How much river rock you need
The math is the same for any decorative stone — only how you get the square footage changes between a rectangular bed and a round one:
Cubic feet = Area (sq ft) × (Depth in ÷ 12)
Cubic yards = Cubic feet ÷ 27
Tons = Cubic yards × stone density (tons per cubic yard)
Rectangular example: a 10 ft × 8 ft border bed, filled 2 inches deep with river rock (1.3 tons/yd³):
10 × 8 × (2 ÷ 12) = 13.33 ft³ → 13.33 ÷ 27 ≈ 0.49 yd³
0.49 × 1.3 ≈ 0.64 tons
Circular example: an 8 ft diameter tree ring (radius 4 ft), filled 2 inches deep with pea gravel (1.4 tons/yd³):
π × 4² ≈ 50.27 sq ft, × (2 ÷ 12) ≈ 8.38 ft³ → 8.38 ÷ 27 ≈ 0.31 yd³
0.31 × 1.4 ≈ 0.43 tons
The weight for the same footprint changes with what you pick, because density isn't fixed across stone types — heavier stone means more tons for the same volume, and lighter, more porous stone (like lava rock) means fewer tons for the same coverage. Treat any density figure as an estimate; ask your supplier for their actual tons-per-yard number if you're ordering close to the wire.
For an L-shaped or curved bed, split it into two or three rectangles (or a rectangle plus a rough circle), calculate each piece separately, and add the tons together — it's close enough for ordering purposes and far simpler than trying to measure an irregular outline directly. Most stone yards sell river rock in bulk by the ton with a delivery minimum, often around half a ton to a ton, so it's worth rounding your total up slightly rather than ordering a fraction short and paying for a second delivery.
How deep should decorative rock be?
2 inchesis the standard depth for river rock, pea gravel, and similar decorative stone laid over landscape fabric. That's enough to fully hide the fabric underneath and give solid, even coverage without paying for stone depth that adds nothing to the look.
A few adjustments worth knowing before you order: go slightly deeper — around 3 inches — if you're using larger stone that looks sparse at 2 inches, or if the bed will see foot traffic and some settling over time. Edging (metal, plastic, or paver edging) along the border keeps rock from migrating into the lawn or beds next to it, which matters more for coverage retention than adding extra depth does. And skipping landscape fabric isn't a depth problem so much as a maintenance one — the rock volume stays the same, but weeds and gradual sinking into bare soil become more likely.
River rock vs. pea gravel vs. lava rock
These three decorative stones look and perform differently enough that picking the wrong one changes both the finished look and how much material actually covers your bed:
- River rockis smooth, rounded stone in a range of larger sizes (roughly 3/4 in to 3+ in), typically mixed colors from natural rivers. At around 1.3 tons per cubic yard, it's the go-to for borders, dry creek beds, and accent areas where you want individual stones to show.
- Pea gravelis smaller — about 3/8 in — and settles into a denser, more walkable surface, which is why it's common for paths and play areas. It runs slightly heavier at around 1.4 tons per cubic yard, so the same depth needs a bit more tonnage than river rock.
- Lava rockis porous, lightweight volcanic stone at around 0.5 tons per cubic yard — less than half the weight of river rock for the same volume. That makes it cheap to ship and easy to move, and a ton covers far more ground, but it's also more prone to breaking down and fading over time than solid stone.
Because coverage per ton swings this much between stone types — roughly 125 sq ft per ton for river rock versus 300+ sq ft per ton for lava rock at the same 2-inch depth — always calculate with the density of the specific product you're buying rather than assuming one number applies across the board.
Beyond looks, the choice affects upkeep too. River rock and pea gravel are dense and heavy enough to stay put through rain and wind, and they don't break down, which suits high-visibility beds and xeriscaped yards where drainage and low maintenance matter. Lava rock's lighter weight makes it a common pick around fire features and grills, since it's naturally heat-resistant, but the same porous structure that makes it light also lets it crack and crumble faster underfoot over the years — worth factoring in if the bed will see regular foot traffic.