How much mulch do I need?
Enter the bed size and the depth you want. The tool converts to cubic yards and to bags, and flags the depth if it goes past what is good for the plants.
How deep, and why it matters
Mulch is not a decoration you can lay on thick. Penn State Extension's number for organic mulch around landscape trees is 2 to 4 inches and no more, less where the soil drains poorly, and only 1 to 2 inches for finely textured material, which packs down and keeps oxygen from the roots.
The other rule is horizontal, not vertical: keep it off the trunk and leave the root flare visible. Mulch heaped against bark traps moisture on it and gives carpenter ants, termites and voles somewhere to work unseen. The bed mulching guide covers timing, drip-line spread and fixing volcano mulch without buying more bags.
- Organic mulch — 2–4 inches (1–2 for fine textures)
- Root flare — keep mulch off the trunk; bark stays dry
- Volcano mulch — piled against bark traps moisture and invites pests
The formula
cubic feet = area in sq ft × (depth in inches ÷ 12)
cubic yards = cubic feet ÷ 27
Example. A 20 ft × 6 ft bed is 120 sq ft. At 3 inches that is 120 × 0.25 = 30 cubic feet, or 1.1 cubic yards — 15 bags at 2 cubic feet each.
Bags or bulk
A cubic yard is 27 cubic feet: 13.5 of the 2-cubic-foot bags, or 9 of the 3-cubic-foot ones. Past roughly a yard, bulk delivery usually wins on price and on the number of trips; under that, bags are easier to handle and store.
Questions people actually ask
How deep should mulch be?
Penn State Extension says 2 to 4 inches of organic mulch and no more, less if the soil drains poorly. Finely textured mulches go 1 to 2 inches, because deeper layers of fine material restrict oxygen getting to the roots.
How many bags of mulch are in a cubic yard?
A cubic yard is 27 cubic feet, so it is 13.5 bags at the common 2 cubic foot size, or 9 bags at 3 cubic feet. Bulk delivery is usually cheaper past about a yard, bagged is easier for small beds.
Can I pile mulch against the trunk?
No. Extension guidance is to keep mulch off the trunk entirely and leave the root flare visible — roughly 3 to 5 inches of clearance on young trees and 8 to 10 inches on mature ones. Mulch packed against bark holds moisture against it and gives insects and voles cover to chew through it.
Do I remove the old mulch first?
Measure what is already there. If the existing layer is still 2 inches deep, you are topping up an inch, not laying 3. Layers that keep getting stacked year over year are how beds end up smothered.
Sources
- Penn State Extension — Mulching Landscape Trees: 2–4 inches of organic mulch, 1–2 inches for fine textures, and keeping mulch off the trunk with the root flare showing.
- University of Maryland Extension — Excess Mulch Problems: what goes wrong when mulch is piled too deep or against the trunk.