Decorative vs. drainage: two very different jobs
River rock does two jobs that call for very different depths, which is why this calculator has a mode toggle instead of a single default. As a decorative material, a shallow 2 to 3 inch layer over a bed or border is plenty, the same range as the general gravel calculator uses. As drainage material, river rock is doing structural work, moving water away from a foundation or across a low spot, and that calls for real depth: about 12 inches of stone around a French drain pipe, or 4 to 6 inches through a dry creek bed. Sizing a drainage project like a decorative bed is the most common way people underorder river rock for a French drain, since the visible surface area looks the same either way but the trench underneath needs far more material.
How to size a French drain
A standard residential French drain trench holds about 12 inches of washed stone around a 4 inch perforated pipe: roughly 4 inches of stone below the pipe to let water drain freely underneath, the pipe itself laid with its perforations down, and another 4 inches of stone on top before the trench is capped or covered. The trench itself is often dug a bit deeper than that, commonly 18 to 24 inches total once you account for slope and any soil cap over the top, but the stone volume this calculator computes is the 12 inch backfill figure around the pipe. Trench width matters too, typically 12 to 18 inches, since that width times the trench length times the 12 inch stone depth is what actually determines how many cubic yards you need.
Why washed rock, specifically
French drains must use washed stone, not standard fill rock. Washed stone has been rinsed to remove sand, dust, and fine particles, leaving open void space between the pieces for water to move through. Unwashed rock still carries fines that settle into those gaps over time and can clog the pipe's perforations within a season, turning a drain into a buried, useless pipe. If a supplier does not specify their river rock as washed, ask before you order it for a drainage project, even though the same unwashed rock would be perfectly fine for a decorative bed where drainage performance is not the point.
Sizing a dry creek bed
A dry creek bed is a shallower, more visible drainage feature, a shaped channel that directs surface water during heavy rain and reads as a landscape feature the rest of the time. Plan for about 4 to 6 inches of total depth, often built as a 3 to 4 inch base layer of larger stone topped with 1 to 2 inches of a smaller decorative layer along the surface. Measure the channel's length and average width to get the area, the same rectangle-and-circle approach used throughout this site, then apply that depth. Rock size also matters for how natural the finished bed looks: larger 2 to 5 inch stone along the bends and banks, smaller 1 to 2 inch stone through the center where water actually flows, mimics how a real creek naturally sorts material by size.
Why river rock is sold by the ton, and why density varies
Like other bulk aggregate, river rock is priced and delivered by weight, since trucks have a payload limit that matters more than a volume limit for bulk delivery. A cubic yard of typical river rock weighs around 2,650 pounds, about 1.33 tons, but that figure shifts with stone size and roundness: larger, rounder stones pack together with more air gaps between them than smaller or more angular material, so the same volume of large round river rock can weigh somewhat less than a denser crushed stone. Treat 1.33 tons per cubic yard as a reasonable planning default, not a guarantee, and confirm the actual density with your supplier for anything beyond a small decorative order.
Order about 10 percent extra for a straightforward bed or drain run, more for a dry creek bed with a lot of curves or a French drain trench with an uneven grade. If part of the job is a compacted base or a decomposed granite path rather than loose decorative or drainage rock, that math needs a separate compaction allowance, see the crushed stone & decomposed granite calculator.