for a circle i would guess it would be

3.14 X Radius squared X .25/27= Cubic Yards

so a 10 foot diameter circle would have a radius of 5 feet from center to edge so

3.14 X 5' X 5'=78.5 sq ft

78.5 X .25= 19.625 cubic ft

19.625/ 27= 0.7268 cubic yards

so 3/4 of a yard or just get a yard...or you could just eyeball it

I think thats all right