How to use the stone calculator
Pick the rock, enter the bed size and the depth, and you get tons to order plus how many square feet a single ton covers at that depth. That coverage figure is handy at the landscape yard, where rock is often priced per ton and the person at the counter will ask how much ground you're covering.
The rock type matters more than anything else. Lava rock weighs roughly half as much as river rock, so a ton of it goes nearly twice as far. If your supplier gives a weight per yard, choose "My supplier's figure" and enter it.
The formula
cubic feet = area (ft²) × depth (ft)
tons = (cubic feet ÷ 27) × density
coverage per ton = (27 ÷ density) ÷ depth (ft)
Worked example: a 15 × 8 ft bed of river rock, 3 inches deep
- Area: 15 × 8 = 120 ft².
- Volume: 120 × (3 ÷ 12) = 30 ft³, which is 30 ÷ 27 = 1.11 yd³.
- Weight at 1.4 tons per yard: 1.11 × 1.4 = 1.56 tons.
- Coverage: one ton is 27 ÷ 1.4 = 19.3 ft³, and spread 0.25 ft deep that covers 77 ft².
Rock doesn't compact or rot the way mulch does, so this page starts with no extra allowance. Add 5% if the ground under the bed is uneven.
How deep to lay rock
| Rock | Typical size | Depth |
|---|---|---|
| Pea gravel, small chips | ⅜-½ in | 2 in |
| Decorative crushed stone | ¾-1 in | 2-3 in |
| River rock | 1-3 in | 3-4 in |
| Lava rock | ¾-2 in | 2-3 in |
| Cobbles | 3-8 in | One stone deep, about 1.5× the size |
The calculator warns you if the depth is less than about one and a half times your largest stones, because big rock laid too thin leaves fabric showing between stones.
Laying a rock bed
- Dig out first. Remove 2 to 4 inches of soil so the rock sits level with the lawn or path and doesn't spill.
- Use landscape fabric. Rock doesn't block weeds by itself. A woven or non-woven fabric underneath keeps the stone clean and stops it sinking into the soil.
- Edge it. Steel, aluminium or stone edging stops rock migrating into the lawn, where it damages mowers.
- Think about heat. Large rock beds in full sun get hot and reflect heat onto plants and walls. Mulch is kinder to most plants.
Common mistakes
- Buying by the bag for a big bed. 0.5 ft³ bags are convenient but cost far more per ton than bulk. Past about 20 bags, check the bulk price.
- Mixing up volume and weight. A "yard" of rock is a volume. A ton is a weight. Ask your supplier which one their price is for.
Questions people ask
How many square feet does a ton of river rock cover?
At 2 inches deep a ton of river rock covers about 115 square feet; at 3 inches about 77, and at 4 inches about 58. Larger rock needs more depth, so it covers less.
How deep should landscape rock be?
A common rule of thumb is about twice the size of the largest stones, so the rock covers the fabric with no gaps. That works out to 2 to 3 inches for ¾ to 1 inch stone, and 3 to 4 inches for 1 to 3 inch river rock.
How much does a yard of landscape rock weigh?
Most decorative stone and river rock weigh about 1.3 to 1.5 tons per cubic yard. Lava rock is far lighter, around 0.65 to 0.85 tons, so a ton of it covers almost twice the area.
Is the rock calculator the same as a gravel calculator?
The maths is the same, but decorative rock is usually laid in thinner, single-purpose beds and sold in a wider range of densities. Use the gravel calculator for driveways and bases, and this one for beds and borders.
How many bags of rock do I need?
Bagged decorative stone usually comes in 0.5 cubic foot bags, so you need 54 bags per cubic yard. Check the bag, because some brands sell by weight instead.
Sources and assumptions
- Vulcan Materials and Luck Stone product calculators: decorative stone and river rock about 1.3-1.45 tons/yd³
- Supplier density charts: lava rock about 0.65-0.85 tons/yd³, large cobbles about 1.5-1.75 tons/yd³
- The depth of about twice the stone size is a widely used landscaping rule of thumb, not a standard.
Last reviewed 26 October 2026. Every formula on this page is checked against the worked example above in our automated tests. Found a problem? Tell us.