Mulch calculator

Mulch by bed area and spread depth. Cubic yards or cubic metres, the count of standard 2 ft³ bags, and the bag-vs-bulk break-even most calculators leave you to work out at the register.

01 How the math works

Mulch volume is bed area times spread depth, with the depth converted to the same unit as the area. For a rectangular bed that is length × width; for a round bed it is π × radius². Multiply by the depth in feet and you have cubic feet; divide by 27 for cubic yards.

Bags are the raw volume plus a waste factor, divided by the bag size and rounded up to the whole bag. The standard big-box bag is 2 ft³; some color-enhanced lines are 1.5 ft³ or 3 ft³, so the field is editable. Bulk mulch is sold by the cubic yard, which is 27 ft³ — the same 13.5 of those 2 ft³ bags. That last number is the entire bag-vs-bulk comparison, and it is the one most calculators never show you.

02 Worked example

Take a foundation border bed, 20 ft × 4 ft (6.10 m × 1.22 m), mulched 3 in (76 mm) deep — the middle of the recommended range for bark mulch.

Area is 80 ft² (7.43 m²). Volume: 80 × 0.25 ft = 20 ft³ = 0.74 yd³ (0.57 m³). With a 10% waste factor that is 22 ft³, which at 2 ft³ per bag is exactly 11 bags.

The bulk alternative rounds 0.81 yd³ up to the yard's half-yard step: 1.0 yd³. But one cubic yard is 13.5 bags of mulch, and the bed only needs 11. On a bed this size the bags are the tighter fit, and you only pay for what the bed takes. Bulk wins once the bed is large enough that its half-yard rounding is a rounding error rather than a third of the order.

03 When this calculator is wrong

The volume is exact arithmetic. What it does not know is the state of your bed or the shape of your real planting area, and those are where the order goes off.

• It assumes bare soil, not an existing bed. An established bed usually still holds 1–2 in of last year's mulch, partly broken down. Topping it back up to the 2–3 in range takes roughly half to a third of a fresh install — closer to 1 in of new material, not the full 3. Entering the full depth every spring over-orders year after year. For a refresh, enter the depth you are adding, not the finished depth.
• Deeper is not better, and the calculator will let you type any number. The recommended organic-mulch depth is 2–4 in (51–102 mm): 2–3 in for fine or composted material, 3–4 in for coarse bark. Past 4 in the layer mats, sheds water, and starves roots of oxygen; piled against a trunk it rots the bark. That guidance is from the ISA and university extension services, not from us. A depth over 4 in orders mulch that works against the plants it is meant to protect.
• It treats the bed as a clean rectangle or circle. Real beds curve around downspouts, trees, and paths. Measuring the bounding box over-orders; breaking an irregular bed into rectangles and quarter-circles and adding them is closer. The round-bed mode is for a tree ring or an island bed, not an L-shaped border.
• Loose settling and compaction vary by material. Shredded hardwood knits and settles; coarse nuggets stay loose and shift. Bagged volume is measured loose, so a freshly spread bag reads a hair deeper than the same mulch a month later. This is small — under the 10% waste factor — but it is why the bed looks thin by late summer.

04 What to do with the result

Compute the volume, add the waste factor, and round up — to the whole bag for bagged, or to the yard's increment for bulk. Rounding up is the right call on every material order, because a second trip for one more bag costs more than the bag. For mulch there is a split in that rule worth knowing: unopened bags are returnable at most garden centers, so rounding bags up is nearly free, but a bulk pile dumped in your driveway is not returnable, so round bulk up by the smallest scoop the yard sells rather than a full extra yard.

To decide between bags and bulk, use the break-even the calculator prints: one cubic yard is 13.5 of the 2 ft³ bags. Multiply the bag price by 13.5 and compare it to the yard's bulk price per cubic yard, delivery included. Below about 0.5 yd³ — one small bed — bags almost always win, because the bulk delivery fee has nothing to spread across.

05 Common questions

How many bags of mulch are in a cubic yard?

A cubic yard is 27 ft³, so it holds 13.5 of the standard 2 ft³ bags, 18 of the 1.5 ft³ bags, or 9 of the 3 ft³ bags. Read the bag — volume varies by product line.

How much does a 2 cubic foot bag of mulch cover?

At the recommended depths, one 2 ft³ bag covers 12 ft² at 2 in, 8 ft² at 3 in, and 6 ft² at 4 in (1.11 / 0.74 / 0.56 m²). Coverage is just volume divided by depth, so halving the depth doubles the coverage.

How deep should mulch be?

2–4 in (51–102 mm): 2–3 in for fine or composted mulch, 3–4 in for coarse bark. On poorly drained clay, hold to 1–2 in so roots still get oxygen. Keep it off the trunk and root flare — mulch piled against bark is the common way an otherwise healthy tree declines.

Is it cheaper to buy mulch in bags or in bulk?

Bulk is cheaper per cubic yard once the bed is big enough to justify delivery. The break-even is arithmetic: bulk wins when its price per cubic yard, delivery included, is below 13.5 × the price of one 2 ft³ bag. For one or two small beds, bags usually win; for a whole yard's worth of beds, bulk pulls ahead.

How much mulch do I need for a flower bed?

Multiply the bed's length by its width for the area, then by the depth in feet (3 in is 0.25 ft) for cubic feet, and divide by 27 for cubic yards. A 20 × 4 ft bed at 3 in is 20 ft³, or about 11 bags with waste. Round beds use π × radius² for the area.