River Rock Explained: How Much You Need, Sizes, and Coverage

River rock — smooth, rounded stone, tumbled by water — is laid as ground cover, drainage, and decoration, and it is bought by weight and laid by depth. The quantity question is therefore a conversion: the area times the depth gives the volume, the volume times the density gives the weight, and the weight plus a waste allowance gives the order. The stone size decides the depth: small pebbles sit shallow, large cobbles sit deep, and the depth must be at least twice the stone’s size to cover the ground. This guide covers the quantity arithmetic, the size and depth table, the uses, and the delivery checks.

How much river rock: area, depth, volume, weight

Measure the area in square feet — length times width for a rectangle, split into rectangles for an irregular bed — and choose the depth in inches. Volume in cubic feet is area × depth ÷ 12. A 200 sq ft bed at 3 inches: 200 × 3 ÷ 12 = 50 cubic feet. Cubic yards, the unit bulk suppliers often quote, are cubic feet ÷ 27: 50 ÷ 27 = 1.85 cubic yards. The River Rock Calculator runs the same steps and adds the waste allowance, so the figure it reports is the order quantity, not the neat volume.

Weight is volume times density. River rock densities run from about 85 lb per cubic foot for light, porous stone to about 110 lb per cubic foot for dense granite and basalt, with 100 lb per cubic foot a common planning figure. At 100 lb per cubic foot, 50 cubic feet weighs 5,000 lb — 2.5 US tons. At 85 lb it weighs 4,250 lb (2.13 tons); at 110 lb it weighs 5,500 lb (2.75 tons). The spread is 20%, which is the difference between a delivery that covers the bed and one that falls short, and it is why the supplier’s density figure — not a generic one — should be used for the order. Ask the supplier for the weight per cubic yard of the specific stone, and use that.

Waste and settling add 10% as a planning allowance. Uneven ground takes more stone in the hollows; edges spill; the stone settles into the soil and the fabric beneath it. Ten percent covers the ordinary cases; a steep slope, a soft base, or a bed that will be walked on wants more, and a flat bed on compacted base with edging wants less. The allowance is added to the weight, not to the depth: order 10% more tons, not 10% deeper, because the depth is set by the stone size, and extra depth is not the same as extra coverage.

Stone sizes and depths: the table

River rock is sold by size range, and the size range sets the minimum depth. The rule is depth at least twice the largest stone’s size: 1-inch stone needs 2 inches, 2-inch stone needs 4 inches, 3-inch stone needs 6 inches. Below that, the ground shows between the stones, and the bed looks thin however much stone is in it. Above it, the stone is stable — large stones lock against each other — and the bed reads as a surface rather than a scattering.

The common sizes and their uses: pea gravel and small pebbles (3/8–3/4 inch) for paths and between pavers, at 2–3 inches; 1–2 inch river rock for beds and borders, at 3–4 inches; 2–3 inch cobbles for drainage and large beds, at 4–6 inches; and 3–5 inch cobbles and boulders for accents, dry creek beds, and erosion control, placed individually rather than by depth. The smaller the stone, the more it migrates — walked on, washed by rain, blown by leaf blowers — and the more often it needs topping up. The larger the stone, the more it stays, and the less it covers per ton, because large stones leave larger voids.

Colour is a choice, not a quantity, but it has a quantity consequence: light stone shows dirt and algae sooner and needs cleaning or topping up more often; dark stone absorbs heat and can scorch adjacent planting in hot climates. Both are sold washed, and both arrive dusty from transport — the first rain, or a hose, restores the colour. The supplier’s sample, wet, is the colour to judge by, because the stone will be wet half its life.

Coverage figures are approximate at 100 lb per cubic foot; the supplier’s density governs the actual order.

Uses, fabric, edging, and delivery checks

River rock’s uses divide into three. Ground cover: beds and borders where it suppresses weeds, retains moisture, and does not decompose like mulch — laid over landscape fabric, at the depth the stone size requires, with edging to contain it. Drainage: French drains, dry creek beds, and downspout aprons, where the voids between stones carry water and the stone’s weight resists washout — larger stone, deeper, and no fabric where the water must pass through. And decoration: accents, borders, and water features, where the stone is placed, not spread, and the quantity is counted, not weighed.

Landscape fabric belongs under ground-cover rock and not under drainage rock. Under a bed, it suppresses weeds and stops the stone sinking into the soil; under a drain, it blocks the water the drain exists to carry, and the drain should be lined with fabric on its sides only, or with a graded gravel envelope. Edging — steel, stone, or timber — contains the rock and keeps it out of the lawn, where a mower will throw it. Without edging, the bed loses stone at its perimeter every season, and the topping-up never ends.

