How much mulch you need is a volume question with three inputs: the bed area, the depth, and the unit you buy in, cubic yards for bulk or bags for bagged. Multiply length by width by depth, convert to cubic yards, and you have the bulk order. Divide the cubic feet by the bag size for the bag count. This guide works through the formula, the depth decision, the bulk-versus-bags comparison, and the coverage table for common bed sizes, so the order arrives right the first time.

The formula: area times depth, converted

For a rectangular bed, volume in cubic feet is length in feet times width in feet times depth in inches divided by 12, because depth in inches must become feet. Cubic yards divide cubic feet by 27. Combined, cubic yards equal length times width times depth in inches, divided by 324, since 12 times 27 is 324. The calculator does this for rectangles, circles, and irregular beds, but the rectangle formula is the one to memorise.

The worked example: a 10 ft by 20 ft bed is 200 square feet. At 3 inches deep, cubic feet are 200 times 3 divided by 12, or 50, and cubic yards are 50 divided by 27, or 1.85. Order 2 cubic yards bulk, because bulk is sold by the yard and a little extra covers settling and uneven spreading. For bags of 2 cubic feet, 50 cubic feet divided by 2 is 25 bags exactly.

Circular beds use pi times radius squared for area, then the same depth conversion. A circle 10 feet across has a radius of 5 feet, an area of about 78.5 square feet, and at 3 inches needs about 19.6 cubic feet, or 0.73 cubic yards, or 10 bags of 2 cubic feet. Irregular beds are measured as several rectangles added together, which is accurate enough for ordering.

Depth: the decision that matters most

Two to four inches is the standard depth range for most beds, and 3 inches is the usual choice. Two inches suppresses weeds lightly and conserves moisture modestly, and it needs topping up sooner. Three inches gives solid weed suppression and moisture retention for a season. Four inches suits new beds and coarse mulches, where settling is greater.

Deeper than 4 inches causes problems: it can suffocate roots by limiting oxygen, keep the soil too wet, and encourage root growth upward into the mulch instead of down into the soil. Piled against a trunk or stem, deep mulch causes rot and pest harbourage: keep mulch a few inches away from trunks and stems, in a donut rather than a volcano.

Existing mulch counts toward depth. If 1 inch remains from last year, add 2 inches to reach 3, not 3 more. Top up annually to maintain depth, because mulch decomposes and settles, losing roughly an inch a year depending on material and climate. The calculator depth input should be the added depth, not the total depth, when topping up.

Coverage table: common beds at 3 inches

The table gives cubic yards and 2-cubic-foot bag counts at the standard 3-inch depth. Scale proportionally for other depths: at 2 inches multiply by two-thirds, at 4 inches by four-thirds.

Bulk versus bags: cost, handling, and waste

Bulk mulch, delivered by the cubic yard, is cheaper per yard and suits large beds, but it needs a driveway or verge for the pile, a wheelbarrow, and a day of spreading. A cubic yard covers about 100 square feet at 3 inches, and weighs roughly 400 to 800 pounds depending on material and moisture, so delivery placement matters: a pile far from the beds means many barrow trips.

Bagged mulch, usually 2 or 3 cubic feet per bag, is dearer per yard but easier to handle, cleaner to store, and simpler to count: the bag count is the order, and unopened bags keep for later. Bags suit small beds, top-ups, and gardens without delivery access. The calculator gives both figures, so the comparison is direct: bulk yards against bag count for the same bed.

Waste and settling argue for rounding up, not down. Order the next whole yard bulk, or the next few bags, beyond the computed figure, because beds are rarely exact rectangles, edges need extra, and mulch settles after rain. A small surplus is useful for topping up; a shortage means a second delivery or a second trip to the store.

Materials: bark, wood, stone, and rubber

Bark and wood mulches decompose, feeding the soil as they break down, which is a benefit and a cost: they need annual topping up. Hardwood bark lasts longer than softwood, and shredded bark knits together and resists washing away on slopes better than chips. Pine straw is light, acidic, and popular in the southeast, but it decomposes fast.

Stone mulches, gravel and river rock, do not decompose and need no topping up, but they do not feed the soil, they retain heat, and they migrate into lawns and beds. They suit paths, drainage areas, and xeriscaping, not planting beds where soil improvement matters.

Rubber mulch, from recycled tyres, does not decompose, does not attract insects, and cushions falls, which is why it appears on playgrounds. It does not feed soil, it can retain heat and odour, and its use near plants is debated. The calculator volume formula is identical whatever the material: volume is geometry, and material affects only weight, longevity, and suitability.

Rubber mulch and playground safety: a note

Rubber mulch is kept here as a safety section, not as planting advice. On playgrounds, loose-fill surfacing depth matters for fall cushioning, and the standard is depth maintained at the rated level, commonly 6 to 12 inches depending on the product and the fall height, with regular raking and topping up. A thin or compacted layer does not cushion as rated, whatever the material.

For planting beds, rubber mulch is a surfacing choice with trade-offs, not a soil amendment, and it should not be mixed into soil. Where children play, the relevant question is the maintained depth and the product rating, and the supplier documentation, not the volume formula.

Frequently Asked Questions

How much mulch do I need?

Multiply bed area by depth and convert: cubic yards equal length times width times depth in inches, divided by 324. A 10 ft by 20 ft bed at 3 inches needs 1.85 cubic yards, or 25 bags of 2 cubic feet.

How deep should mulch be?

Two to 4 inches is standard, with 3 inches the usual choice. Deeper than 4 inches can suffocate roots and cause rot against trunks. Keep mulch a few inches away from stems.

How many bags of mulch do I need?

Divide cubic feet by bag size. A 10 ft by 20 ft bed at 3 inches needs 50 cubic feet, or 25 bags of 2 cubic feet. Round up for edges and settling.

Is bulk or bagged mulch cheaper?

Bulk is cheaper per cubic yard and suits large beds, but needs delivery access and barrowing. Bagged is dearer per yard but easier to handle and count, and suits small beds and top-ups.

How often should I top up mulch?

Annually, to maintain depth, because mulch decomposes and settles, losing roughly an inch a year. Measure the remaining depth and add only the difference.

Does mulch go against the tree trunk?

No. Keep mulch a few inches away from trunks and stems, in a donut shape. Piled mulch against bark causes rot and pest harbourage.