River Rock Calculator

River rock is a larger, rounder, naturally tumbled stone, typically 1 to 5 in across, while pea gravel is small, pea-sized (3/8 in) rounded stone. Because the larger cobbles nest tighter with less air space between them, river rock weighs slightly more per cubic foot (about 105 lb per cu ft loose, vs 100 lb per cu ft for pea gravel). River rock is more decorative and stays put under foot traffic, so it is the usual pick for drainage features, dry creek beds, and around downspouts. Pea gravel is more comfortable to walk barefoot on and is the standard for patios, playgrounds, and path fill. The calculator uses 105 lb per cu ft by default, but you can override the density for an unusually light or heavy rock type.

Decorative ground cover (around plantings, along borders) works at 2 to 3 in deep, enough to hide the soil and weed barrier without burying the stone's color. Pathways and walking areas need 3 to 4 in so feet do not push down to the dirt. French drains, downspout splash zones, and dry creek beds need 4 to 6 in to actually carry water and stay in place during heavy runoff. Always lay landscape fabric underneath: river rock without a weed barrier will sink into soft soil within a year or two, and you will be topping it up sooner than you planned.

The rule of thumb is 1 cubic yard. Under 1 cu yd, the 50 lb bags from a home center are usually cheaper once you factor in the flat delivery fee (typically $50 to $150 in metro areas). At or above 1 cu yd, the per-ton price of bulk wins decisively, and most yards will not deliver smaller loose orders anyway. The calculator flips its recommended delivery mode at the 1 cu yd line and tells you the exact tonnage to ask the yard for. A cubic yard of river rock weighs about 2,835 lb, so plan your driveway: a half-ton pickup is at its rated payload with one cubic yard in the bed, no margin for tools or passengers.

Smaller cobbles (1 to 2 in) pack with less air space between them, so the loose-fill density runs around 105 to 110 lb per cu ft. Larger cobbles (3 to 5 in) leave more void space, especially when freshly dumped, so the loose-fill number drops to about 95 to 100 lb per cu ft. Specialty stones like Mexican beach pebbles or polished black river rock can run a bit denser because they are more uniform. The calculator defaults to 105 lb per cu ft (a middle-of-the-road number that matches what most US landscape yards quote), but you can override it if your supplier publishes a different density for the specific rock you are buying.

Yes, almost always. River rock laid directly on soil sinks into the dirt within a season, especially in beds you water or in areas with foot traffic. A woven landscape fabric (4 to 6 oz weight) under 2 to 4 in of rock keeps stones on top, blocks most weeds, and lets water through. Skip the cheap perforated plastic sheeting, which traps water and rots roots. For dry creek beds and drainage, use a heavier non-woven geotextile sized for the job. Add 10 percent to your fabric square footage for overlap at seams, and pin every 12 to 18 in along the edges so wind and water do not lift the corners before the rock goes down.