Work out how many cubic yards or bags of mulch a bed needs from its area and the depth you are spreading, for rectangular, circular and triangular beds.
Measure the bed, not the border. Mulch is volume divided by depth, so a few extra feet along curved edges changes the order by several bags on a shallow refresh.
Mulch is sold two ways and the two do not line up in your head: bulk by the cubic yard, bagged by the cubic foot. A cubic yard is 27 cubic feet, so one yard is 13.5 bags of the standard 2 cu ft bag, 9 bags of a 3 cu ft bag, or 18 bags of a 1.5 cu ft bag.
That means the arithmetic is only ever two steps: area × depth in feet gives cubic feet, and cubic feet ÷ 27 gives yards, or ÷ the bag size gives bag count. Coverage per cubic yard depends only on depth, 324 sq ft at 1 in, 162 at 2 in, 108 at 3 in, 81 at 4 in.
The reason to care about coverage rather than volume is that bags are roughly twice the price of bulk per cubic yard once you add delivery, and the crossover is usually somewhere around 2–3 cubic yards. Below that, bags are genuinely cheaper because there is no delivery fee; above it, bulk wins.
| Depth | Per cubic yard | Per 2 cu ft bag | Use |
|---|---|---|---|
| 1 in | 324 sq ft | 24 sq ft | Top-up over existing mulch |
| 2 in | 162 sq ft | 12 sq ft | Vegetable beds, annuals, shallow feeder roots |
| 3 in | 108 sq ft | 8 sq ft | Perennial beds and shrubs, the usual target |
| 4 in | 81 sq ft | 6 sq ft | New beds, tree rings, playgrounds |
Doubling the depth halves the coverage: a cubic yard does 324 sq ft at 1 in and 162 sq ft at 2 in.
| Bag size | Bags per cubic yard | Buy this many |
|---|---|---|
| 1 cu ft | 27 | 27 |
| 1.5 cu ft | 18 | 18 |
| 2 cu ft | 13.5 | 14 |
| 3 cu ft | 9 | 9 |
Always round bags up, retail does not sell half a bag, and the leftover tops up the spots that settle. Bag volume is printed on the front of the bag, and mulch sold in the US carries a Mulch and Soil Council certification mark confirming the stated volume.
Depth is a horticultural decision, not a cosmetic one, and too much is worse than too little. Three inches is the usual target for perennial beds and around shrubs. One to two inches suits vegetables and annuals, whose shallow roots need water and warmth to reach them. Tree rings take about three inches, kept clear of the trunk.
Past about four inches, mulch stops being a benefit: it sheds water instead of absorbing it, suffocates roots, and holds moisture against stems and trunks where it causes rot. Piling mulch into a cone against a tree trunk, a mulch volcano, is one of the most common causes of decline in urban trees.
Mulch also settles. What arrives at 3 in thick can compress to 2 in within a week under rain and foot traffic, so a 3 in target is almost always the right number to enter here.
The volume arithmetic comes from the definition of the cubic yard as 27 cubic feet, which produces the 324 ÷ depth-in-inches coverage figure. Bag-to-yard conversions (13.5 bags of 2 cu ft, 9 of 3 cu ft, 18 of 1.5 cu ft) are simple division of 27 by the bag volume. Recommended application depths follow extension-service guidance, three inches for perennial beds and shrubs, one to two for vegetables and annuals, and nothing over four because deeper mulch sheds water and damages trunks. Bagged mulch volume labelling is standardised by the Mulch and Soil Council.
Multiply the bed area by the depth in feet to get cubic feet, then divide by 27 for cubic yards or by the bag size for a bag count. A 200 sq ft bed at 3 in deep is 200 × 0.25 = 50 cu ft, which is 1.85 cubic yards or 25 bags of 2 cu ft.
13.5 bags of the standard 2 cubic foot size, so buy 14. That is 18 bags of 1.5 cu ft, 9 bags of 3 cu ft, or 27 bags of 1 cu ft. One cubic yard is 27 cubic feet whichever way you split it.
Three inches for perennial beds and around shrubs, one to two inches for vegetables and annuals, and about three inches for tree rings kept clear of the trunk. Never go past four inches: thick mulch sheds water instead of absorbing it, suffocates roots and holds moisture against stems.
324 sq ft at 1 in deep, 162 at 2 in, 108 at 3 in and 81 at 4 in. Coverage is inversely proportional to depth, so the shortcut is 324 divided by the depth in inches.
Only if it is still loose and useful. A bed with an inch of fresh bark has effectively already had an inch applied, so a 3 in target means 2 in of new material. Mulch that has decomposed into soil does not count, it behaves like soil, not like mulch.
Bulk is roughly half the price per cubic yard, but it carries a delivery fee and usually a 2–3 cubic yard minimum. Below about 2–3 yards the bags frequently come out cheaper and they store indefinitely; above it, bulk wins clearly.