River rock densities — and why lava rock breaks every chart
Most rock calculators apply one density to everything, which works until it badly doesn’t. Typical bulk densities for the common decorative products:
| Product | Typical range (t/yd³) | Preset | Note |
|---|---|---|---|
| River rock (3/4″–2″) | 1.2–1.5 | 1.30 | Rounded stone packs looser than crushed |
| Large river rock / cobbles (2″–6″) | 1.2–1.45 | 1.25 | More void space between big stones |
| Pea gravel (1/4″–3/8″) | 1.4–1.6 | 1.45 | Small rounds pack tight |
| Lava rock | 0.5–0.8 | 0.65 | About half the weight of river rock — it’s vesicular (full of gas bubbles) |
| Decomposed granite, loose | 1.2–1.5 | 1.35 | As delivered |
| Decomposed granite, compacted | 1.5–1.8 | 1.65 | For pathways compacted in lifts — order to this if you’ll compact |
| Crushed granite (3/8″–3/4″) | 1.3–1.6 | 1.45 | Angular; locks together |
| Riprap (3″–12″) | 1.5–1.7 | 1.60 | Ordering estimate only — see disclaimer |
The lava rock row is the one that saves money. Because the stone is volcanic foam, a yard of it weighs roughly 1,100–1,500 lb against river rock’s 2,400–2,800 lb. Calculators that quietly apply a river-rock density to lava rock overestimate the tonnage by about double — and since lava rock often sells by the ton, that’s a doubled order. (If your supplier sells lava rock by the yard or the bag, the volume figure is the one to use anyway.)
How much does a ton of river rock cover?
Computed straight from the density (coverage = 27 × 12 ÷ depth ÷ density ÷ … — or just: one ton at 1.3 t/yd³ is 0.77 yd³ spread over the depth you choose):
| Product (preset density) | 2″ deep | 3″ deep | 4″ deep |
|---|---|---|---|
| River rock (1.30) | 125 sq ft/ton | 83 sq ft/ton | 62 sq ft/ton |
| Pea gravel (1.45) | 112 sq ft/ton | 74 sq ft/ton | 56 sq ft/ton |
| Lava rock (0.65) | 249 sq ft/ton | 166 sq ft/ton | 125 sq ft/ton |
| Decomposed granite, loose (1.35) | 120 sq ft/ton | 80 sq ft/ton | 60 sq ft/ton |
| Crushed granite (1.45) | 112 sq ft/ton | 74 sq ft/ton | 56 sq ft/ton |
Depth guidance: 2″ suits pea gravel and small rock over fabric; 3″ is the standard for 3/4″–1.5″ river rock beds; 4″+ for large cobbles, dry creek beds and anywhere the rock must hide the ground completely. As a rule, the bed should be about twice the largest stone dimension deep.
Flagstone coverage per ton
Flagstone is sold by the ton but laid by the square foot, and the bridge between them is the stone’s thickness. Typical coverage:
| Thickness | Coverage per ton | Use |
|---|---|---|
| ~1″ (patio-grade) | 100–120 sq ft | Mortared over a slab, or light-traffic paths on a good base |
| ~1.5″ | 80–90 sq ft | The standard dry-laid patio thickness |
| ~2″ | 60–80 sq ft | Dry-laid, heavy traffic, stepping pads in lawn |
These figures already absorb typical cutting waste from irregular pieces. A 300 sq ft dry-laid patio in 1.5″ stone therefore needs roughly 300 ÷ 85 ≈ 3.5 tons. Denser stone types run to the low end of each range, lighter sandstones to the high end — ask the yard for the coverage of the specific pallet.
Stepping stone path spacing
Stepping stones are spaced to a walking stride: 24–26 inches center-to-center feels natural for most adults (a leisurely garden pace; tighten toward 22″ for paths children use). The count is the path length in inches divided by the spacing, plus one for the starting stone:
Set each stone so its top sits flush with or a touch above grade, on a couple of inches of compacted sand or fines — and if the stones float in a river-rock bed, calculate the rock for the full area and let the stones displace what they displace; the 10% overage covers the difference.
Ordering: yards or tons?
Suppliers quote decorative rock both ways. The calculator gives both so the comparison is direct: yards for volume-priced products and bagged material, tons for scale-priced bulk. When comparing two quotes in different units, convert with the density — and notice that for lava rock a “cheap” per-ton price can be expensive per yard of coverage if the seller used a heavy-rock density.