How much mulch or soil to buy
Mulch, topsoil and compost are sold by the cubic yard in bulk and by the cubic foot in bags, while your garden beds are measured in feet and inches. The calculator converts the bed size and depth into a volume, adds an allowance for settling, and tells you how many bags you need and what bulk delivery would cost instead.
The formula
- Volume = length × width × depth × (1 + settling allowance)
- Cubic yards = cubic feet ÷ 27
- Bags = volume ÷ bag size, rounded up
- Quick rule: 1 cubic yard covers 324 sq ft at 1 in, 108 sq ft at 3 in
Worked example
A 20 × 10 ft flower bed gets 3 in of shredded hardwood mulch, with 10% extra for settling.
- Volume: 200 sq ft × 0.25 ft = 50 cubic feet, + 10% = 55 cubic feet.
- In cubic yards: 55 ÷ 27 = 2.04 cubic yards.
- In 2 cu ft bags: 28 bags, about $112 at $4 a bag.
- In bulk at $40 per cubic yard: about $81, before delivery.
Volume by depth
For the same 200 sq ft bed:
| Depth | Cubic yards | 2 cu ft bags |
|---|---|---|
| 2 in | 1.36 | 19 |
| 3 in | 2.04 | 28 |
| 4 in | 2.72 | 37 |
| 6 in | 4.07 | 56 |
| 8 in | 5.43 | 74 |
| 12 in | 8.15 | 111 |
Recommended depths
- Mulch: 2 to 3 in suppresses weeds and holds moisture. More than 4 in can starve roots of air. Keep mulch a few inches away from trunks and stems: “mulch volcanoes” around trees cause rot.
- Topsoil for a new lawn: 4 to 6 in over loosened subsoil.
- Raised vegetable beds: 8 to 12 in of a soil and compost blend.
- Compost top dressing: 1/4 to 1/2 in on a lawn, 1 to 2 in on beds.
Bags or bulk
Bags are convenient for small jobs and easy to carry into a backyard. For more than about 2 cubic yards, bulk is usually cheaper even with a delivery fee, and you avoid a pile of plastic. Bulk topsoil is heavy, about 1 to 1.5 tons per cubic yard, so check how close the truck can get and lay a tarp where it dumps. Fresh mulch and topsoil both settle, which is why the calculator adds 10% by default.