How much mulch do I need?

For a bed, you need area in square feet × depth in inches ÷ 324 cubic yards of mulch. A 12 × 6 ft bed (72 sq ft) at 3 inches deep needs 72 × 3 ÷ 324 = 0.67 cubic yards, which is 18 cubic feet or 9 bags of 2 cu ft. The 324 comes from converting units: a cubic yard is 27 cubic feet, and there are 12 inches in a foot, so 27 × 12 = 324. For bags, the sum is area × depth ÷ 12 to get cubic feet, then divide by the bag size and round up.

The two formulas

Cubic yards = area (sq ft) × depth (in) ÷ 324

Bags = area (sq ft) × depth (in) ÷ 12 ÷ bag size (cu ft), rounded up

Most bagged mulch in the US is sold in 2 cu ft bags, but 1.5 and 3 cu ft bags exist. Check the label before you do the sum.

To find the area, multiply length by width. For an irregular bed, split it into rectangles and add them. For a circle around a tree, use 3.14 × radius × radius: a ring 4 ft out from the trunk is 3.14 × 4 × 4 ≈ 50 sq ft (subtract the small area right around the trunk, which you leave bare anyway).

Bags and cubic yards by bed size

Bags are 2 cu ft, rounded up to whole bags. Cubic yards are rounded to two decimal places.

Example of one row worked out: 100 sq ft × 3 in ÷ 12 = 25 cu ft. 25 ÷ 2 = 12.5, so 13 bags. In yards: 100 × 3 ÷ 324 = 0.926.

Bags or bulk?

A cubic yard is 27 cu ft, or 13.5 bags of 2 cu ft. So:

• Under about half a yard (roughly 7 bags or fewer), bags are simpler. No delivery, no tarp, no pile on the driveway.
• Around 1 to 3 yards and up, bulk is often cheaper per cubic foot, but bulk suppliers usually charge delivery and some have a minimum order. Compare the delivered total, not just the per-yard price, against the bag count from the table.
• Bulk you pick up in a pickup truck saves delivery, but a full cubic yard of wet wood mulch is heavy. Ask the supplier how much your truck can safely carry.

Bulk mulch varies more in quality than bagged. Ask what it is made of and how long it has been aged; very fresh, wet wood can smell sour and, in rare cases, damage tender plants.

How deep should it be?

Fresh mulch settles, so a 3 inch layer of shredded bark may be closer to 2 inches after a few weeks of rain. That is fine.

Mistakes to avoid

• Volcano mulching. Piling mulch up against tree trunks in a cone keeps the bark wet, invites rot and pests, and can encourage roots to circle in the mulch. Keep mulch a few inches away from trunks and shrub stems, so it looks like a doughnut, not a volcano.
• Too deep. More than about 4 inches can starve roots of air and keep water from reaching the soil, especially with fine mulches that mat.
• Mulching over weeds. Pull or cut perennial weeds first. Mulch smothers seedlings; it does not stop established bindweed or Bermuda grass.
• Plastic underneath. Plastic sheeting stops water and air. If you want a barrier, use a few layers of plain cardboard or a woven landscape fabric, and know that weeds will still root in the mulch on top.
• Dyed mulch made from scrap wood. Some colored mulches are made from recycled pallets or construction wood. Ask what is in it.
• Cocoa bean hulls if you have a dog. They contain theobromine, the same compound that makes chocolate toxic to dogs, and they smell appealing.
• Topping up every year by habit. Organic mulch breaks down, but if you still have 2 inches, an inch on top is enough.

When to mulch

Spring, once the soil has warmed a little, and right after planting are the most common times. A fall layer helps protect roots in cold areas from freeze-thaw heaving. Water new plants well before mulching; mulch keeps moisture in but also slows light rain from getting through.

The free mulch calculator does these sums for any bed size and depth. If you are planting a new bed, FloraVista's plant list includes the mulch quantity for your bed alongside the plants: start with a photo.