Landscape Rock Calculator

How to use the landscape rock calculator

1. Measure the bed. For curves, split it into rectangles and circles, or estimate an average width.
2. Choose a depth about twice the average rock size so no fabric or soil shows through.
3. Select the rock type. Dense stones are close to 100 lb/ft³; porous lava rock is much lighter, which is why it is sold by the yard more often than by the ton.
4. Add 10% for rock that settles into soil and uneven edges.

The formula

Cubic yards = area (sq ft) × depth (in) ÷ 12 ÷ 27

Tons = cubic yards × 27 × density (lb/ft³) ÷ 2,000

Defaults: most solid decorative rock is set to 100 lb/ft³ (1.35 t/yd³), in line with The Engineering ToolBox's figure for crushed stone (1,602 kg/m³). Decomposed granite is set to 105 lb/ft³, the ToolBox value for dry gravel. Lava rock (scoria) is full of gas bubbles; suppliers commonly quote roughly 0.5–0.75 tons per cubic yard, so the default here is 45 lb/ft³ (0.61 t/yd³). If your supplier gives a number, type it in.

Recommended depths by rock type

• Decomposed granite paths: 3 inches, compacted, with edging.
• Pea gravel and small rock (under 1 inch): 2 inches in beds.
• 3/4–1.5 inch decorative rock or crushed granite: 2–3 inches.
• 1–3 inch river rock or beach pebbles: 3–4 inches.
• Lava rock: 2–3 inches; it is light, so it can blow or float in heavy rain on slopes.

Worked example

A front bed is 25 ft long and 4 ft wide. You want decorative rock 3 inches deep, plus 10%:

• Area: 25 × 4 = 100 sq ft
• Volume: 100 × 3 ÷ 12 = 25 cu ft; with 10% = 27.5 cu ft
• Cubic yards: 27.5 ÷ 27 = 1.02 yd³
• Weight at 100 lb/ft³: 2,750 lb = 1.38 tons
• Bagged: 27.5 ÷ 0.5 = 55 bags

The same bed in lava rock weighs only about 0.62 tons, but you still need the same 1.02 cubic yards.

Coverage per ton

At 100 lb/ft³, one ton of landscape rock is 20 cubic feet, enough for about 120 sq ft at 2 inches, 80 sq ft at 3 inches or 60 sq ft at 4 inches. A ton of lava rock covers more than twice that area.

Installation tips

• Remove grass and weeds and rake the soil smooth first; rock will not smother established weeds.
• Use woven landscape fabric so rock does not sink into soil, and pin seams with staples every 3–4 ft.
• Install edging 1 inch above the rock surface to keep stones out of the lawn.
• Keep rock a few inches back from tree trunks and siding to limit heat buildup and moisture traps.
• For larger beds or dry creeks, the river rock calculator covers cobble sizes in more detail.

Frequently asked questions