Mulch Calculator
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About the Mulch Calculator
Mulch is ordered in cubic yards (bulk) or bags (2 or 3 cu ft), but gardens are measured in feet and depth in inches — three unit systems in one purchase, which is why guessing goes wrong. The volume math is identical to gravel; only the depths and the reasons differ: mulch is about moisture retention and weed suppression, not load-bearing.
Enter the bed's length and width (split irregular beds into rectangles and sum), pick your depth — 3″ is the standard refresh-to-suppression sweet spot — and get yards, cubic feet, bag count, and cost at your price. The bag/bulk comparison is built in: run it both ways with your local prices before ordering.
Mulching over a stone or gravel path project too? Volume math for stone lives in the Gravel Calculator
The 324 Rule
One constant to memorize — everything else is multiplication:
Cubic yards = (sq ft × depth in inches) ÷ 324 Bags (2 ft³) = cubic yards × 13.5 Bags (3 ft³) = cubic yards × 9
Worked example: a 20 × 10 ft bed at 3″ needs 200 × 3 ÷ 324 = 1.85 cubic yards — 25 two-cubic-foot bags, or a small bulk delivery. At $6 a bag that's $150; bulk at $40/yd³ is about $74, which is the usual story past a yard or so.
Coverage by Depth
What one cubic yard covers, and yards per 100 sq ft, at the common depths:
| Depth | 1 yd³ covers | Per 100 sq ft | 2-ft³ bags per 100 sq ft |
|---|---|---|---|
| 1″ (top-dress) | 324 sq ft | 0.31 yd³ | 5 |
| 2″ (refresh) | 162 sq ft | 0.62 yd³ | 9 |
| 3″ (standard) | 108 sq ft | 0.93 yd³ | 13 |
| 4″ (weed control) | 81 sq ft | 1.23 yd³ | 17 |
Bag math tip: 13.5 two-cubic-foot bags equal one cubic yard — past roughly a yard of total need, bulk delivery almost always wins on price.
How Deep to Mulch
Depth is a horticultural decision, not just a volume one:
- Annual refresh over existing mulch: 1–2″ — just enough to restore color and cover thin spots.
- New beds around perennials and shrubs: 3″ — the balance point of moisture retention, weed suppression, and cost.
- Weed-prone areas and around trees: 3–4″ — but keep mulch 3–6″ away from trunks and stems (no “mulch volcanoes”; piled bark holds moisture against bark and invites rot and rodents).
- Vegetable beds: 2–3″ of straw or shredded leaves, pulled back where direct-sown seeds need bare soil.
More than 4″ suffocates roots and sheds water instead of soaking it through — deeper is not better past that line.
Frequently Asked Questions
How many bags of mulch are in a cubic yard?
13.5 of the common 2-cubic-foot bags, or 9 of the 3-cubic-foot size (one yard = 27 cubic feet). That conversion is the fastest bulk-vs-bag price check: multiply the bag price by 13.5 and compare to the bulk per-yard quote plus delivery.
How much mulch do I need for 100 square feet?
At the standard 3″ depth: 100 × 3 ÷ 324 ≈ 0.93 cubic yards — call it one yard bulk, or 13 two-cubic-foot bags. At a 2″ refresh, 0.62 yards (9 bags).
How deep should mulch be?
3″ is the standard for beds — enough to hold moisture and block most weeds without smothering roots. Use 1–2″ for an annual color refresh over existing mulch, up to 4″ for weed-prone areas, and never more; past 4″ mulch sheds water and starves roots of air.
Is bulk or bagged mulch cheaper?
Bulk wins past roughly one cubic yard even after delivery fees — a yard is 13.5 bags, and per-bag pricing rarely survives that multiplication. Bags win for small refreshes, tight access, and when you need to store leftover material dry.
Should mulch touch plant stems and tree trunks?
No — pull it back 3–6″ from stems and trunks. Mulch piled against bark (the classic “volcano” around trees) traps moisture, invites rot, borers, and rodents, and can girdle young trees. Flat donut, not volcano.
How often should mulch be replaced?
Organic mulches break down at roughly 1″ per year depending on climate and material — most beds want a 1–2″ top-up annually and a full refresh every 2–3 years. That decomposition feeds the soil; it's a feature you're budgeting for, not a defect.
Methodology. This calculator uses standard construction and material-estimation formulas. It is reviewed and maintained by the Vast Calculators editorial team.
Last updated · July 2026
Results are estimates for general use; verify critical figures independently.