Multiply length × width × depth in feet, divide by 27 for cubic yards, then multiply by 1.65 to get tons of riprap. A 30 × 5 ft slope at 12 inches deep needs about 5.6 cubic yards — roughly 9.2 tons.
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Order – tons (includes your waste allowance) so compaction and spillage don’t leave you short.
Riprap Calculator Guide
Riprap is large, angular stone placed to armor slopes, shorelines, channels, and outfalls against the erosive force of moving water. It’s the heaviest common aggregate — 1.65 tons per cubic yard — because the stone is big, dense, and packs with relatively little void space once it settles into place. This calculator sizes riprap projects with a 15% waste allowance built in by default, since irregular slope faces rarely use material as efficiently as a flat area.
Riprap sizing is a function of water energy, not preference — undersized stone simply washes away in the next storm.
How Much Riprap Do I Need Per Slope Length?
| Slope width | Layer depth | Tons per 10 linear ft |
|---|---|---|
| 4 ft | 12 in | 2.4 tons |
| 6 ft | 12 in | 3.7 tons |
| 4 ft | 18 in | 3.7 tons |
| 6 ft | 18 in | 5.5 tons |
These figures already include a 15% allowance for the uneven placement typical of slope work. For a channel or shoreline that runs 100+ feet, multiply the per-10-ft figure by 10 rather than re-measuring the whole run at once — it’s faster and just as accurate.
Sizing Riprap to the Water, Not the Look
- Low-flow swales and small streams: 3–6 inch stone, 12-inch layer.
- Shoreline and pond edge protection: 6–12 inch stone, 12–18 inch layer.
- Culvert outlets and high-velocity channels: 12–24 inch or engineered stone, sized per a drainage calculation, not a rule of thumb.
Always lay woven geotextile fabric before placing riprap — it’s what stops fines from washing out from beneath the stone, which is how most riprap slopes actually fail even when the visible stone never shifts. For smaller, decorative water features, river rock is the better-looking, lighter-duty alternative; for structural drainage layers beneath riprap, see the #57 stone calculator.
Frequently Asked Questions
How thick should a riprap layer be?
Layer thickness should be at least 1.5 times the largest stone diameter — so 12–18 inches for typical 6–12 inch class riprap, and up to 3 feet for large stone protecting high-velocity channels.
How much does riprap weigh per cubic yard?
Riprap runs about 1.65 US tons per cubic yard — the heaviest common aggregate, because it’s large, dense, angular stone with relatively little void space once placed.
Do I need fabric under riprap?
Yes, always. Woven geotextile fabric between riprap and soil stops fines from being washed out from underneath, which is the main way riprap slopes fail over time even when the stone itself never moves.
What size riprap do I need?
Match stone size to water velocity: small streams and swales use 3–6 inch stone, moderate flow and shoreline protection use 6–12 inch, and high-energy areas like culvert outlets need 12–24 inch or larger — check local drainage specs before ordering.