'x3'x3')...or something else again! Makes a big difference. I also came up with 18 cubic feet to fill a space of those dimensions.

Usually a 'yard' refers to a cubic yard. Like a 10 yard dump truck can carry 10 cubic yards. A cubic yard is a volume measurement. A square yard has no relevance to volume ....it only describes surface area. Square yard is simply length times width. (3x3)

The space described is 18 cubic feet or .667 cubic yards.(there are 27 cubic feet in one cubic yard)
If buying bags, the bag label will state the volume in cubic yards or cubic feet. Ignore the bag weight and use the volume measurement given on the bag.
Jan

A yard is a cubic yard. I can tell you (from my own topsoil purchase this spring) that THREE yards makes a heck of a pile on a big tarp in the yard :-)

Yep, I think it's supposed to be also. But get your yardstick out and measure it; you won't find it works out to that in reality. At least I've never seen it do so in my experience. Then listen to 'em yammer on about 'settling'--LOL!