When you order topsoil it generally comes to you by the cubic yard. How many yards do you need? Here is a simple formula to make your order easy on the mind, if not on the back!
To figure out how many square feet one yard of soil will cover simply divide 36 by how many inches of coverage you want and multiply by 9. If you are looking to add a foot of soil divide 36 by 12 inches which gives you 3 and times that by 9 for a sum of 27. This means one yard of soil will cover 27 square feet (just over a five foot by five foot patch) in one foot of soil.
For those of us who think in metric the formula would be: for one cubic metre of soil divide 100 by how thick you want your soil topping in centimetres. The answer will be for square metres of garden space.
Hope that helps!