Mulch calculator
Mulch is sold by the cubic yard in bulk and by the 2 cubic foot bag at retail, and those are not the same price per volume. The other thing that matters is depth: 3 inches suppresses weeds, 6 inches kills the plants it is supposed to protect.
Bed area
How this is calculated
- Area = length × width.
- Volume in cubic feet = area × (depth ÷ 12).
- With settlement = volume × (1 + allowance).
- Bags = ⌈cubic feet ÷ bag size⌉. Cubic yards = cubic feet ÷ 27.
One cubic yard covers 324 sq ft at 1 inch deep, 108 sq ft at 3 inches, and 54 sq ft at 6 inches. That single relationship answers most mulch questions without a calculator.
Worked example
A 40 × 4 ft bed at 3 inches, 10% settlement, 2 cu ft bags:
- Area = 160 sq ft
- Volume = 160 × 0.25 = 40 cu ft
- With settlement = 44 cu ft = 1.63 cu yd
- Bags = ⌈44 ÷ 2⌉ = 22 bags
Twenty-two bags is 44 cu ft, or 1.63 cubic yards. Bulk mulch is usually sold in whole or half yards, so you would order 2 yards and have about a third left over — which is why the bag count and the bulk order rarely match.
Depth by what the bed is for
| Bed type | Depth | Why |
|---|---|---|
| Established perennials | 2–3″ | Enough to suppress weeds without smothering crowns |
| New bed over bare soil | 3–4″ | Covers the soil surface completely |
| Annuals | 2″ | Deeper mulch keeps soil too cool for germination |
| Trees and shrubs | 2–3″, pulled back from the trunk | Mulch against bark holds moisture and invites rot |
| Erosion control on a slope | 4–6″ | Needs mass to stay in place |
More than 4 inches is almost always wrong. Deep mulch sheds water instead of absorbing it, keeps oxygen out of the root zone, and encourages surface roots that dry out faster in a drought. If a bed already has 3 inches, top it up by an inch rather than adding three more.
What this does not account for
- It does not handle curved or irregular beds. Measure the area by breaking the bed into rectangles and triangles first.
- It does not deduct plant crowns or hardscape. Subtract large shrub areas and paths before entering the area.
- It does not convert to weight. Bulk mulch is sold by volume, but a delivery ticket may show tons; wet hardwood mulch runs 600–800 lb per cubic yard.
- It does not choose a mulch type. Bark, straw, leaf mould and rubber have different depths and settlement rates.
Questions people actually ask
How many bags of mulch is a cubic yard?
Thirteen and a half 2-cubic-foot bags. Since bags are sold whole, plan on 14 bags per cubic yard, or order bulk if you need more than about 2 yards.
How deep should mulch be?
Two to three inches for most beds, four for a new bed over bare soil. Beyond four inches mulch sheds water, starves roots of oxygen and encourages surface rooting.
How much does a cubic yard of mulch cover?
324 square feet at one inch deep, 108 square feet at three inches, 54 square feet at six inches. Divide 324 by your depth in inches.
Should I remove old mulch before adding more?
Only if it has formed a water-shedding mat or smells sour. Otherwise turn it with a rake and add an inch of fresh material to reach the target depth.