Mulch is sold by volume (cubic yards or bagged cubic feet) but spread over an area, so depth is the hinge between the two. Get the depth right and the yardage follows. This guide gives the formula, a coverage chart, and the bag-to-yard conversion that saves money on big beds.
What’s in this guide
The mulch volume formula
The magic number is 324. Because a cubic yard is 27 cubic feet and there are 12 inches in a foot, one cubic yard spread 1 inch deep covers exactly 27 × 12 = 324 square feet. Divide 324 by your depth to get coverage at that depth.
Coverage per cubic yard by depth
| Depth | Coverage per cubic yard | Cubic yards per 100 sq ft |
|---|---|---|
| 1 inch | 324 sq ft | 0.31 yd³ |
| 2 inches | 162 sq ft | 0.62 yd³ |
| 3 inches | 108 sq ft | 0.93 yd³ |
| 4 inches | 81 sq ft | 1.23 yd³ |
Worked example: refreshing a bed
A curved bed measures roughly 25 ft × 16 ft (400 sq ft) and needs a 3-inch layer:
| Step | Value |
|---|---|
| Bed area | 25 × 16 = 400 sq ft |
| Depth | 3 inches |
| Volume | 400 × 3 ÷ 324 = 3.70 yd³ |
| Rounded order | 4 cubic yards (bulk) |
| Or in bags | 3.70 × 13.5 ≈ 50 bags (2 cu ft) |
For curved or kidney-shaped beds, split into rectangles and a half-circle, or use the ellipse and circle calculators, then add the areas before applying depth.
Bags vs. bulk yards
| Bag size | Cubic feet | Bags per cubic yard | Coverage at 3″ |
|---|---|---|---|
| 1 cu ft bag | 1.0 | 27 | 4 sq ft |
| 2 cu ft bag | 2.0 | 13.5 | 8 sq ft |
| 3 cu ft bag | 3.0 | 9 | 12 sq ft |
Recommended depths
| Use | Depth | Note |
|---|---|---|
| Annual top-up | 1 inch | Refreshes color over existing mulch |
| Established beds | 2–3 inches | Standard weed & moisture control |
| New beds / weed suppression | 3 inches | Full coverage |
| Around trees | 2–3 inches | Keep mulch off the trunk (no volcanoes) |
| Playground / paths | 4–6 inches | Safety cushioning |
Common mistakes
Depth is in inches but area is in feet. The 324 divisor already handles the unit change — don't convert twice.
Two beds of the same area need different amounts if their depths differ. Always include depth.
Fresh mulch settles 10–20%. If you want a firm 3-inch finish, spreading slightly deeper is fine for paths, not for plant crowns.
Key takeaways
- Cubic yards = area × depth(in) ÷ 324.
- One yard covers 324 ÷ depth square feet.
- A cubic yard ≈ 13.5 two-cubic-foot bags.
- Spread 2–3 inches in most beds; keep mulch off trunks and stems.
Related calculators & guides
Frequently asked questions
- How many cubic yards of mulch do I need?
- Multiply the bed area in square feet by the depth in inches, then divide by 324. For example, 400 sq ft at 3 inches deep = 400 × 3 ÷ 324 = 3.7 cubic yards.
- How much area does a cubic yard of mulch cover?
- One cubic yard covers 324 sq ft at 1 inch deep, 162 sq ft at 2 inches, 108 sq ft at 3 inches, and 81 sq ft at 4 inches. The deeper the layer, the less ground one yard covers.
- How many bags equal a cubic yard of mulch?
- Bagged mulch is usually 2 cubic feet per bag, and a cubic yard is 27 cubic feet, so it takes about 13.5 bags to equal one cubic yard. Bulk (by the yard) is cheaper for large beds.
- How deep should mulch be?
- Use 2–3 inches for most beds and around trees, and 1 inch for annual top-ups. Deeper than 4 inches can suffocate roots and hold excess moisture against stems.