Mulch Calculator

How the mulch calculator works

A 20 × 6-ft garden bed mulched 3 inches deep takes 1.25 cubic yards — or 17 two-cubic-foot bags. That's 120 sq ft × 0.25 ft = 30 cu ft, or 33 cu ft with a 10% waste factor, which is 1.22 cubic yards rounded up to the quarter-yard increment bulk suppliers sell. At that size bags are usually the cheaper option, since bulk delivery carries a $50–$100 minimum.

The math: we calculate the bed area (length × width for a rectangle, π × radius² for a circle), multiply by your depth, and convert to the selected unit (cubic yards in imperial, cubic meters in metric). For bags, we divide the volume by your bag size. The waste factor defaults to 10%, which covers the mulch that ends up on the lawn, in the wheelbarrow, and packed into the corners of the bag — not extra depth. Bulk orders are then rounded up to the nearest quarter yard, because that is the smallest increment most suppliers will actually load.

How much ground does a cubic yard cover?

A cubic yard is 27 cubic feet, so its coverage depends entirely on depth. This is the single most useful table for planning a mulch order, and it is worth checking against your own bed before you buy:

Those are raw figures with no waste allowance. Add the calculator's 10% and a cubic yard realistically dresses about 98 sq ft at 3 inches. In bag terms: a cubic yard is 13.5 two-cubic-foot bags or 9 three-cubic-foot bags, and a single 2 cu ft bag covers 8 sq ft at 3 inches deep — roughly a 3 × 2.5-ft patch.

How deep should the mulch be?

Cooperative Extension programs converge on the same answer here: 2 to 4 inches of organic mulch, and no more. Deeper is not better, and the failure mode is expensive.

• 2 in / 5 cm — refresh coat on a bed that already has mulch; minimum for aesthetics. Also the right depth for fine-textured mulch like double-shredded hardwood, which packs tighter than nuggets.
• 3 in / 7.5 cm — the sweet spot for new beds: weed suppression, moisture retention, and it still breathes. This is the calculator's default.
• 4 in / 10 cm — maximum for most plants; reasonable for coarse nuggets or aggressive weed areas where light has to be fully blocked.
• Deeper than 4 in / 10 cm — the mulch itself starts behaving like a soil layer. Roots grow up into it, then dry out in the first hot spell. Water sheds off the crust instead of soaking in, and the zone against the stems stays permanently damp.
• Under 1.5 in / 4 cm — too thin to stop weed germination. You get the look and none of the function.

Measure what is already there before you order. The most common overbuy is treating a refresh like a new bed. If a bed has settled to 1.5 inches of old mulch and you want 3 inches, you need mulch for 1.5 inches — half the volume. Push a finger or a trowel down to soil in three or four spots and use the average.

Which mulch to buy

Type affects how long the job lasts far more than it affects the volume you order. Prices below are 2026 US planning ranges for bulk delivered material and move a lot by region; confirm locally and enter your real number in Advanced options to get a cost estimate.

Stone is the other option, and it changes the math completely — it is ordered by weight, not by bags, and it never needs replacing. If you are weighing the two, the gravel vs. mulch comparison runs the cost over ten years, and the gravel calculator handles the tonnage. For filling a bed rather than dressing it, use the topsoil calculator or the raised bed calculator instead — mulch is a surface layer, not a fill.

Bags vs. bulk: where the break-even actually falls

Below about 3 cubic yards (~2.3 m³), bags usually win. Above that, bulk pulls away fast. The reason is the delivery fee, and you can find your own crossover in one line of arithmetic.

Bagged mulch at $4.50 per 2 cu ft bag works out to $60.75 per cubic yard (13.5 bags). Bulk at $40 per yard plus a $75 delivery fee costs 40N + 75 for N yards. Setting those equal: 60.75N = 40N + 75, so N ≈ 3.6 cubic yards. Under that, bags are cheaper and you skip the pile in the driveway. Over it, every additional yard saves you about $21. Plug in your own supplier's numbers — a low delivery fee can move the crossover to under 2 yards.

Two practical notes bags don't advertise. First, a 2 cu ft bag of damp hardwood weighs 40–50 lb, so 30 bags is 1,300 lb — most of a half-ton pickup's payload, and a full afternoon of lifting. Second, bagged mulch that has sat sealed and wet on a pallet can go anaerobic. If a freshly opened bag smells of vinegar, ammonia, or sulfur, that is sour mulch: its pH has crashed and spreading it straight onto bedding plants can scorch them. Spread it on a tarp, turn it once, and let it air out for a day before use.

Two worked examples

An L-shaped foundation bed. The calculator does one rectangle at a time, so add the legs yourself: a 30 × 4-ft run plus a 12 × 4-ft return is 120 + 48 = 168 sq ft. Enter any rectangle with that area — 42 × 4 works — and at 3 inches deep you get 46.2 cu ft with waste, or 1.75 cubic yards / 24 bags.

A ring around a tree. Switch to Circle and enter the radius of the finished ring, not the trunk. An 8-ft radius gives 201 sq ft, and at 3 inches that is 2.25 cubic yards / 28 bags. The calculator draws a full circle while your ring is really a donut, but a 1-ft-radius trunk flare only removes 3 sq ft — about 1.5%, comfortably inside the waste factor. Do not stack mulch against the bark to make up the difference.

The mulch volcano, and why it kills trees

Mulch piled in a cone against a trunk is the most common landscaping mistake in North America, and it is fatal on a five-to-ten-year delay. Bark is not root tissue: kept constantly wet it rots, which opens the tree to decay fungi and gives voles a covered place to chew a girdle. Worse, the tree responds to the buried trunk by sending roots out into the mulch, and those adventitious roots circle back and strangle the trunk they came from.

Do it the other way around: spread the mulch flat, 2–4 inches deep, out toward the drip line, and keep a bare gap of 3–6 inches of visible soil all the way around the trunk flare. The flare — the point where the trunk widens into the roots — should be plainly visible. A wide flat ring is also better for the tree than a narrow deep one, because you are protecting root zone, not decorating a stem.

Four things people get wrong

• "Wood mulch steals nitrogen from my plants." Only at the thin boundary where mulch touches soil, which established plants root well below. The real problem is tilling fresh wood chips into the soil — there, microbes decomposing the wood genuinely do outcompete plants for nitrogen. On the surface, wood mulch is fine, and it feeds the soil as it breaks down.
• "Put landscape fabric under it." Under stone, yes. Under organic mulch it usually backfires: the mulch decomposes into a fertile layer sitting on top of the fabric, weeds root happily in that, and the fabric works its way to the surface in a few years while blocking organic matter from reaching the soil. Skip it and mulch a little deeper.
• "Remove all the old mulch first." Rarely necessary. Loosen the crust with a rake — often that alone restores the look — and top up to depth. Strip it only if the total has crept past 4 inches, or if it has matted into a water-repellent crust.
• "Any mulch is fine next to the house." In wildfire-prone areas, many local codes and fire-agency guidelines call for non-combustible material — stone, or bare mineral soil — within 5 feet of the structure. Shredded bark against the siding is a direct ignition path. Check your jurisdiction before mulching a foundation strip.

When to add mulch

Spring (after the ground warms, so you are not insulating cold soil) and fall (before first frost) are both fine. Avoid mulching immediately after heavy rain — you will trap excess moisture against stems. Most beds want a refresh annually or every other year, depending on how fast the material breaks down; the table above is a reasonable guide. If you are budgeting a season of beds rather than one, the mulch cost guide works through per-yard, per-bag, and installed pricing.

Frequently asked questions