Mulch Calculator

2-3 inches. That is enough to hold moisture in and keep weeds down, without burying roots or piling against trunks. Deeper is not better here: mulch heaped against a tree trunk holds damp bark against the tree and rots it.

13.5 bags, at the standard 2 cubic foot bag size, because a cubic yard is 27 cubic feet. In practice you buy 14. The calculator above works it out from your bed size so you do not have to.

Bulk, by a good margin, once you are past about 3-4 cubic yards. Below that, delivery minimums eat the saving and bags are easier to move around a garden anyway. Bags also win for small or awkwardly shaped beds, where a dumped pile is more work than it saves.

Usually not. A couple of inches of old, broken-down mulch is fine to top up over. Once the total gets past 4-5 inches, rake some out first, because a thick mat of old mulch sheds water instead of letting it through to the roots.