Work through the order
What the 10 × 10 foot example means
First measure the clear stone area. Subtract large planters, paths, and other spaces that will stay uncovered. Multiply 100 square feet by a 2-inch layer (2 ÷ 12 foot) to get 16.67 cubic feet. Divide by 27 to get 0.62 cubic yard. That volume stays the same whether the stone is red or black, but the bulk weight can change with particle size, moisture, and the supplier's stock.
At the example 1,500 lb/cu yd, 0.62 cubic yard weighs about 926 lb, or 0.46 US short ton. Half-cubic-foot bags need whole-number rounding: 16.67 ÷ 0.5 = 33.33, so buy 34 before any chosen extra. The 34 bags contain 17 cubic feet in total. A supplier may sell bulk in half-yard or whole-yard steps, so ask about the smallest available order and delivery charge.
Cubic feet = area (sq ft) × depth (in) ÷ 12
Cubic yards = cubic feet ÷ 27
US short tons = order yards × supplier lb/yd³ ÷ 2,000
Choose landscape lava rock for the job
Scoria and product size
Landscape “lava rock” is usually vesicular volcanic stone, often sold as scoria. Its many voids make it lighter than dense river stone by volume. Check the actual red or black product's particle size: a coarse decorative bed has a different look and handling from fine cinders. The space between pieces is part of loose bulk volume, so a mineral's solid density cannot stand in for supplier bulk weight.
Depth and uneven beds
Set the depth for the appearance and function you want, then check the largest pieces against it. A thin layer can reveal fabric or soil; a deeper one takes proportionally more material. For curves, divide the bed into smaller rectangles or measure its area directly. If the ground slopes or has low spots, estimate those separately and use the optional extra field only after inspecting the site.
Plants and upkeep
Rock does not decompose like bark, but leaves, dust, and soil can collect in its gaps and support weeds. Remove debris periodically. Dark rock can warm soil in sun, and stone supplies no organic matter; match it to the plants and climate. A fabric layer can separate rock from soil in some beds, yet it is not a permanent weed cure and can complicate future planting.
Fire features need approved media
This calculator sizes a landscape bed. It does not specify media for a gas burner. A landscape bag may be marked unsuitable for fire pits. If you are filling an appliance, use the maker's approved media, fill depth, and vent clearance. Wet media can pop; follow the appliance manual's drying and lighting directions and keep clear until it is safe. Never transfer the bed-depth recommendation into a burner pan.
Use the right next calculation
Lava rock planning questions
How much lava rock do I need for a 10 × 10 foot bed?
At 2 inches deep, a 100-square-foot bed needs 16.67 cubic feet, or 0.62 cubic yard, before extra. That is 34 whole bags if each bag holds 0.5 cubic foot. At the example loose bulk weight of 1,500 pounds per cubic yard, the net volume weighs about 0.46 US short ton.
How many cubic yards are in one ton of lava rock?
It depends on the actual loose bulk weight. At an example 1,500 pounds per cubic yard, one US short ton is about 1.33 cubic yards. At 1,250 pounds per yard it is 1.60 yards. Ask the supplier for the weight of the product and size you will receive.
What depth should I enter for a lava rock bed?
Enter the finished depth you want on the ground. A 2-inch example is useful for a decorative layer, but the right depth depends on stone size, site, and planting. Measure the surface after grading; if the bed is uneven, add an allowance based on your site rather than assuming a universal percentage.
Does lava rock stop weeds or cool plant roots?
A maintained rock layer can reduce light at the soil, but windblown soil and leaves can collect between stones and support new weeds. Dark rock can warm soil in strong sun. Keep rock away from plant crowns, remove debris, and decide whether fabric is useful for your planting plan.
Can I put landscaping lava rock in a gas fire pit?
Do not assume a landscaping bag is approved for a burner. Some retail landscape-lava products explicitly exclude fire-pit use. Choose media approved by the appliance maker and follow its fill-depth, vent, moisture, and lighting instructions. Wet fire media can pop.
Product and field references
Bulk weights are examples from Landscape Centre (1,250 lb/yd³), Palmer Coking Coal (1,300–1,500 lb/yd³), and Sutherland's black lava (1,420 lb/yd³). Vigoro's red landscape lava listing gives 0.5 cu ft and 3 sq ft at 2 in, and excludes fire-pit use. Iowa State Extension discusses rock mulch, soil warmth, and maintenance. For gas fire features, consult the specific appliance manufacturer's media and moisture instructions.