For most organic mulch the answer is 2 to 4 inches, and not more than 4. The right point in that range depends on how coarse the mulch is and what it is going around. Everything below comes from university extension services, named for each figure.
Depth by situation
| Situation | Depth | Who says so |
|---|---|---|
| Around trees and shrubs | 2 to 4 in | Nebraska, Illinois, Penn State |
| Fine wood or bark mulch, composted mulch | 2 to 3 in | Florida, Clemson, Illinois |
| Coarse mulch such as pine bark nuggets | 3 to 4 in | Illinois, Clemson |
| Gravel or small stone | 1 to 2 in | Nebraska, Clemson |
| Around annual and perennial flowers, and vegetables | 1 to 2 in | Nebraska only |
| Finely textured or double-shredded mulch | 1 to 2 in | Penn State only |
The last two rows rest on one source each. The mulch calculator lists only the rows that two or more sources support.
Why not deeper
Penn State puts it plainly: apply a 2 to 4 inch layer of organic mulch “and no more”, and use less where the soil drains poorly. It warns that more than 4 inches may harm a tree’s root system. Clemson gives the reason for the lower limit on fine mulch. Many wood and bark mulches are made of fine particles and should not be more than 2 to 3 inches deep, while coarse nuggets can go to 4.
Topping up
Florida’s guidance is to keep mulch at 2 to 3 inches and replenish it as needed to hold that depth. So measure what is already on the bed. If an inch is left and you want three, order for two.
What depth does to the quantity
Clemson gives the sum: one cubic yard covers 324 square feet with an inch of mulch. Each extra inch needs the same amount again.
| Depth | One cubic yard covers | One 2 cu ft bag covers |
|---|---|---|
| 1 in | 324 sq ft | 24 sq ft |
| 2 in | 162 sq ft | 12 sq ft |
| 3 in | 108 sq ft | 8 sq ft |
| 4 in | 81 sq ft | 6 sq ft |
Going from 2 inches to 3 is half as much mulch again. Decide the depth first, then use the calculator for the beds you have.
What the sources do not say
None of these publications gives a figure for how much mulch settles after it is spread, or for how much extra to buy. The calculator therefore adds nothing unless you enter a percentage yourself.