How Much Mulch Do I Need for a Large Garden Bed?

Mulch is a volume problem dressed up as an area problem. Multiply bed area in square feet by depth in feet — inches ÷ 12 — then divide by 27 to get cubic yards. A 600 ft² bed at 3 in deep is 600 × 0.25 = 150 ft³, and 150 ÷ 27 = 5.6 cubic yards.

The useful shortcut is coverage per yard. Because a cubic yard is 324 ft² spread 1 in deep, it covers 324 ft² at 1 in, 162 ft² at 2 in, 108 ft² at 3 in, and 81 ft² at 4 in. Divide your bed area by the right number and you have the yardage without any conversion at all: 600 ÷ 108 = 5.6 yards.

Bagged mulch is 2 ft³ per bag, so a cubic yard is 13.5 bags. Our 5.6 yard bed would take 76 bags. The break-even against bulk delivery is usually around 2 to 3 cubic yards — past that, bulk is far cheaper per yard even after the delivery fee, and it saves you flattening 76 plastic bags.

Choose the depth deliberately. Two to three inches is the sweet spot for weed suppression and moisture retention in most beds. Four inches is reasonable for pure weed control on a bare bed but starts to restrict water and oxygen reaching the soil. Always keep mulch 2–3 in clear of trunks and stems — the volcano of mulch piled against a tree trunk holds moisture against the bark and kills trees slowly.

Finally, account for what is already there. Existing mulch that has broken down to a thin crust can be topped up with 1–2 in rather than a full 3 in, cutting your order by a third or more. Rake the old layer to break the crust before topping so water can penetrate, and skip landscape fabric under organic mulch — it stops the mulch feeding the soil and eventually surfaces as ugly frayed plastic.