Delivery checks: the tonnage on the ticket against the order, the stone size against the sample, and the depth after spreading against the plan. A delivery that is 10% light on a 3-inch bed leaves the bed at 2.7 inches, which shows ground through a 2-inch stone — the shortfall is visible, and it should be visible before the truck leaves, not after the stone is spread. Spread a test patch, measure it, and call the supplier while the truck is still there if the depth is short. A ton of stone spread thin is not a bargain; it is a second delivery. For the loose-stone relative — the gravel that fills driveways and paths by the cubic yard — see the guide to gravel quantities, which covers the same volume-to-weight conversion for angular stone.

Worked orders: three beds, from area to tons

Three beds, all at 3 inches deep, at 100 lb per cubic foot, with the 10% waste allowance. A 100 sq ft bed: 25 cubic feet, 2,500 lb, 1.25 tons, plus 10% gives 1.38 tons to order. A 200 sq ft bed: 50 cubic feet, 5,000 lb, 2.5 tons, plus 10% gives 2.75 tons. A 400 sq ft bed: 100 cubic feet, 10,000 lb, 5 tons, plus 10% gives 5.5 tons. The pattern is linear — double the area, double the tons — because the depth and density are fixed, and the waste allowance scales with the order. A supplier who quotes by the cubic yard: the three beds need 0.93, 1.85, and 3.70 cubic yards before waste, and the tons follow from the supplier’s density, not from a generic one.

The order quantity should be rounded up to the supplier’s delivery unit, not down to the neat figure. A supplier who delivers in whole tons, or in half-ton increments, turns 1.38 tons into 1.5 tons and 2.75 tons into 3 tons, and the rounding is not waste: the extra stone tops up the bed, fills the hollows, and covers the settling. A supplier who delivers by the cubic yard in whole yards turns 1.85 yards into 2, and the extra 0.15 yards is the same insurance. Rounding down, by contrast, guarantees a short bed, because the neat figure already assumes a level base, no spillage, and no settling — three assumptions that never all hold.

Split deliveries suit large orders. A 400 sq ft bed at 5.5 tons is a full truckload for a small tipper, and the stone should be tipped where it will be spread from, not where it is convenient for the truck, because moving 5 tons by wheelbarrow across a lawn is a day’s work and a damaged lawn. Two deliveries of 3 tons, a week apart, let the first half be spread and the depth checked before the second is ordered — and the second can then be adjusted for the actual depth achieved, which is the figure the first delivery measured. The delivery plan is part of the order, and it should be agreed before the truck leaves the yard.

Density 100 lb per cubic foot; use the supplier’s density for the specific stone.

Colour, cleaning, and topping up

Colour is chosen wet, because the stone is wet half its life. A sample viewed dry in the supplier’s yard looks lighter than the same stone after rain, and the difference can be two shades — enough to clash with the house or the planting. Wet the sample, view it in the light the bed will receive, and judge it then. Light stone — white, cream, pale grey — brightens a shaded bed and shows dirt, algae, and leaf stain sooner; dark stone — charcoal, basalt, dark granite — recedes, absorbs heat, and can scorch adjacent planting in hot sun. Both are honest choices, and the choice should be made for the bed’s light and its neighbours, not for the yard’s display.

Cleaning is a hose and a rake, not a chemical. Algae and dirt wash off with water and agitation; a leaf blower on a low setting clears leaves without scattering the stone, while a high setting scatters the stone with the leaves. Weeds grow in the silt that accumulates between the stones, not through the fabric, and they are pulled or hoed before they seed — a bed weeded twice a season stays clean, and a bed left for a year becomes a weed bed with stones in it. The fabric suppresses weeds from below; it does nothing about the silt from above, and the maintenance is the silt’s removal, not the fabric’s replacement.

Topping up is annual, small, and expected. A bed loses stone at its edges, into the soil, and to the mower, and a topping of half an inch a year — a fraction of the original order — keeps the depth at the plan. Keep a bag of the same stone, from the same supplier, for the topping: stone from a different source differs in colour and size, and a patch of mismatched stone is visible for years. The topping is also the moment to check the depth properly, with a ruler in several places, and to re-level the hollows before they become bare patches. A bed maintained this way looks new for a decade; a bed left alone looks thin in three years.

Frequently asked questions