Riprap / Rock Calculator
Find out exactly how much riprap or rock your erosion control or slope project needs — tons, cubic yards, truckloads and cost.
Your area
18 in is typical for medium riprap.
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Coverage breakdown
9.0 tons covers 300 sq ft at 18 in thick
Delivery & hauling
3.0
Dump truck loads, ~10 tons each
Fabric & thickness planner
The short answer
A 300 sq ft area covered 18 inches deep with medium riprap needs about 9 tons of rock. Work it out precisely by finding the area, multiplying by layer thickness in feet to get volume, adding 5–10% waste, then converting to weight using the rock’s placed density — around 115 to 125 lb per cubic foot depending on class.
Riprap needed by area and class
Figures below use each class’s own recommended layer thickness, no waste added.
| Rock class | Thickness | Volume | Cubic yards | Tons |
|---|---|---|---|---|
| Light (4-8 in) | 12 in | 100 cu ft | 3.7 | 5.75 |
| Medium (8-18 in) | 18 in | 150 cu ft | 5.56 | 9.0 |
| Heavy (18-24+ in) | 24 in | 200 cu ft | 7.4 | 12.5 |
Riprap is almost always delivered and priced by the ton rather than by the bag or by volume alone.
Choosing a rock class
| Light | Medium | Heavy | |
|---|---|---|---|
| Stone size | 4-8 in | 8-18 in | 18-24+ in |
| Placed density | ~115 lb/cu ft | ~120 lb/cu ft | ~125 lb/cu ft |
| Recommended thickness | ~12 in | ~18 in | ~24 in or more |
| Best for | Low-flow ditches, mild slopes | Typical channel banks, shorelines | High-flow channels, wave action |
How to calculate how much riprap you need
Riprap is priced by weight, so the calculation converts a volume into tons.
- Find the area. Length × width for a rectangle, or π × radius² for a circle.
- Convert thickness to feet. Divide the layer thickness in inches by 12.
- Find the volume. Multiply area by thickness in feet to get cubic feet.
- Add waste. 5–10% covers settling and slope irregularities.
- Convert to weight. Multiply the volume by your rock class’s placed density.
- Convert to tons. Divide the total weight in pounds by 2,000.
Worked example
A 30×10 ft slope, covered 18 inches deep with medium riprap:
- Area30 × 10 = 300 sq ft
- Thickness in feet18 ÷ 12 = 1.5 ft
- Volume300 × 1.5 = 450 cu ft
- Plus 10% waste495 cu ft
- Weight (× 120 lb/cu ft)59,400 lb
- Convert to tons59,400 ÷ 2,000 = 29.7
- Cubic yards495 ÷ 27 = 18.3
Almost 30 tons is roughly 3 truckloads on a standard 10-ton dump truck.
Placing riprap: a quick checklist
- Grade the slopeShape the subgrade to the planned slope angle before placing any rock.
- Lay filter fabricGeotextile fabric under the riprap keeps soil from washing out between the stones.
- Place from the bottom upStarting at the toe of the slope and working upward keeps rock from rolling downhill.
- Key the toe into the groundAn embedded toe keeps the bottom row from undermining and shifting.
- Interlock the rockWell-placed riprap has stones nested together, not just dumped in a loose pile.
- Check thickness across the whole areaThin spots are the first places erosion breaks through.
Common mistakes
- Skipping filter fabricWithout it, soil migrates up through the gaps between rocks and the slope can undermine from below.
- Undersizing the rock classRock too small for the water’s flow velocity can wash away, defeating the purpose of the riprap.
- Layer too thinA layer thinner than the rule of thumb leaves gaps where erosion can start.
- No toe protectionAn unprotected toe at the base of the slope can erode and undermine the whole installation.
- Dumping instead of placingLoosely dumped riprap has more voids and less interlock than hand- or machine-placed rock.
- Ordering by volume aloneSince riprap is priced by weight, a volume-only estimate can under- or over-order the actual tonnage needed.
What this calculator assumes
Adjust your result if your project differs from these.
- Placed densities of roughly 115, 120 and 125 lb per cubic foot for light, medium and heavy riprap
- Areas treated as simple rectangles or circles at a uniform layer thickness
- A dump truck holding about 10 tons per load
- A loader bucket holding about 1 ton per pass
- Filter fabric area matching the riprap footprint, with no extra overlap allowance
- The rock class comparison table uses each class’s own recommended thickness, not your custom depth
- Prices are whatever you enter; they are not live market rates
Common questions
How much riprap do I need for erosion control?
Multiply the area by the recommended layer thickness in feet, add 5 to 10 percent waste, then convert to weight using the rock’s density. 100 square feet at an 18 inch layer of medium riprap needs about 9 tons.
What size riprap should I use?
Rock size is chosen based on water flow velocity and slope steepness: light riprap (4 to 8 in) suits low-flow areas, medium (8 to 18 in) suits typical channel banks, and heavy riprap (18 to 24 in or larger) handles high-flow or high-energy sites.
How thick should a riprap layer be?
A common rule of thumb is a layer thickness of about 1.5 to 2 times the largest stone size. Light riprap typically needs about 12 inches, medium about 18 inches, and heavy riprap about 24 inches or more.
Do I need filter fabric under riprap?
Yes, in most cases. Geotextile filter fabric under riprap keeps the underlying soil from washing out through the gaps between the rocks, while still letting water pass through.
How much does riprap weigh?
Placed riprap typically weighs around 115 to 125 lb per cubic foot, or roughly 1.5 to 1.7 tons per cubic yard, accounting for the voids between stones.
Is riprap sold by the ton or by the yard?
Riprap is almost always sold and delivered by weight, in tons, since irregular rock sizes make volume measurement less consistent than with sand or gravel.
What is the difference between riprap classes?
Riprap classes group stone by size range and typical weight, matched to the energy of the water or slope they need to resist. Larger class riprap costs more but withstands higher flow velocities.
Can I install riprap on a steep slope myself?
Small, gentle slopes can sometimes be hand-placed, but riprap for steep banks or high-flow channels is heavy work usually done with an excavator or skid steer, and often needs engineering input.
How much does riprap cost per ton?
Cost varies by rock class, source quarry and delivery distance. Larger, heavier riprap classes generally cost more per ton than smaller classes. Use your own local supplier pricing for an accurate total.
How many tons of riprap fit on a dump truck?
A standard tandem dump truck typically hauls around 10 to 12 tons of riprap per load, though capacity varies by truck size.
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Related calculators
Riprap / Rock Estimate
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materialcalculate.com/tools/riprap-rock-calculator/
Project
| Area shape | — |
| Dimensions | — |
| Layer thickness | — |
| Rock class | — |
| Waste allowance | — |
Volume & weight
| Volume | — |
| Tons | — |
| Truckloads (~10 tons each) | — |
| Filter fabric needed | — |
Cost
| Estimated cost | — |
Notes
Estimate generated by materialcalculate.com. Assumes rectangular or circular areas at uniform depth, and placed rock densities of roughly 115 to 125 lb per cubic foot depending on class. Confirm quantities with your supplier before buying.