How deep should mulch be?
Use the right mulch depth for fresh beds, old beds, trees, and shrubs without smothering plants.
Mulch depth changes the estimate fast. One extra inch over a large bed can mean a lot more bags.
For refreshing an existing bed
About 1 to 2 inches is often enough if there is already mulch in place. Rake the old mulch first so you can see what is actually needed.
For a new bed
About 2 to 3 inches is a common planning range. That is usually enough to help with moisture and weeds without making a heavy mat.
Around trees
Keep mulch away from the trunk. The volcano-shaped pile around a tree looks common, but it traps moisture against the bark and can cause problems.
Near siding and stems
Leave a little breathing room around siding, plant stems, and foundations. Mulch should cover soil, not bury things that need air.
Bagged or bulk?
Bags are easy for small jobs and cleanups. Bulk mulch can be cheaper for bigger beds, but you need somewhere to drop it and a way to move it.
Most bagged mulch is sold in cubic feet. The calculator lets you change bag size because 1.5 cu ft and 2 cu ft bags are both common.