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.