Most jobs need more than one material. A patio needs concrete or pavers, a gravel base, edging and tools; a fence needs posts, rails, pickets, concrete and hardware. These calculators take the measurements once and work out every line of the shopping list, using the same formulas as our single-material calculators.
Each result shows the quantity to buy in the units the store sells, with the assumption behind it, so that you can change any number and see what happens. They are planning estimates: check the structural and code questions for your project with your building department.