Article — River Rock Calculator
River Rock Calculator: Volume, Tons, and Coverage
A cubic yard of river rock covers 108 square feet at 3 inches deep and weighs about 1.35 US tons. To calculate how much you need, multiply length by width by depth (in feet), divide by 27, and add a 10% to 15% waste factor.
River rock pricing in the United States runs $25 to $75 per ton at the quarry or yard, with multi-color decorative blends sitting at the top of that range. The exact amount you need depends on three things: the area you are covering, how deep the layer should be, and which size grade you pick. The calculator above handles the unit conversions and density math; this article explains the numbers behind it.
What is river rock?
River rock is naturally rounded stone shaped by water transport in streams and rivers. The U.S. Geological Survey describes the process as a combination of abrasion (stones grinding against each other), attrition (corner-wearing during transport), and chemical weathering. Stones that travel hundreds of miles downstream from headwaters develop the smooth, egg-shaped profile that defines decorative river rock.
Suppliers sell it in five common size grades: pea gravel at 3/8 inch, small at 1 to 2 inches, medium at 2 to 5 inches, large at 5 to 12 inches, and boulders over 12 inches. Each grade has its own density because larger stones leave more void space but also pack more tightly under their own weight. The pea-gravel-to-boulder range covers densities from roughly 1.30 to 1.425 short tons per cubic yard.
The river rock volume formula
The math is straightforward. Volume equals length times width times depth, all in the same unit. The trick is that depth is usually given in inches while length and width are in feet, so the depth value needs to be divided by 12 before multiplying.
Cu ft = L × W × (D / 12)Cu yd = Cu ft / 27Tons = Cu yd × densityWith waste × 1.10 to 1.15For a 10-foot by 12-foot patio at 3 inches of depth: 10 × 12 × 0.25 = 30 cubic feet, divided by 27 = 1.11 cubic yards. Multiplied by 1.35 tons per cubic yard for small river rock, that is 1.5 tons of material. Add a 12% waste factor and you should order 1.7 tons.
River rock density by size grade
Density rises with stone size, which seems counterintuitive at first. The reason is that larger stones leave bigger void spaces between individual rocks, but the rocks themselves are heavier per piece. Geological survey work on construction aggregate finds the net effect favors size: bigger stones pack denser overall.
- Pea gravel (3/8 in) = 2,600 lb/yd³ (1.30 tons)
- Small (1-2 in) = 2,700 lb/yd³ (1.35 tons) — most common grade
- Medium (2-5 in) = 2,750 lb/yd³ (1.375 tons)
- Large (5-12 in) = 2,800 lb/yd³ (1.40 tons)
- Boulders (12+ in) = 2,850 lb/yd³ (1.425 tons)
- Cubic feet per ton = roughly 14.8 ft³ for small river rock
The roundness of river rock is a direct record of how far it traveled. Geomorphologists measure pebble roundness on the Krumbein scale from 0.1 (sharp angular fragments) to 0.9 (perfect spheres). Highly rounded rocks have typically traveled 100 miles or more from their parent bedrock.
How deep should river rock be?
Depth depends on the job. Decorative ground cover only needs to hide the landscape fabric underneath. Drainage applications need enough depth to handle water flow and prevent erosion. Dry creek beds combine both visual and functional requirements.
- Decorative beds = 2 to 3 inches
- Around foundations = 4 to 6 inches (drainage)
- Dry creek beds = 6 to 8 inches
- Under water features = 8 to 12 inches
- Slope erosion control = 4 to 8 inches over fabric
Going shallower than 2 inches almost guarantees you will see soil and weeds through the rocks within a few months. Going deeper than 6 inches is wasteful for purely decorative use because nobody sees the bottom layer.
River rock coverage chart
One cubic yard contains 27 cubic feet. Spread thinner, it covers more area. Spread thicker, it covers less. This single number drives every coverage chart you will see at landscape supply yards.
- 1 in deep = 324 sq ft per cu yd
- 2 in deep = 162 sq ft per cu yd
- 3 in deep = 108 sq ft per cu yd
- 4 in deep = 81 sq ft per cu yd
- 6 in deep = 54 sq ft per cu yd
- 12 in deep = 27 sq ft per cu yd
Most landscape yards sell by the ton, not the cubic yard, because trucks weigh material at the scale. Use the calculator's tons output for your order; the cubic yard number is for visualizing the pile size.
River rock vs. crushed stone
River rock and crushed stone are not interchangeable. River rock is rounded and decorative; crushed stone is angular and structural. The shape difference matters because angular crushed stone interlocks under pressure, which is exactly what you want under a driveway or paver base. River rock shifts and rolls underfoot, which is bad for foundations but good for decorative use where you want a soft, natural look.
Common river rock pitfalls
Most river rock complaints trace back to a small handful of mistakes. None of them are hard to avoid, but they all add cost if you skip the prep work.
River rock laid directly on soil will be invaded by weeds within one growing season. Quality 4 to 6 mil landscape fabric under the rock is non-negotiable for any bed you do not want to weed every few weeks.
The second-most-common mistake is underestimating waste. A 10% buffer is the floor; complex shapes with curves and tight corners, sloped sites, and small jobs benefit from 15%. Running out mid-project means a second delivery fee, and a bag of rock at the home improvement store costs about three times what bulk delivery costs per ton.
Third: density assumptions. Plugging in 1.35 tons per cubic yard when you ordered boulders gives an undercount of about 5%. Plugging in 1.40 when you ordered pea gravel gives an overcount of the same size. The calculator's grade selector handles this automatically, but it is worth knowing why it matters.