How to calculate mulch and topsoil for your beds
Mulch math trips people up for one simple reason: beds are measured in feet, depth is pictured in inches, bulk deliveries come in cubic yards, and store bags are labeled in cubic feet. This calculator crosses all four units in one pass. Measure each bed's length and width in feet (for a circle, just the widest distance across), pick your depth — 3 inches is the sweet spot for most mulch — and read off both the bulk order in yards and the bag count in every common size. The multi-bed tab exists because almost nobody mulches a single bed: list every bed in the yard, and buy for the whole job in one trip. A 5% cushion covers settling, uneven raking, and the corners that always drink up more material than the tape measure promised.
The mulch coverage formula, explained
Underneath it's the universal volume formula, plus a handy inversion for coverage:
Coverage per yard (sq ft) = 324 ÷ Depth (in)
Worked example: a 10 × 4 ft border bed mulched 3 inches deep is 10 × 4 × 0.25 = 10 cubic feet — about 0.37 cubic yards, or five standard 2-cubic-foot bags before any allowance. The coverage inversion answers the store-aisle question directly: at 3 inches deep, one cubic yard spreads over 324 ÷ 3 = 108 square feet; at 2 inches, 162 square feet.
Bag sizes and the bags-per-yard rule
Bagged mulch and soil come in three standard retail sizes, and the conversion to bulk is fixed arithmetic:
| Bag size | Bags per cubic yard | Coverage per bag at 3" deep |
|---|---|---|
| 1.5 cu ft (common for soil) | 18 | 6 sq ft |
| 2 cu ft (standard mulch bag) | 13.5 | 8 sq ft |
| 3 cu ft (large format) | 9 | 12 sq ft |
The rule of thumb worth memorizing: 13.5 standard bags equal one cubic yard. Once a job climbs past a dozen bags, it's usually worth pricing a bulk delivery — one scoop from a landscape yard replaces a cart-load of plastic, and bulk material is typically fresher than bags that sat on a pallet all season.
How deep should mulch and topsoil be?
For mulch, 2–4 inches is the working range and 3 inches is the standard: deep enough to suppress weeds and hold soil moisture, shallow enough that water and air still reach the roots. Keep mulch pulled a few inches back from tree trunks and plant stems — the notorious "mulch volcano" traps moisture against bark and invites rot. Topsoil depths depend on the job: 4–6 inches for a new garden bed, 2–3 inches when amending existing soil, and about a quarter to half an inch when top-dressing a lawn before overseeding. Raised beds are the exception — those you fill to the height of the frame, so just enter the frame's actual depth.
Frequently asked questions
How many bags of mulch are in a cubic yard?
13.5 of the standard 2-cubic-foot bags, since a yard is 27 cubic feet. In 1.5-cubic-foot bags it's 18, and in the large 3-cubic-foot format it's 9.
How much area does a yard of mulch cover?
108 square feet at the standard 3-inch depth. The general formula is 324 divided by depth in inches — so 162 sq ft at 2 inches, and 81 sq ft at 4 inches.
Is it cheaper to buy mulch in bags or in bulk?
Per cubic foot, bulk almost always wins once quantity is meaningful — you're not paying for plastic, palletizing, and handling. Bags win on convenience for small jobs and for keeping unopened leftovers. The practical crossover for most people is around one cubic yard, or roughly 13–14 standard bags.
How do I calculate mulch for several beds at once?
Use the multi-bed tab: enter each bed's length and width, set one depth, and the calculator sums every bed into a single order. It handles up to ten beds — enough for most yards — and applies your extra allowance to the combined total rather than padding each bed separately.
Should I remove old mulch before adding new?
Usually not. Organic mulch decomposes into the soil, so the routine is a yearly top-up of about an inch to return to full depth. Only strip the old layer if it's matted into a water-shedding crust or the combined depth would exceed 4 inches, which starts suffocating roots.
How much topsoil do I need for a raised garden bed?
Treat the frame as the bed: multiply its length, width, and full height. A classic 4 × 8 ft bed with 10-inch sides holds about 27 cubic feet — almost exactly one cubic yard. Enter the frame height as the depth and the calculator does the rest.