How the rock calculator works
Every rock calculator, and every landscape supply yard taking an order over the phone, uses the same three steps:
- Area. Length × width for a rectangle, or 3.1416 × radius² for a circle, in square feet.
- Volume. Area × depth in feet (inches ÷ 12) gives cubic feet. Add your allowance for waste and uneven ground.
- Cubic yards and tons. Cubic feet ÷ 27 gives cubic yards. Cubic yards × the rock’s weight per yard gives pounds; divide by 2,000 for tons.
cubic yards = length × width × (depth ÷ 12) ÷ 27
tons = cubic yards × pounds per cubic yard ÷ 2,000
Worked example
A 20 × 10 ft bed covered in 1–3 inch river rock, 4 inches deep, with 10% extra:
- Area: 20 × 10 = 200 sq ft
- Volume: 200 × (4 ÷ 12) = 66.7 cu ft, plus 10% = 73.3 cu ft
- Cubic yards: 73.3 ÷ 27 = 2.72 cu yd
- Tons: 2.72 × 2,700 lb ÷ 2,000 = 3.67 tons
- Bags: 73.3 ÷ 0.5 = 147 bags of 0.5 cu ft
Most yards would sell that as 2¾ or 3 cubic yards, or about 3.7 tons. For an odd-shaped bed, split it into rectangles and circles, run each one, and add them up. The how much rock do I need guide has more worked examples, including L-shaped beds, rings around trees and slopes.
How deep should rock be? The 2× rule
Loose rock needs to be deep enough that the stones overlap and hide the soil or fabric underneath. A reliable rule is depth ≈ 2 × the average stone size. One layer of stones leaves gaps you can see through; two layers close them. For very small rock the rule gives less than 2 inches, which is too thin to stop weeds or stay put, so 2 inches is the floor.
When you pick a rock size in the calculator, it fills in the depth from this table. You can still type your own.
| Rock size | Average stone | 2 × average | Depth the calculator uses |
|---|---|---|---|
| 3/8”–1/2” chips or pebbles | about 0.45” | 0.9” | 2” (the minimum) |
| 3/4” rock | 0.75” | 1.5” | 2” (the minimum) |
| 1”–1½” rock | 1.25” | 2.5” | 2.5” |
| 1”–3” rock (most river rock) | 2” | 4” | 4” |
| 2”–4” rock | 3” | 6” | 6” |
| 3”–5” cobble | 4” | 8” | 5”: one stone deep instead |
Cobbles are the exception. Stones bigger than about 3 inches are normally set one layer deep, packed close together, so the depth is roughly the size of the largest stones. Doubling them would waste a lot of heavy rock that nobody sees.
Going thinner than the rule saves money but shows more of what is underneath. Going deeper than about 4 inches in a bed you walk through makes the rock shift underfoot.
How much does rock weigh per cubic yard?
Loose rock is roughly 55–65% stone and the rest air, so most types land in a narrow band. Lava rock is the outlier: it is full of gas bubbles, so a yard of it weighs about half as much as a yard of river rock. These are the figures the calculator uses. Your supplier’s number for their own stone is better.
| Rock type | About lb per cu yd | About tons per cu yd | Typical range (lb) |
|---|---|---|---|
| Landscape rock (general) | 2,700 | 1.35 | 2,500–2,900 |
| River rock | 2,700 | 1.35 | 2,500–2,900 |
| Mexican beach pebbles | 2,800 | 1.40 | 2,600–2,900 |
| Crushed granite (decorative) | 2,700 | 1.35 | 2,500–2,900 |
| Decomposed granite (loose) | 2,600 | 1.30 | 2,400–2,900 |
| Marble chips | 2,700 | 1.35 | 2,500–2,800 |
| Cobblestone | 2,600 | 1.30 | 2,400–2,800 |
| Lava rock | 1,200 | 0.60 | 900–1,500 |
The rock weight calculator converts between cubic yards, tons, cubic feet and pounds for each of these.
Coverage of one cubic yard and one ton
A cubic yard is 27 cubic feet whatever it is made of, so its coverage only depends on depth. A ton depends on how heavy the rock is.
| Depth | 1 cu yd covers | 1 ton of river rock covers | 1 ton of lava rock covers |
|---|---|---|---|
| 2 inches | 162 sq ft | about 120 sq ft | about 270 sq ft |
| 3 inches | 108 sq ft | about 80 sq ft | about 180 sq ft |
| 4 inches | 81 sq ft | about 60 sq ft | about 135 sq ft |
| 6 inches | 54 sq ft | about 40 sq ft | about 90 sq ft |
Starting from a pile you already have? The rock coverage calculator runs the sum backwards.
Bulk or bags?
A cubic yard is 54 bags of 0.5 cu ft. A 0.5 cu ft bag of river rock weighs about 50 lb; the same bag of lava rock is about 22 lb. Bags make sense for a few square feet around a downspout or a container. Past about one cubic yard, bulk delivery from a landscape yard is usually cheaper and saves you lifting 2,700 lb of rock one bag at a time. Many yards also sell half yards for pickup in a pickup truck, but check your truck’s payload first: a full yard of river rock is more than most half-ton pickups are rated to carry.
Before you order
- Measure twice, in feet. Convert inches to feet (9 inches = 0.75 ft) before multiplying.
- Ask how they sell it. Landscape centers usually price rock by the cubic yard; quarries and many delivery firms price by the ton. The landscape rock cost guide shows how to compare the two.
- Lay fabric or prepare the base. Rock over bare soil sinks over time. A permeable landscape fabric slows that down and keeps weeds back.
- Edge the bed. Steel, stone or plastic edging stops rock migrating into the lawn.
The weights here are averages for dry, loose stone. Wet rock straight from the yard can weigh several percent more, and stone from different quarries varies. If your supplier quotes a weight per yard for their stone, multiply the calculator's cubic yards by their figure and divide by 2,000 for tons.
Questions people ask
- How do I calculate how much rock I need?
- Multiply length by width in feet to get square feet, multiply by the depth in inches divided by 12 to get cubic feet, then divide by 27 to get cubic yards. Multiply cubic yards by about 1.35 to get tons for most landscape rock, or by about 0.6 for lava rock.
- How deep should landscape rock be?
- About twice the average size of the stones, and never less than 2 inches. That is 2 inches for 3/4 inch rock, about 2.5 inches for 1 to 1.5 inch rock and about 4 inches for 1 to 3 inch river rock. Cobbles bigger than about 3 inches are laid one stone deep.
- How many square feet does a ton of rock cover?
- For typical landscape rock at about 2,700 pounds per cubic yard, one ton is about 20 cubic feet. That covers about 120 square feet at 2 inches deep, 80 at 3 inches and 60 at 4 inches.
- How much does a cubic yard of rock weigh?
- Most landscape rock, river rock and decorative stone weighs about 2,500 to 2,900 pounds per loose cubic yard, roughly 1.25 to 1.45 tons. Lava rock is much lighter, about 900 to 1,500 pounds per cubic yard.
- How many bags of rock make a cubic yard?
- 54 bags of 0.5 cubic feet, or 27 bags of 1 cubic foot. A 0.5 cubic foot bag of river rock weighs about 50 pounds.