Concrete is batched in ¼ yard increments and it is a good idea to order at least 10% more than it calculates to avoid costly short load charges for a cleanup.
It impossible for a person to excavate and prepare for a pour of any size and have all the measurements exactly right. This is especially true for depth as grade is the most likely thing to be off. Just an average of ¼ to ½ inch variance in depth can cause a big change to the number of cubic yards you would need.
We would be pleased to go over your calculations when you call to order concrete.