How to Calculate River Rock
River rock is smooth, tumbled stone quarried from riverbeds and sized by screening. It is used for decorative landscaping, dry creek beds, tree rings, drainage, and around water features. Unlike crushed gravel, river rock does not compact under load and is not suitable for structural bases like driveways or paver underlayment. Calculating the right volume means multiplying length, width, and depth in consistent units, then converting to tons using the size-specific density.
Volume Formulas
Weight Conversion
River rock weighs 90 to 100 pounds per cubic foot. Larger stones have more air space and lower effective density:
River Rock Sizes and Coverage
Coverage per ton depends on both rock size and installation depth. Larger rocks cover less area per ton but fewer rocks are needed per square foot:
| Size | Range | Density (lb/cu ft) | Coverage per Ton | Best Use |
|---|---|---|---|---|
| Small (pea) | 3/8 - 3/4 in | 100 | 120 sq ft (at 2") | Walkways, play areas, ground cover |
| Medium | 1 - 2 in | 100 | 100 sq ft (at 2") | Landscape beds, drainage swales |
| Large | 2 - 4 in | 95 | 70 sq ft (at 3") | Dry creek beds, erosion control |
| Jumbo | 4 - 8 in | 90 | 40 sq ft (at 4") | Focal points, water features |
Depth Recommendations
- Ground cover / weed suppression: 2 inches of small pea-sized rock
- Decorative landscape beds: 2-3 inches of medium (1-2 in) rock
- Walkways and paths: 3 inches of small rock over compacted base
- Dry creek beds: 3-4 inches of mixed large rock with occasional jumbo accents
- Drainage ditches: 6 inches of medium to large angular stone (not river rock - use crushed)
- Focal point accents: jumbo rock placed as single-layer feature
River Rock vs Crushed Gravel
River rock and crushed gravel look similar but perform completely differently. Know which you need:
| Property | River Rock | Crushed Gravel |
|---|---|---|
| Shape | Smooth, rounded | Angular, sharp |
| Compaction | None - stays loose | Locks under load |
| Best Use | Decorative, dry creek | Structural base, driveway |
| Walkability | Rolls underfoot | Stable surface |
| Typical Cost | $60-100 / ton | $30-50 / ton |
Bulk vs Bagged Cost Comparison
Retail bagged river rock (0.5 cu ft per bag, ~50 lbs) costs 4 to 6 dollars per bag. Forty bags equal 1 ton - so bagged costs 160 to 240 per ton. Bulk delivery runs 60 to 100 per ton for standard varieties, with delivery fees of 50 to 150. Break-even is around 0.5 to 0.75 ton depending on local delivery rates. Premium varieties (Mexican beach pebble, polished black, Montana rainbow) cost 150 to 400 per ton in bulk.
Installation Timeline
- Day 1: excavate 2-4 inches below finish grade, level the subgrade
- Day 1: install edging (steel, plastic, or stone), stake in place
- Day 1: roll out landscape fabric, overlap seams, pin with staples
- Day 1-2: dump and spread rock, rake level to specified depth
- Year 1: may need to top off after settling (1/2 to 1 inch)
- Year 3-5: rake and refresh - some rocks migrate into soil