Rock is the go-to ground cover for low-water and fire-wise landscapes. Colorado State University Extension calls gravel mulch the best option for making landscapes more fire-resistant, and notes that pea gravel improves water infiltration in short, intense storms. Rock is also heavy and usually sold by the ton, so turning a bed's size into an order takes two steps: volume, then weight. This landscape rock calculator works out the cubic yards for a rectangular or round bed at your chosen depth, adds extra for settling and uneven ground, and converts to tons at an editable density. It also gives the number of 0.5 ft³ bags for small jobs.
For wood mulch, use the mulch calculator. For driveways and base layers, use the gravel calculator.
How to use the landscape rock calculator
- Measure the bed. Split irregular beds into rectangles and circles and add the results.
- Choose a depth. CSU Extension gives 4 inches as the depth for gravel and other mulches to suppress weeds. Decorative top-dressing over a weed barrier is often 2–3 inches.
- Keep about 10% extra for stone that settles into the soil.
- Set the density from your supplier’s quote. 1.4 tons per cubic yard is a mid-range default for dense stone; lava rock is far lighter.
- Enter a price per ton, per yard or per bag to compare bulk delivery with bags.
Formula
Round beds use π × (diameter ÷ 2)² for the area. Density depends on the stone type, size and moisture, so the default is only a starting point. Suppliers usually list a "tons per yard" figure for each product.
Worked example
A 20 × 6 ft border, 3 inches deep, with 10% extra and stone at 1.4 tons/yd³.
- Area: 120 ft². Volume: 120 × 0.25 = 30 ft³ = 1.11 yd³.
- With 10%: 1.22 yd³.
- Weight: 1.22 × 1.4 = 1.71 tons (3,422 lb).
- In bags: 1.22 × 27 ÷ 0.5 = 66 bags of 0.5 ft³, so bulk delivery is usually the better buy at this size.
At CSU's 4-inch weed-suppressing depth, the same border needs 1.63 yd³, about 2.28 tons.
Coverage per ton (at 1.4 tons/yd³)
| Depth | ft² per ton | ft² per yd³ |
|---|---|---|
| 2 in | 116 | 162 |
| 3 in | 77 | 108 |
| 4 in | 58 | 81 |
| 6 in | 39 | 54 |
Coverage per cubic yard doesn't depend on density; coverage per ton does. Lighter stone such as lava rock covers far more area per ton, so ask your supplier for their figure.
Choosing stone and what goes underneath
Stone size changes how rock mulch behaves. CSU Extension says stones used as mulch should be smaller than ½ inch across to conserve water and suppress weeds. Larger stone doesn't work well as mulch and can even inhibit plant growth, though it can add landscape interest. Gravel also transfers more heat to the soil and nearby structures than wood chips, which helps some plants in cold climates and stresses others against hot walls.
On weed barriers, CSU does not recommend black plastic or woven polypropylene fabric beneath landscape mulch. Plastic blocks oxygen and water. Woven fabric eventually clogs, and weeds germinate in debris on top and root through it. CSU says mulch applied at the correct depth doesn't need underlayment. Edge the bed with steel, stone or concrete so the rock stays put. CSU also notes that gravel can transfer heat to buildings, so think about rock placed against sunny walls.
Caveats and assumptions
- Density varies a lot by stone type and moisture: lava rock is much lighter than river rock or granite. Use your supplier’s figure.
- Bag sizes differ between brands; check the cubic-foot figure on the bag.
- Large rock sizes (2 in and up) leave voids, so you may need extra depth to hide the soil.
- Delivery trucks have minimums and maximum loads; ask before ordering a partial yard.
Frequently asked questions
How much landscape rock do I need?
Multiply length × width × depth in feet, divide by 27 for cubic yards, add about 10%, then multiply by the stone’s density (often around 1.4 tons per yard) for tons.
How many square feet does a ton of river rock cover?
At 1.4 tons per yard, about 77 ft² at 3 inches deep, or 58 ft² at 4 inches. Lighter stone covers more.
How deep should landscape rock be?
Colorado State University Extension gives 4 inches as the depth for gravel and other mulches to suppress weeds. Decorative layers over a barrier are often 2–3 inches.
Should I put landscape fabric under rock?
CSU Extension does not recommend black plastic or woven weed fabric beneath landscape mulch. It says rock at the correct depth doesn’t need underlayment.
How many bags of rock make a yard?
A cubic yard is 27 ft³, so 54 bags of 0.5 ft³.
Sources
- Colorado State University Extension — Mulching (CMG GardenNotes) — 4 in depth, stone under ½ in, fabric and plastic, fire resistance, infiltration
- U.S. Geological Survey — Crushed Stone Statistics and Information — types of crushed stone
- NIST — Unit Conversion — cubic foot, cubic yard and ton definitions
This calculator gives estimates for planning and education. Check measurements, local codes, supplier specs and your school's official policy before relying on a result. See our disclaimer and how we build and check calculators.