River Rock Calculator
River rock is sold by the ton but planned by the look, and the look depends on depth. Small 3/8-inch stones cover fine at 2 inches; big 3-to-5-inch rock needs 4 inches or you'll see landscape fabric peeking through all summer. This tool ties those together. Pick your rock size, enter the bed dimensions, and you'll get tons, cubic yards, and bag counts, plus a flag that tells you whether your chosen depth actually works for that stone.
Depth follows the stone, not the other way around
River rock coverage charts disagree with each other because they hide one variable: the stone has to sit at least twice its own size deep to cover. Small 3/8-inch pebbles blanket the ground at 2 inches. A 3-to-5-inch cobble sitting 2 inches deep is just rocks scattered on fabric. Pick the size first, and let the depth follow. The tool's depth check does exactly that comparison and flags an order that won't look right.
Coverage per ton by size and depth
| Rock size | Workable depth | One ton covers | Typical use |
|---|---|---|---|
| 3/8 - 3/4 in | 2 in | ~95 sq ft | Paths, between steppers, around plants |
| 1 - 2 in | 3 in | ~65 sq ft | Beds, borders, general ground cover |
| 3 - 5 in | 4 in | ~50 sq ft | Dry creek beds, swales, downspout runs |
| Oversize / boulders | single layer | by the piece | Accents, retaining edges |
Density runs 1.3 to 1.4 tons per cubic yard depending on how much air the stones trap, and the tool matches the density to the size you pick.
A dry creek bed, worked
A 30-foot drainage swale, 4 feet wide, in 3-to-5-inch rock at 4 inches deep: 120 square feet times a third of a foot is 40 cubic feet, about 1.6 yards with waste, which is 2.1 tons. Most yards will round to 2, and delivery beats 84 bags by a mile. Line it with fabric first and set the biggest stones along the edges like banks; the middle takes the water.
Rock over mulch: the five-year math
River rock costs three to five times what mulch does up front, $100 to $250 a ton against $30 to $60 a yard. Then mulch decomposes and gets topped up every year or two, and rock just sits there. Run the numbers over five years and rock usually wins anywhere you don't plant annuals. The catch: rock reflects heat onto plants, and once it's down, changing your mind means moving tons by hand. Beds you re-dig often should stay mulched, and the mulch calculator prices that side of the choice.
Ordering notes
- Fabric first, always. Woven landscape fabric with 6-inch overlapped seams keeps a season's rock from becoming next year's buried gravel.
- Buy 10% over. Rock migrates to low spots as it settles, and matching a color lot a year later rarely works.
- Check the source pile in person if the color matters. "Mixed river rock" varies wildly between quarries and even between deliveries.
- For anything that moves water, size up. Flowing water carries small rock away; a wet-weather creek wants the 3-to-5-inch grade.
Common questions
At 2 inches deep, roughly 100 square feet for small stone. At 4 inches, closer to 50. Big 3-to-5-inch rock covers 40 to 55 square feet a ton because it has to sit deeper to look right.
3/8 to 3/4 inch for paths and around plants, 1 to 2 inch for beds and general cover, 3 to 5 inch for dry creek beds and drainage swales. Bigger rock stays put in rain; smaller is kinder underfoot.
Yes. Without it the rock works its way into the soil in a season or two and you're topping up forever. Use a woven landscape fabric, overlap seams 6 inches, and pin it.
Up front, no: rock runs $100 to $250 a ton against mulch at $30 to $60 a yard. But rock doesn't decompose, so over five years or more it usually wins. The cost fields in both tools let you compare your local prices.
Estimates for planning. Verify against your supplier's units and your local building codes before ordering or building. For anything structural, follow your engineer or local code.