Which one do I need?
If you are buying material by volume — concrete, mulch, gravel, topsoil — start with the calculator named after the material; each one converts to the unit the supplier actually sells in. If you are counting pieces, use the calculator for the piece: pavers, tiles, sheets, pickets, rafters.
Every calculator prints a materials list and includes a waste allowance, because the yard will not deliver the second half of a pour on the same afternoon.