The method in five steps
- Measure the area in feet. Length and width for rectangles; diameter for circles. Convert inches to feet (6 in = 0.5 ft, 9 in = 0.75 ft).
- Work out square feet. Length × width, or 3.1416 × radius² for a circle. Split odd shapes into simple ones.
- Pick the depth. About twice the average stone size, never less than 2 inches. 3/4 inch rock → 2 in; 1–1½ inch → 2.5 in; 1–3 inch → 4 in.
- Convert to cubic yards. Square feet × depth in inches ÷ 12 = cubic feet. Cubic feet ÷ 27 = cubic yards. Add 10% extra.
- Convert to tons if the yard sells by weight. Cubic yards × about 1.35 for most rock, about 0.6 for lava rock.
The rock calculator does steps 2 to 5 for you. The examples below show the arithmetic so you can check it.
Example 1: a rectangular bed
A 12 × 8 ft bed in 3/4 inch decorative rock at 2 inches:
- 12 × 8 = 96 sq ft
- 96 × 2 ÷ 12 = 16 cu ft → 16 ÷ 27 = 0.59 cu yd
- With 10%: 0.65 cu yd, or 0.65 × 1.35 = 0.88 tons
- In bags: 17.6 cu ft ÷ 0.5 = 36 bags of 0.5 cu ft
That is right on the line between bags and bulk. Many yards sell half or three-quarter yards for pickup.
Example 2: an L-shaped bed
An L-shaped bed along two sides of a patio: one leg 20 × 4 ft, the other 12 × 4 ft, sharing a 4 × 4 ft corner. Split it so the corner is counted once: 20 × 4 = 80 sq ft, plus (12 − 4) × 4 = 32 sq ft.
- Total: 112 sq ft of 1–3 inch river rock at 4 inches
- 112 × 4 ÷ 12 = 37.3 cu ft = 1.38 cu yd
- With 10%: 1.52 cu yd, about 2.05 tons
Example 3: a circle
A 10 ft circle around a fire pit area, in 3/4 inch rock at 2 inches:
- Radius 5 ft: 3.1416 × 5 × 5 = 78.5 sq ft
- 78.5 × 2 ÷ 12 = 13.1 cu ft = 0.49 cu yd
- With 10%: 0.53 cu yd, about 0.72 tons, or 29 bags
For a ring (a circle with the middle left open), work out the outer circle, subtract the inner circle, and use the difference.
Example 4: a triangular corner
A corner bed with two straight sides of 10 ft meeting at a right angle:
- Area of a right triangle: ½ × 10 × 10 = 50 sq ft
- In 1–1½ inch rock at 2.5 inches: 50 × 2.5 ÷ 12 = 10.4 cu ft = 0.39 cu yd, about 0.52 tons before extra
A curved corner (a quarter circle with a 10 ft radius) is 3.1416 × 10 × 10 ÷ 4 = 78.5 sq ft, quite a bit more than the triangle.
Example 5: an irregular bed
Lay a tape along the longest side. Every 3 or 4 feet, measure the width of the bed at right angles to the tape. Add the widths, divide by how many you took, and multiply that average width by the length. A 30 ft bed with widths of 3, 5, 6, 4 and 2 ft has an average width of 4 ft and an area of about 120 sq ft.
Example 6: rock on a slope
Measure the slope along its surface, not as it looks from above, because that is the area the rock has to cover. On slopes steeper than about 3:1, round river rock tends to roll; angular rock or crushed granite holds better, and some people set larger stones in the soil as a stop at the bottom. For a creek bank or anything that carries water, see the riprap calculator.
Example 7: a big job in tons
A 1,000 sq ft front-yard xeriscape in 1–1½ inch rock at 3 inches:
- 1,000 × 3 ÷ 12 = 250 cu ft = 9.26 cu yd
- 9.26 × 1.35 = 12.5 tons
- With 10%: 10.2 cu yd, about 13.75 tons
That may be more than one truckload. Ask the yard how much their truck carries and what each load costs to deliver, and have it tipped close to where it is going.
Quick reference: rock per 100 square feet
| Depth | Cubic feet | Cubic yards | Tons (about 1.35 per yd) | 0.5 cu ft bags |
|---|---|---|---|---|
| 2 inches | 16.7 | 0.62 | 0.83 | 34 |
| 2.5 inches | 20.8 | 0.77 | 1.04 | 42 |
| 3 inches | 25 | 0.93 | 1.25 | 50 |
| 4 inches | 33.3 | 1.23 | 1.67 | 67 |
| 6 inches | 50 | 1.85 | 2.5 | 100 |
Multiply by your area ÷ 100. For lava rock, use the cubic yards column and multiply by about 0.6 instead of 1.35.
Common mistakes
- Mixing inches and feet. 3 inches is 0.25 ft, not 3 ft. Divide inches by 12 before multiplying.
- Using square yards instead of cubic yards. Rock is sold by volume (cubic yards) or weight (tons). Square yards measure area only.
- Using the wrong weight. Lava rock at river-rock weight leaves you with twice the rock you expected. Check the rock weight calculator.
- Spreading too thin. Thin rock shows fabric and soil and needs topping up sooner. Stick to the 2× rule.
These are estimates for dry, loose rock. Your supplier's own weight per yard and loading increments are the final word.
Questions people ask
- How do I figure out how much rock I need?
- Measure the area in feet and multiply length × width for square feet. Choose a depth of about twice the average stone size (at least 2 inches). Multiply square feet by depth in inches ÷ 12 for cubic feet, divide by 27 for cubic yards, and multiply by about 1.35 for tons.
- How much rock do I need for 100 square feet?
- About 0.62 cubic yards (0.83 tons) at 2 inches deep, 0.93 cubic yards (1.25 tons) at 3 inches and 1.23 cubic yards (1.67 tons) at 4 inches, before adding 10 percent extra.
- How many tons of rock do I need?
- Work out cubic yards first, then multiply by the rock's weight in tons per cubic yard: about 1.35 for most landscape rock and river rock, about 0.6 for lava rock. A 1,000 square foot area at 3 inches deep is 9.26 cubic yards, or 12.5 tons.
- Should I round up when ordering rock?
- Yes. Add 5 to 10 percent for uneven ground and settling, then round up to the yard's loading increment, usually a quarter or half yard or ton. A second small delivery usually costs more than a little extra rock.