Multiply the square footage of the bed by the depth you want in inches and divide by 324 — that is the mulch order in cubic yards. A 10×10 ft bed is 100 sq ft, so at 3 inches deep it takes 100 × 3 ÷ 324 = 0.93 cubic yards, which is 25 cubic feet, or 13 bags of the standard 2 cu ft bag. The same bed at 2 inches needs 0.62 cubic yards (9 bags) and at 4 inches 1.23 cubic yards (17 bags). One cubic yard covers 108 sq ft at 3 inches, 162 sq ft at 2 inches and 81 sq ft at 4 inches — and 2 to 3 inches is the depth the extension services keep landing on, with 3 inches the practical ceiling on a soil surface and 2 inches if the mulch is finely ground.
The formula, in three steps
- Get the area. Length × width in feet. An L-shaped bed is two rectangles added together, and a curved bed is close enough to a rectangle for ordering purposes.
- Turn the depth into feet. Divide the inches by 12 — 2 in = 0.167 ft, 3 in = 0.25 ft, 4 in = 0.333 ft.
- Do the volume math. Cubic feet = area × depth in feet. Divide by 27 for cubic yards, or by the cubic footage printed on the bag for a bag count. Round up.
| Unit you want | Formula |
|---|---|
| Cubic feet | sq ft × depth in inches ÷ 12 |
| Cubic yards (bulk) | sq ft × depth in inches ÷ 324 |
| 2 cu ft bags | cubic feet ÷ 2, rounded up |
| Cubic yards from cubic feet | cubic feet ÷ 27 |
The 324 is nothing exotic: it is 27 cubic feet per yard × 12 inches per foot, which is why the same number works for any area. The mulch calculator applies it to rectangular beds and returns yards and bags at once, and the square footage calculator handles circles and triangles when the bed is not a rectangle.
How much one cubic yard covers
| Depth | Coverage per cubic yard | Coverage per 2 cu ft bag |
|---|---|---|
| 1 in | 324 sq ft | 24 sq ft |
| 2 in | 162 sq ft | 12 sq ft |
| 3 in | 108 sq ft | 8 sq ft |
| 4 in | 81 sq ft | 6 sq ft |
| 6 in | 54 sq ft | 4 sq ft |
A standard 2 cu ft bag covering about 8 sq ft at a 3-inch depth is the number retailers publish, and it matches the arithmetic exactly: 2 cubic feet spread 0.25 ft deep covers 2 ÷ 0.25 = 8 sq ft. A cubic yard is always 13.5 bags at that size, whatever the depth, because the bag size does not change with how thick you spread it.
How deep should mulch be
| Situation | Depth | Source |
|---|---|---|
| Most landscape beds | 2–4 in | Illinois Extension |
| Composted or finely ground mulch | 2–3 in (2 in maximum) | Illinois Extension; UK Extension |
| Coarse mulch: wood chips, pine bark nuggets | 3–4 in | Illinois Extension; Clemson |
| Ceiling on any soil surface | 3 in | UK Extension |
| Grass clippings or shredded leaves | never deeper than 2 in | Clemson |
| Gravel, pebbles or crushed stone | 1 in | Clemson |
The reason the range has a lid on it rather than a floor is that overshooting causes the damage: more than 4 inches restricts the oxygen reaching the roots and pushes roots up into the mulch layer itself, and fine-textured material mats into a sheet that sheds water instead of passing it. Clemson singles out the two materials that mat worst — grass clippings and shredded leaves — and caps them at 2 inches, while allowing the coarse materials that stay airy, such as pine bark nuggets, to run to 4 inches.
Worked examples you can copy
| Bed | Area | 2 in deep | 3 in deep | 4 in deep |
|---|---|---|---|---|
| 10 × 10 ft | 100 sq ft | 0.62 yd³ · 9 bags | 0.93 yd³ · 13 bags | 1.23 yd³ · 17 bags |
| 10 × 20 ft | 200 sq ft | 1.23 yd³ · 17 bags | 1.85 yd³ · 25 bags | 2.47 yd³ · 34 bags |
| 20 × 25 ft | 500 sq ft | 3.09 yd³ · 42 bags | 4.63 yd³ · 63 bags | 6.17 yd³ · 84 bags |
| 40 × 25 ft | 1,000 sq ft | 6.17 yd³ · 84 bags | 9.26 yd³ · 125 bags | 12.35 yd³ · 167 bags |
Two real-world cases that fall outside the grid:
- Foundation bed, top-up depth. A run of 60 ft × 3 ft is 180 sq ft. Topping it up at 2 inches takes 180 × 2 ÷ 324 = 1.11 cubic yards, or 30 cubic feet, or 15 bags. This is the classic bagged-mulch job: one cubic yard already buys 13.5 bags, so anything under a yard or so is usually cheaper and easier to handle than a bulk delivery.
- Shrub border, new bed. A 40 ft × 4 ft border is 160 sq ft. At the standard 3 inches that is 40 cubic feet = 1.48 cubic yards = exactly 20 bags of 2 cu ft.
Bags vs. bulk: pick the unit first
Bag sizes are not all the same, and the count per yard changes with them:
| Bag size | Coverage at 2 in | Coverage at 3 in | Bags per cubic yard |
|---|---|---|---|
| 1.5 cu ft | 9 sq ft | 6 sq ft | 18 |
| 2 cu ft (most common) | 12 sq ft | 8 sq ft | 13.5 |
| 3 cu ft | 18 sq ft | 12 sq ft | 9 |
Read the cubic footage off the label before you do the division — 2 cu ft is the usual retail size, but 3 cu ft bags are also sold, and the difference is five bags per yard. Bulk mulch is quoted in cubic yards, so if you order a yard and plan to spread it yourself, know that you are handling 13.5 bags’ worth in one loose pile.
Tree rings and round beds
Circle area is π × radius². Clemson sizes a mulch ring for an established tree at about 2 feet of diameter per inch of trunk diameter, so a 6-inch trunk gets a 12 ft ring: radius 6 ft, area π × 36 = 113 sq ft. At 3 inches deep that is 113 × 3 ÷ 324 = 1.05 cubic yards, or 15 bags. Subtract the trunk clear zone — a 4-inch radius circle is only about a third of a square foot, so it changes nothing in your order — and keep that zone bare. For most trees, Illinois Extension caps the ring at a 3 ft radius, which is 28 sq ft, or 0.26 cubic yards at 3 inches: 4 bags.
Top up, do not rebuild
Organic mulch is not a yearly purchase. Clemson puts the useful life of pine needles and bark mulches at roughly 2 to 4 years before they need replenishing, and UK Extension’s guidance is to replace mulch only when it has completely broken down. Check the depth in the bed first: if 2 to 3 inches are already sitting there, adding more pushes you past the 3-inch ceiling, and raking the surface to break up matted layers is what restores the look.
Quick checklist
- Measure each bed and add the areas together.
- Choose 2 to 3 inches for a general bed, 3 to 4 inches for coarse wood chips or nuggets, and never more than 2 inches for fine or matting material.
- Cubic yards = square feet × depth in inches ÷ 324; cubic feet = the same number × 4 (or square feet × depth ÷ 12).
- Bags = cubic feet ÷ bag size, rounded up; one yard is 13.5 bags of 2 cu ft.
- Keep the mulch at least 4 inches clear of trunks, stems and the house wall, and rake rather than add a new layer if the depth is already right.
If the same bed is getting a stone finish instead, the gravel calculator runs the identical area × depth math in tons and 0.5 cu ft bags, and the cubic yards calculator converts any of these volumes into cubic feet, cubic meters or gallons if your supplier quotes differently.