Mulch math is genuinely simple — area × depth ÷ 12, divided by 27 for yards. What trips people up is everything around the formula: what depth is right, how bags convert, and when bulk delivery beats the trunk of a sedan.
The formula, once
Cubic feet = square feet × depth in inches ÷ 12. Cubic yards = cubic feet ÷ 27.
A 20×4 ft bed at 3 inches: 80 × 3 ÷ 12 = 20 cu ft ≈ 0.74 yards — one yard with a little left over. Run your own numbers with the mulch calculator; it also converts to 2-cu-ft bags, which is how bagged mulch actually sells.
Depth is the real decision
| Depth | Coverage per cubic yard | Use it when |
|---|---|---|
| 2 inches | ~162 sq ft | Annual top-up on healthy beds |
| 3 inches | ~108 sq ft | The standard — weed suppression + moisture |
| 4 inches | ~81 sq ft | New beds, bare soil, heavy weed pressure |
More than 4 inches suffocates: roots climb into the damp pile, and trunks buried in "mulch volcanoes" rot. If last year's mulch still reads 2+ inches thick, you're topping up, not remulching — buy accordingly.
Bulk vs bags: the breakeven
Bagged mulch at $3–5 per 2-cu-ft bag works out to roughly $40–70 per cubic yard; bulk delivered often lands at $30–50/yd plus delivery. The honest breakeven is around 1.5–2 cubic yards — below that, bags save you the delivery fee and the shovel-heavy afternoon; above it, bulk usually pays. Count bags before you decide: 13.5 bags per yard is the number to price against.
Three mistakes worth avoiding
- Mulching too early in spring. Cold soil stays cold under fresh mulch; wait until soil warms and perennials wake up.
- Piling against trunks and stems. Keep a mulch-free collar a hand-width wide around every woody plant.
- Ignoring settlement. Wood mulch loses roughly a third of its depth to compaction and decay each season — that's why top-ups are forever.
A note on buying
Bagged mulch varies: some "2 cu ft" bags are 1.5, some premium bark is sold by the cubic foot. Read the fine print on the bag — the volume, not the picture — and let the calculator's bag count (assuming 2 cu ft) be your scale, adjusted for whatever your store stocks.