Why lava rock needs its own calculator
Landscape lava rock is mostly scoria: volcanic rock that cooled while full of gas bubbles. Those holes make each piece light for its size, and a pile of it has air both inside the stones and between them. The result is a rock that weighs about 900–1,500 lb per cubic yard, compared with about 2,700 lb for river rock.
The volume you need is the same as for any rock. The weight is not. If you order lava rock by the ton using a river rock conversion, you will get about twice as much rock as you planned.
cubic yards = square feet × (depth in inches ÷ 12) ÷ 27
tons = cubic yards × about 0.6 (range 0.45–0.75)
Worked example
A 20 × 10 ft bed in 3/4–1½ inch lava rock at 2.5 inches:
- 200 sq ft × (2.5 ÷ 12) = 41.7 cu ft, plus 10% = 45.8 cu ft
- 45.8 ÷ 27 = 1.70 cu yd
- 1.70 × 1,200 ÷ 2,000 = 1.02 tons (between about 0.76 and 1.27 tons across the weight range)
- 45.8 ÷ 0.5 = 92 bags, each roughly 22 lb
Lava rock sizes and depths
The calculator uses the same rule as the rest of the site: about twice the average size, rounded up to the nearest half inch, at least 2 inches deep.
| Size | Average piece | Depth | 1 cu yd covers |
|---|---|---|---|
| Mini, 3/8”–3/4” | about 0.6” | 2” (minimum) | 162 sq ft |
| Small, 3/4”–1½” | about 1.1” | 2.5” | 130 sq ft |
| Large, 1½”–3” | about 2.25” | 4.5” | 72 sq ft |
Size names vary between suppliers, so check the actual range in inches.
How far does a ton of lava rock go?
Because the weight varies so much, the honest answer is a range. The middle column is what the calculator uses.
| Depth | 1 ton at 1,500 lb/yd | 1 ton at 1,200 lb/yd | 1 ton at 900 lb/yd |
|---|---|---|---|
| 2 inches | about 216 sq ft | about 270 sq ft | about 360 sq ft |
| 2.5 inches | about 173 sq ft | about 216 sq ft | about 288 sq ft |
| 3 inches | about 144 sq ft | about 180 sq ft | about 240 sq ft |
| 4 inches | about 108 sq ft | about 135 sq ft | about 180 sq ft |
This is why it is usually better to buy lava rock by the cubic yard. A yard is a yard whatever the rock weighs, so you know exactly what you are getting.
Lava rock weight compared with other rock
| Rock | About lb per cu yd | About tons per cu yd | 0.5 cu ft bag |
|---|---|---|---|
| Lava rock | 900–1,500 | 0.45–0.75 | about 17–28 lb |
| River rock | 2,500–2,900 | 1.25–1.45 | about 46–54 lb |
| Mexican beach pebbles | 2,600–2,900 | 1.3–1.45 | about 48–54 lb |
| Crushed granite | 2,500–2,900 | 1.25–1.45 | about 46–54 lb |
The light weight is a real advantage on roof gardens, balconies and planters, where load matters, and it makes bags much easier to carry. The rock weight calculator converts any amount between yards, tons and pounds.
Living with lava rock
- Red and black are the common colors. Red lava rock can fade in strong sun over the years; black tends to hold its color better. Look at an older installation if you can.
- It is sharp. The rough texture stays put on slopes better than round river rock, but it is not comfortable to kneel on or walk over barefoot. Wear gloves when spreading it.
- It is hard to clean. Leaves catch in the pores and gaps. A leaf blower on a low setting helps. If you might remove it later, lay fabric underneath so it lifts out in one layer.
- Garden rock is not fire-pit rock. For gas fire pits and fire features, use only rock sold for that purpose and follow the appliance maker’s instructions. Porous rock that has absorbed water can crack when heated.
Lava rock varies more than any other landscape rock. If your supplier sells by the ton, ask them how many cubic yards are in a ton of their product and use that instead of our 1,200 lb figure.
Questions people ask
- How much does a cubic yard of lava rock weigh?
- About 900 to 1,500 pounds, depending on how porous the rock is, its size and whether it is wet. That is roughly 0.45 to 0.75 tons per cubic yard. The calculator uses 1,200 pounds (0.6 tons).
- How many square feet does a yard of lava rock cover?
- The same as any cubic yard: 162 square feet at 2 inches deep, 108 at 3 inches and 81 at 4 inches. Lava rock only differs from other rock when you buy it by weight.
- How much area does a ton of lava rock cover?
- At about 1,200 pounds per cubic yard, a ton is about 1.67 cubic yards. That covers roughly 270 square feet at 2 inches deep or 180 square feet at 3 inches.
- How deep should lava rock be in a flower bed?
- About twice the average size of the pieces, and at least 2 inches. That is 2 inches for mini lava rock, about 2.5 inches for 3/4 to 1½ inch rock and about 4.5 inches for 1½ to 3 inch pieces.
- How much does a bag of lava rock weigh?
- A 0.5 cubic foot bag weighs roughly 17 to 28 pounds, about half the weight of the same bag of river rock.